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Microwave-based medical diagnosis using particle swarm optimization algorithm

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MICROWAVE-BASED MEDICAL DIAGNOSIS USING
PARTICLE SWARM OPTIMIZATION ALGORITHM
by
Arezoo Modiri
APPROVED BY SUPERVISORY COMMITTEE:
Kamran Kiasaleh, Chair
Lakshman S. Tamil
Randall E. Lehmann
Rashaunda M. Henderson
c 2013
Copyright Arezoo Modiri
All rights reserved
This Dissertation
is dedicated to my beloved parents
and brother.
MICROWAVE-BASED MEDICAL DIAGNOSIS USING
PARTICLE SWARM OPTIMIZATION ALGORITHM
by
AREZOO MODIRI, BS, MS
DISSERTATION
Presented to the Faculty of
The University of Texas at Dallas
in Partial Fulfillment
of the Requirements
for the Degree of
DOCTOR OF PHILOSOPHY IN
ELECTRICAL ENGINEERING
THE UNIVERSITY OF TEXAS AT DALLAS
December 2013
UMI Number: 3606254
All rights reserved
INFORMATION TO ALL USERS
The quality of this reproduction is dependent upon the quality of the copy submitted.
In the unlikely event that the author did not send a complete manuscript
and there are missing pages, these will be noted. Also, if material had to be removed,
a note will indicate the deletion.
UMI 3606254
Published by ProQuest LLC (2013). Copyright in the Dissertation held by the Author.
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ACKNOWLEDGMENTS
As the author, it is almost impossible to express gratitude in mere words for the wonderful
people who have helped and supported me during my doctoral journey. I feel blessed for
having every single one of them in my life.
First and foremost, I would like to thank my PhD advisor, Professor Kamran Kiasaleh,
for his continuous guidance and support. For me, it was an amazing opportunity to get
engaged in several distinct, thrilling, professional activities during my PhD, and Professor
Kiasaleh’s leading role as a thoughtful mentor was indeed a significant factor for making
these opportunities become reachable.
In addition, I am very grateful to professors Lakshman S. Tamil, Randall E. Lehmann and
Rashaunda M. Henderson for their persistent, considerate support throughout my PhD and
also for serving in my dissertation committee. I am also very thankful to Dr. Robert Glosser
and Dr. Ricardo Saad for their sincere, continuous help and Dr. Dennis Miller for his great
help to my research project. I have learned much more than course material from my UTD
professors, especially those for whom I have served as teacher assistant. Their friendly and
meanwhile professional presence in my PhD life has thought me invaluable lessons.
I should thank the friendly and thoughtful UTD staff, as well. They have been helping me
out with every single step during my education. For sure, being with talented faculty and
staff in UTD has been a rewarding experience and wonderful learning opportunity for me,
and unfortunately, it is not possible to list the names of all people I want to thank in this
page.
I am specially thankful to my lab mates, Ali Montazeri, Gaurav Sureka, Anish Nair, Zhengjie
Liu, Raffat Khan, Xiajiang Tian, Allen Webb, Damin Cao, Michael Cole and Jaskaran Sing
v
who have made my PhD journey much more pleasant and memorable. In addition, I am
grateful to my very good friends Miad Faezipour and Soudeh Khoubrouy, and of course, to
many other friends, the names of whom cannot be mentioned herein. Good friends make life
full of blessing.
Finally, I would like to emphasize that my educational endeavors have always been strongly
supported by my dearly loved parents and brother in any possible way. They have been
constantly encouraging me and helping me with the challenges of my life as an international
student. No words can express my gratefulness to them.
July 2013
vi
PREFACE
This dissertation was produced in accordance with guidelines which permit the inclusion as
part of the dissertation the text of an original paper or papers submitted for publication.
The dissertation must still conform to all other requirements explained in the “Guide for the
Preparation of Master’s Theses and Doctoral Dissertations at The University of Texas at
Dallas.” It must include a comprehensive abstract, a full introduction and literature review,
and a final overall conclusion. Additional material (procedural and design data as well as
descriptions of equipment) must be provided in sufficient detail to allow a clear and precise
judgment to be made of the importance and originality of the research reported.
It is acceptable for this dissertation to include as chapters authentic copies of papers already
published, provided these meet type size, margin, and legibility requirements. In such cases,
connecting texts which provide logical bridges between different manuscripts are mandatory.
Where the student is not the sole author of a manuscript, the student is required to make an
explicit statement in the introductory material to that manuscript describing the student’s
contribution to the work and acknowledging the contribution of the other author(s). The
signatures of the Supervising Committee which precede all other material in the dissertation
attest to the accuracy of this statement.
vii
MICROWAVE-BASED MEDICAL DIAGNOSIS USING
PARTICLE SWARM OPTIMIZATION ALGORITHM
Publication No.
Arezoo Modiri, PhD
The University of Texas at Dallas, 2013
Supervising Professor: Kamran Kiasaleh
This dissertation proposes and investigates a novel architecture intended for microwave-based
medical diagnosis (MBMD). Furthermore, this investigation proposes novel modifications
of particle swarm optimization algorithm for achieving enhanced convergence performance.
MBMD has been investigated through a variety of innovative techniques in the literature
since the 1990’s and has shown significant promise in early detection of some specific health
threats. In comparison to the X-ray- and gamma-ray-based diagnostic tools, MBMD does
not expose patients to ionizing radiation; and due to the maturity of microwave technology,
it lends itself to miniaturization of the supporting systems. This modality has been shown
to be effective in detecting breast malignancy, and hence, this study focuses on the same
modality. A novel radiator device and detection technique is proposed and investigated in
this dissertation. As expected, hardware design and implementation are of paramount importance in such a study, and a good deal of research, analysis, and evaluation has been
done in this regard which will be reported in ensuing chapters of this dissertation. It is
noteworthy that an important element of any detection system is the algorithm used for
viii
extracting signatures. Herein, the strong intrinsic potential of the swarm-intelligence-based
algorithms in solving complicated electromagnetic problems is brought to bear. This task is
accomplished through addressing both mathematical and electromagnetic problems. These
problems are called benchmark problems throughout this dissertation, since they have known
answers. After evaluating the performance of the algorithm for the chosen benchmark problems, the algorithm is applied to MBMD tumor detection problem. The chosen benchmark
problems have already been tackled by solution techniques other than particle swarm optimization (PSO) algorithm, the results of which can be found in the literature. However, due
to the relatively high level of complexity and randomness inherent to the selection of electromagnetic benchmark problems, a trend to resort to oversimplification in order to arrive at
reasonable solutions has been taken in literature when utilizing analytical techniques. Here,
an attempt has been made to avoid oversimplification when using the proposed swarm-based
optimization algorithms.
To summarize the outline of this dissertation, the first chapter introduces the different elements of the comprehensive study which was explained very concisely in the above paragraph.
Chapters 2 and 3 present the details of the proposed solutions for the benchmark problems
and the MBMD problem, respectively. The remaining chapters are the re-productions of the
outcome of this dissertation, which have been published (or submitted for publication) by
the candidate and her supervisor during her PhD in scholarly, refereed journals and conference proceedings. The first chapter is mostly taken from their published book chapter plus
afterward achievement.
ix
TABLE OF CONTENTS
ACKNOWLEDGMENTS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
PREFACE
v
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
vii
ABSTRACT . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
viii
LIST OF FIGURES . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
xii
LIST OF TABLES . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
xv
CHAPTER 1 INTRODUCTION . . . . . . . . . . . . . . . . . . . . . . . . . . . .
1
1.1
Particle swarm optimization; development, modifications . . . . . . . . . . .
6
1.1.1
Conventional PSO . . . . . . . . . . . . . . . . . . . . . . . . . . . .
6
1.1.2
PSO variations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
10
1.2
Case study 1: PSO in antenna beam forming . . . . . . . . . . . . . . . . . .
16
1.3
Case study 2; PSO in frequency selectivity . . . . . . . . . . . . . . . . . . .
18
1.4
Case study 3: PSO in bend effect compensation . . . . . . . . . . . . . . . .
22
1.5
Breast cancer detection using PSO in MBMD . . . . . . . . . . . . . . . . .
27
1.5.1
Modeling and estimation methodology . . . . . . . . . . . . . . . . .
31
1.5.2
Implementation of microwave diagnosis tool . . . . . . . . . . . . . .
36
1.5.3
Preliminary results . . . . . . . . . . . . . . . . . . . . . . . . . . . .
40
CHAPTER 2 PROPOSED PSO IN ANTENNA DESIGN . . . . . . . . . . . . . . .
44
2.1
Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
44
2.2
PSO in thinned array beam forming1 . . . . . . . . . . . . . . . . . . . . . .
44
2.2.1
Evaluation of the proposed velocity function in RPSO . . . . . . . . .
46
2.2.2
Evaluation of the proposed velocity function in thinned array design .
55
PSO in antenna frequency selectivity2 . . . . . . . . . . . . . . . . . . . . . .
62
2.3.1
Switchable antenna design using modified BPSO . . . . . . . . . . . .
64
2.3.2
Optimization length comparison summary . . . . . . . . . . . . . . .
69
PSO in wearable antenna frequency selectivity3 . . . . . . . . . . . . . . . .
71
2.3
2.4
x
2.5
Modeling antenna bend effect . . . . . . . . . . . . . . . . . . . . . . . . . .
72
2.6
Results . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
76
CHAPTER 3 PROPOSED MBMD ARRANGEMENT . . . . . . . . . . . . . . . .
81
PSO in solving the inverse problem4 . . . . . . . . . . . . . . . . . . . . . . .
82
3.1.1
Problem scope and estimation algorithm . . . . . . . . . . . . . . . .
83
3.1.2
Parameter variation study . . . . . . . . . . . . . . . . . . . . . . . .
87
MBMD proposed radiator design5 . . . . . . . . . . . . . . . . . . . . . . . .
92
3.2.1
The array structure . . . . . . . . . . . . . . . . . . . . . . . . . . . .
93
3.2.2
Analysis results . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
94
Adding a conductor cover . . . . . . . . . . . . . . . . . . . . . . . . . . . .
97
3.1
3.2
3.3
3.4
3.3.1
Proposed structure and 3D model . . . . . . . . . . . . . . . . . . . . 101
3.3.2
Detection mechanism and analysis . . . . . . . . . . . . . . . . . . . . 104
Experimental results6 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 110
3.4.1
Measurement setup . . . . . . . . . . . . . . . . . . . . . . . . . . . . 114
3.4.2
Results and analysis . . . . . . . . . . . . . . . . . . . . . . . . . . . 118
3.4.3
Comparing with a competing MBMD design . . . . . . . . . . . . . . 121
3.4.4
Calibration . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 123
CHAPTER 4 CONCLUSIONS AND FUTURE RESEARCH DIRECTIONS . . . . 132
REFERENCES . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 134
VITA
xi
LIST OF FIGURES
1.1 Fitness values of the four introduced methods averaged over one thousand runs .
15
1.2 Probability density functions of the final fitness values . . . . . . . . . . . . . .
15
1.3 Fitness value trends of the introduced methods averaged over 700 runs . . . . .
18
1.4 The fifteen and twenty segment microstrip antennas . . . . . . . . . . . . . . . .
21
1.5 Average fitness value trend and the best optimization results . . . . . . . . . . .
24
1.6 Measurement arrangements for the wearable antenna . . . . . . . . . . . . . . .
25
1.7 |S11 | measurement results . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
27
1.8 Simplified two dimensional model . . . . . . . . . . . . . . . . . . . . . . . . . .
35
1.9 Average fitness function trends for the two scenarios. . . . . . . . . . . . . . . .
36
1.10 Comparison of the actual values with the estimated values . . . . . . . . . . . .
37
1.11 The preliminary radiator structure . . . . . . . . . . . . . . . . . . . . . . . . .
39
1.12 Digital breast phantom along with the proposed radiator structure. . . . . . . .
39
1.13 Cumulative magnitude difference of reflection coefficient . . . . . . . . . . . . .
42
1.14 Cumulative magnitude difference of transmittance coefficient . . . . . . . . . . .
43
2.1 AIN behaviour of the four methods . . . . . . . . . . . . . . . . . . . . . . . . .
48
2.2 AFV behaviour of the four methods . . . . . . . . . . . . . . . . . . . . . . . . .
49
2.3 AIN . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
50
2.4 AFV . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
51
2.5 Fitness value of the four methods averaged over 1000 runs . . . . . . . . . . . .
52
2.6 Average sample particle position . . . . . . . . . . . . . . . . . . . . . . . . . .
53
2.7 Fitness value of the four methods averaged over 500 runs . . . . . . . . . . . . .
54
2.8 Fitness value of the four methods averaged over 500 runs with 30 particles . . .
55
2.9 Symmetric half-wavelength thinned linear array. . . . . . . . . . . . . . . . . . .
57
2.10 AIN behaviour in a 10D problem . . . . . . . . . . . . . . . . . . . . . . . . . .
58
2.11 AFV behaviour in a 10D problem . . . . . . . . . . . . . . . . . . . . . . . . . .
58
xii
2.12 Non-symmetric thinned planar antenna array. . . . . . . . . . . . . . . . . . . .
59
2.13 AIN and AFV behaviour in a 9D problem . . . . . . . . . . . . . . . . . . . . .
60
2.14 Probability density function in a 9D problem . . . . . . . . . . . . . . . . . . . .
61
2.15 Fitness value trend in a 9D problem
. . . . . . . . . . . . . . . . . . . . . . . .
61
2.16 Fitness value trend over 500 runs . . . . . . . . . . . . . . . . . . . . . . . . . .
62
2.17 15-segment reconfigurable microstrip antenna structure. . . . . . . . . . . . . . .
65
2.18 Probability density function of BPSO algorithms . . . . . . . . . . . . . . . . .
66
2.19 Final fitness value of BPSO algorithms . . . . . . . . . . . . . . . . . . . . . . .
67
2.20 Particle position for BPSO algorithms . . . . . . . . . . . . . . . . . . . . . . .
68
2.21 Final fitness value of BPSO algorithms sing 4 particles . . . . . . . . . . . . . .
69
2.22 Measurement arrangement . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
74
2.23 |S11 | measurement results for the flat and bent cases. . . . . . . . . . . . . . . .
74
2.24 Reconfigurable antenna with (a)three and (b) eight switches. . . . . . . . . . . .
75
2.25 |S11 | measurement results . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
76
2.26 Average fitness value trend . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
79
2.27 The final fitness value of each run . . . . . . . . . . . . . . . . . . . . . . . . . .
80
3.1 The block diagram of the proposed PSO . . . . . . . . . . . . . . . . . . . . . .
85
3.2 Mapping function with different smoothing coefficients. . . . . . . . . . . . . . .
86
3.3 The effect of immersion medium . . . . . . . . . . . . . . . . . . . . . . . . . . .
88
3.4 Estimation error . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
89
3.5 The error for thicker tissue . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
89
3.6 Estimation error assuming different maximum velocities/smoothing coefficients .
91
3.7 Estimation error assuming different number of agents. . . . . . . . . . . . . . . .
91
3.8 The Ansoft digital phantom breast tissue and the antenna array. . . . . . . . . .
94
3.9 The reflection coefficient of the five elements of the array. . . . . . . . . . . . . .
95
3.10 The radiation pattern of the radiator . . . . . . . . . . . . . . . . . . . . . . . .
97
3.11 The polarization-dependent radiation pattern of the radiator . . . . . . . . . . .
98
3.12 The electric field distribution in tissue . . . . . . . . . . . . . . . . . . . . . . .
99
3.13 The transmittance coefficient of the array elements. . . . . . . . . . . . . . . . . 100
xiii
3.14 nsoft female body phantom . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 103
3.15 Digital breast phantom along with the proposed radiator structure. . . . . . . . 104
3.16 Tumors’ shapes and locations inside the breast. . . . . . . . . . . . . . . . . . . 107
3.17 The normalized differences of |Ex | . . . . . . . . . . . . . . . . . . . . . . . . . . 108
i
i
3.18 |End
| and arg(End
) for the superficial spherical tumor . . . . . . . . . . . . . . . 110
i
i
3.19 |End
| and arg(End
) for the deep spherical tumor . . . . . . . . . . . . . . . . . . 111
i
i
3.20 |End
| and arg(End
) for the deep small spherical tumor . . . . . . . . . . . . . . . 112
i
i
3.21 |End
| and arg(End
) for the superficial small spherical tumor . . . . . . . . . . . 113
i
i
3.22 |End
| and arg(End
) for the superficial tiny spherical tumor . . . . . . . . . . . . 114
i
i
3.23 |End
| and arg(End
) for the deep tiny spherical tumor . . . . . . . . . . . . . . . 115
i
i
3.24 |End
| and arg(End
) for the superficial cylindrical tumor. . . . . . . . . . . . . . . 116
i
i
3.25 |End
| and arg(End
) for the deep cylindrical tumor. . . . . . . . . . . . . . . . . . 117
i
i
3.26 |End
| and arg(End
) for the superficial gland-shaped tumor. . . . . . . . . . . . . 118
i
i
3.27 |End
| and arg(End
) for the spherical tumor over 101 receiving points. . . . . . . 119
i
i
3.28 |End
| and arg(End
) for the cylindrical tumor over 101 receiving points. . . . . . 120
3.29 The creation process of the two breast phantoms . . . . . . . . . . . . . . . . . 121
3.30 The measurement setup using an ENA. . . . . . . . . . . . . . . . . . . . . . . . 125
3.31 The measured |S11 | of the sixteen antennas . . . . . . . . . . . . . . . . . . . . . 126
3.32 Cumulative phase difference of reflection coefficient . . . . . . . . . . . . . . . . 126
3.33 Cumulative phase difference of transmittance coefficient . . . . . . . . . . . . . . 127
3.34 The MBMD design from Duke University and their phantom. . . . . . . . . . . 128
3.35 Comparing the phantom measurement results . . . . . . . . . . . . . . . . . . . 129
3.36 Comparing signatures achieved from tumor with misplacement . . . . . . . . . . 130
3.37 Digital breast phantom modeling misplacement. . . . . . . . . . . . . . . . . . . 130
3.38 Tumor signature and movement effect on adjacent antennas. . . . . . . . . . . . 131
xiv
LIST OF TABLES
1.1 Optimization cost . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
34
2.1 Required optimization time for a single 25-iteration run in minutes. . . . . . . .
70
3.1 Optimization length comparison . . . . . . . . . . . . . . . . . . . . . . . . . . .
92
xv
CHAPTER 1
INTRODUCTION1
Authors - Arezoo Modiri and Kamran Kiasaleh
Electrical Engineering Department
The University of Texas at Dallas
800 West Campbell Road
Richardson, Texas 75080-3021
1
This chapter is adapted from chapter 5 in <Swarm Intelligence for Electric and Electronic Engineering>
c 2013, IGI Global, www.igi-global.com.
edited by <Girolamo Fornarelli and Luciano Mescia> Copyright Posted by permission of the publisher.
1
2
Microwave-based medical diagnosis (MBMD) is a tremendously complicated electromagnetic problem both hardware wise and software wise. This study reports the promising
results of a preliminary solution to this complex problem. For the sake of clarity, the author
prefers to start with explaining the proposed algorithm, its variants and its responses to
various problems, and, then, proceed with the radiator design, implementation aspects and
measurement results.
A quick survey of the literature indicates that distinct algorithms have been selected to solve
MBMD problem; however, a reasonable approach is to test the chosen algorithm in some
known problems prior to introducing it to the main unknown one.
This chapter underscores the flexibility and the power of the particle swarm optimization
(PSO) algorithm in handling a variety of analytically intractable problems in order to pave
the path for delving into the foremost objective MBMD one. To that end, specific scenarios,
which are representatives of a larger class of problems, are explored using PSO. The chapter
begins by providing a detailed definition of PSO and its modified versions and then continues
with the discussions regarding the challenges of solving realistic problems by means of this
algorithm. In each case study, along with the problem definition, the key characteristics of
the problem are brought to the forefront. This, in fact, tells the reader what other problems
can be solved in the same or similar way. The conventional algorithm, then, is modified,
exclusively to fit the requirements of that specific problem, and, finally, the performance of
the resulting algorithm is presented and evaluated. Although the benchmark problems seem
to be quite distinct from the MBMD one, they possess major common properties, which
make a swarm-based algorithm a viable search mechanism for them. Those properties can
be summarized as follows:
• The inherent random nature of the key variables of the problem.
• The large number of unknowns as compared to the available information.
3
• The necessity of expeditious solution discovery due to the ultimate real-time nature of
the corresponding application.
The outline of this chapter is as follows: In section 1.1, PSO algorithm is introduced, and
several modifications to the conventional format of the algorithm are presented and compared in terms of the final result accuracy and optimization cost. In addition to the results
presented in (Modiri, et. al., 2011b), this section brings together a summary of the other
discoveries regarding PSO, which are presented in the literature, such as those introduced
by (Nakano, et. al., 2007; Clerc, et. al,. 2002). A good list of the articles in this area can
be found in the reference list of (Modiri, et. al., 2011b).
When studying optimization algorithms, it is important to bear in mind that, in general,
there are two approaches one may take. The first approach is dedicated to a suitable solution found expeditiously, whereas the second approach targets the best possible solution
regardless of the optimization time or cost. One needs to compromise with respect to the
features of the problem in hand. Both real-number PSO (RPSO) and binary PSO (BPSO)
algorithms are taken into account. In order to rationalize the introduced modifications, wellknown mathematical benchmark problems are evaluated. This allows the author to set the
stage for the more challenging problems of later sections.
In section 1.2-1.4, based on the studies presented in (Modiri, et. al., 2011a; Modiri, et. al.,
2011d; Modiri, et. al., 2010), the author concisely illustrates how PSO is utilized in a more
challenging (and of course more appealing) scenarios. In particular, the design of reconfigurable and wearable antennas are considered (Liu, et. al., 2005; Jin, et. al., 2008). These
are, in fact, the electromagnetic problems chosen to test PSO prior to introducing it to the
main problem. In order to illustrate how PSO as a viable technique enters the picture, a
brief description of the problem is presented in the next paragraphs.
Reconfigurable antenna problems which are studied herein are useful in a variety of applications where software defined control over the antenna performance is required. Particularly,
4
the ability to change the frequency performance of the antenna (in terms of resonance frequency) is investigated. This feature is appropriate in multi-application devices where each
application may have its own frequency assignment. Instead of incorporating multiple antennas for supporting individual applications, a single reconfigurable antenna can be designed.
The advantage of using PSO algorithm in this problem becomes even more obvious when a
level of unpredictability in the start time of an application is present in the overall design;
see (Jin, et. al., 2008) as an example. In addition to frequency selectivity, the antenna
radiation pattern can also be controlled in reconfigurable antennas, resulting in yet another
demanding application (Gies, et. al., 2003). System designers are often faced with the
problem of designing systems in areas which are jam-packed with wireless networks, such
as hospitals. In such environments, there is a high chance of interference between adjacent
communication links. Therefore, it is more convenient to be able to block the radiation in a
certain direction. Here, PSO algorithm is used to control the radiation pattern of the array
antennas with the goal of beam blocking, see (Jin, et. al., 2007).
Another topic studied here is wearable antennas (WA). Mounting antennas on the fiber of
daily clothing in the form of wearable antennas has recently attracted a great deal of interest.
This is mainly due to the growing demand for body sensor networks. Sensors are dependent
on the unobtrusive radiators in and around the human body in order to constantly trace
bio signatures over the entire monitoring period. This monitoring state, in some cases, may
continue for several months. However, it has been shown in the literature that various seemingly unnoticeable factors, including habitual activities of the user or common daily events
can degrade the WA performance substantially, see chapter 6 in (Hall, et. al., 2006). More
crucially, telecare devices are at risk when used for controlling the vital bio-signs of young,
active children remotely. Hence, circumventing the performance degradation caused by the
environmental variations can play an important role in preserving a high quality active monitoring. This task is done via applying a real-time, self-tuning algorithm. As a possible
5
variation, in section 1.4, we investigate the problem of bending the antenna and its effect on
the antenna performance. A solution technique is introduced for real-time compensation of
this phenomenon using PSO algorithm. The solution to this problem will also pave the way
for solving similar problems with antenna modification.
In the section 1.5, the usefulness of PSO in an even more challenging and attractive scenario
is discussed. In that scenario, the algorithm is used to estimate the electrical properties of
a lossy, multilayer structure. This study becomes more appealing when the medium under
investigation is the human body tissue. The main application of this study in real world,
as it is evident from the title of this dissertation, is detecting malignancy. The author’s research on breast cancer detection via microwave technology, partly published in (Modiri, et.
al. 2011c, 2012, and 2013), is the main focus of this section. Dealing with live tissue has its
intrinsic challenges; many articles in the literature concerning live tissue, e.g., (Lazebnik, et.
al., 2007), have demonstrated the semi-randomness of the electric properties of human tissue. Taking advantage of these studies, in section 1.5, a PSO-based tumor detection method
is reviewed, particularly for breast cancer detection through MBMD at frequencies below
3GHz. In fact, the proposed detection technique is not dependent on the type of the tissue
under investigation, and hence, can be used in other similar applications as well. However,
the limitation is imposed by the penetration depth of electromagnetic energy at the desired
frequency (herein, microwave) in different tissues. In this scenario, both complex number
PSO (CPSO) and discrete PSO (DPSO) algorithms are studied and redesigned in order to
address the requirements of the problem at hand. The main goal is to demonstrate that PSO
can successfully distinguish between the permittivity values of various layers of the human
tissue, enabling one to verify the existence and the possible location of a tumor.
It should be noted that, in addition to real number, binary, complex number and discrete
PSO algorithms studied in this dissertation, other versions of this algorithm have also been
introduced and utilized in distinct electromagnetic problems successfully in the literature.
6
Boolean PSO is an interesting example. Some more examples are introduced by Mahmoud
(2008), Janson (2006), Garcia-Nieto (2011), Jin (2010), Cai (2009), Zhao (2010), Xu (2009),
Ibrahim (2012) and Epitropakis (2012). However, those PSO variants are out of the scope
of this manuscript and the interested reader is referred to the available articles in the literature, such as (Afshinmanesh, et. al., 2008). It is also noteworthy to mention that, different
references do not necessarily use common abbreviations for distinct PSO variations. For
example, some articles use BPSO as the abbreviation for Binary PSO, and some for Boolean
PSO.
Later in section 1.5, the radiator structure proposed for MBMD is briefly introduced along
with the measurement setup and test environment. The proposed structure consists of an
array of curved planar dipole antennas incorporated on the inner surface of a hemispherical
dielectric body. The array is designed so that it creates complete radiation coverage all over
the tissue under the test. It is also shown that adding a conductor reflector sheet improves
the detection chance for most tumor cases and also acts as an electromagnetic shield. Digital
female body phantom is used for the sake of creating accurate simulation ground. For measurement trials, however, tissue phantoms are built and actual breast tumor donated from
a patient is placed inside the cancerous phantom.
Finally, concluding remarks along with a summary of the potential of PSO and the conditions under which such potentials can be achieved are provided in the conclusion section of
this chapter. It is important to emphasize that this chapter reviews all the aforementioned
subjects in brief. More details are presented in chapters 2 and 3.
1.1
1.1.1
Particle swarm optimization; development, modifications
Conventional PSO
As it is explained by Das (2008), Menhas (2011), Liu (2009), Sedighizadeh (2009), Banks
(2007) and many others, PSO is a member of the evolutionary algorithm (EA) family. The
7
idea of EAs originated from the concept of stochastic variation of a group of ’individuals’,
also named ’particles’, in a solution space. Various algorithms have been developed based on
this very concept since 1990’s. The main difference between the existing algorithms, however, resides in the manner by which their operators are defined. A thorough comparison of
EA’s in electromagnetics was presented by Hoorfar (2007), where the following classifications
of EA’s were also introduced: evolutionary programming, evolution strategies, and genetic
algorithms. Each of these classes was introduced to address a specific group of problems for
which such algorithm yields an effective solution. Therein, the author concluded that the
evolutionary programming (EP) class was a very valuable tool for the efficient design of microwave devices due to its simplicity, ease of implementation, and flexibility. PSO algorithm
belongs to EP class, and hence, the aforementioned statement applies to PSO as well.
At this point, let us have a brief review of PSO history. Kennedy and Eberhart (1995)
formulated the PSO algorithm, for the first time, by emulating the behavior of group-living
creatures during their search activities. The algorithm was further developed by (Shi, et.
al., 1999; Eberhart, et. al., 2001; Clerk, et. al., 2002), among others. As shown by Jin
(2007), PSO, with the aid of a variety of analytical and numerical tools, becomes a viable
technique for various electromagnetic applications. Here are some examples of using PSO;
Matekovits (2005), Mikki(2005) and Robinson (2004) have studied how PSO generally can be
used in electromagnetics, while Cui (2005), Jin (2005), Hwang (2009), Chamaani (2010) and
Boeringer (2004) have focused on some specific applications, such as the design of absorbers,
waveguides and antennas. Wang (2005), He (2009), Ko (2009) and Sun (2010) have used
PSO in filter design. Benedetti (2006), Bayraktar (2006), Abu-Al-Nadi (2012) and Ismail
(2010) have shown how PSO is utilized in array design, and of course, many more interesting articles can be found in the literature that use PSO variants in various electromagnetic
applications.
In spite of a relatively large number of articles on EAs, the design of EA algorithms, in
8
general, and PSO, in particular, remains problem dependent. Initial conditions also have
a substantial impact on the performance, especially when optimization length is a significant concern. Eberhart (2001), Nakano (2007), and Camci (2008) effectively depicted the
abovementioned properties of PSO in their investigations. In other words, it is always recommended to choose the optimization technique and set the parameters of the algorithm
accordingly to meet the unique attributes of the problem at hand.
The fundamentals of PSO algorithm have been extensively presented in the literature; for
example, see (Eberhart, et. al., 2001; Jin, et. al. 2007). For the benefit of the reader, a
concise, yet complete definition of the algorithm is provided here.
Let us assume a problem with n unknowns or parameters. In fact, n defines the dimensionality of the problem at hand, which means that the solution space is n-dimensional. The
optimum solution is a single point in this n-dimensional space. The ranges over which problem parameters vary define how wide the solution space is. A search group, consisting of
m individuals, tries to find the optimum solution. Similar to other optimization techniques,
a fitness (or objective) function determines how ’good’ each location in the solution space
is. Although multiple objective functions can exist for one problem, it is often possible to
merge all of them into one. Hence, single-objective optimization is studied herein.
In PSO, each particle remembers the best location, in terms of fitness function, that it has
discovered during its own exploration. This is named the ’personal best’. At each iteration
cycle, the best location found by all particles (the best personal best) is named the ’global
best’. Following steps describe the basic building blocks of the PSO algorithm:
• The ’m’ particles are spread randomly inside the solution space.
• Fitness values are calculated and personal bests and global best are identified.
• With respect to the personal and global bests, a ’velocity’ vector is assigned to each
particle.
9
• The locations of the particles are updated based on the velocity vectors.
• The algorithm loops back to step 2 until the desired fitness value is achieved.
It is important to note that the velocity vector is the only important operator in the PSO
algorithm and this is the main reason for the ease of implementation of PSO. A definition
for the velocity vector and the impact of its variation on the overall performance will be
presented in the ensuing discussion.
Having m particles and n parameters, the positions and velocities are stored in mn matrices.
In the conventional PSO, the velocity is calculated using following equation.
vt = ωvt−1 + c1 η1 (pbestt−1 − xt−1 ) + c2 η2 (gbestt−1 − xt−1 ).
(1.1)
In the above equation, xt , vt , pbest and gbest denote the position, velocity, personal best,
and global best matrices, respectively. t is the iteration number. η1 and η1 are two random
variables uniformly distributed on [0,1], and lastly, ω, c1 and c2 are constants used to move
the algorithm towards achieving the desired goals. As Eberhart (2001) depicts, ω, c1 and
c2 have been, in large part, obtained through trial and error. This also means that these
parameters can be redesigned according to the problem at hand.
In order to initiate Eq.(1.1), it is necessary to know the initial velocity. In the conventional
PSO, the initial velocity matrix, v0 , is composed of the elements shown in Eq.(1.2).
j
j
v0 (i, j) = random number unif ormly distributed on [vminimum
, vmaximum
].
iǫ1, 2, ...m
jǫ1, 2, ...n
(1.2)
10
j
j
In the above, vmaximum
and vminimum
are the maximum and minimum permissible velocj
j
ities over j th dimension. Often, vminimum
is assumed to be equal to vmaximum
in intensity,
j
but in the opposite direction. vmaximum
is generally assumed to be equal to the parameter
range or a fraction of it, for RPSO. As shown by Jin (2007), a mapping function is required
in the case of binary and discrete PSO’s for the sake of quantization.
Another part of the algorithm is the boundary condition, which is necessary in order to set a
bound on the limits of the solution space. Robinson (2004) has explained different boundary
conditions and has claimed that ’invisible wall’ showed a superior performance. Therefore,
invisible wall is mainly used in this chapter. Using this boundary condition, the fitness function calculation will not be performed for the particles that ’fly out’ of the solution space.
A fairly undesired fitness value is assigned to these particles instead. This way, although
all the particles remain intact, the computation complexity is decreased slightly. Upon the
return of the escaped particles, the objective function calculation will be restarted for them.
Yet as a second option, ’absorbing wall’ boundary condition is also taken into account in
this section. An absorbing wall simply stops particles right at the boundary if they try to
fly out of the solution space.
1.1.2
PSO variations
When deciding to employ PSO in time-sensitive electromagnetic problems, there are two
main issues to consider. First, similar to the other global optimization techniques, it is possible that the particles are trapped by a local best solution which, ultimately, prevents them
from reaching the actual best solution. Second, in many demanding electromagnetic problems, the optimization cost can get unacceptably high due to the large number of required
iterations and/or particles.
Due to the aforementioned two facts, basic PSO has been modified in variety of ways to compensate for its drawbacks. A review of the different modifications existing in the literature
11
can be found in (Nakano, et. al.,2007) along with a comparison of their performances. There,
the author has also introduced a new method, namely, tabu-searching PSO or TS-PSO. To
briefly illustrate, in TS-PSO, a second swarm and a number of ’tabu’ conditions are added
to the algorithm. Equations (5)-(7) in (Nakano, et. al., 2007) show that the particles of the
second swarm are led away from the personal best and/or global best positions which satisfy
the tabu condition. In this way, the particles are prevented from getting locked in their local
bests. However, optimization length is shown to increase significantly in TS-PSO due to the
increased computation complexity.
More modified PSO algorithms are briefly introduced in the ensuing paragraphs in which
personal best coefficient (c1 ) and initial velocity (v0 ) definition are particularly varied. The
advantage of the following modifications resides in the non-increasing computation cost.
Looking back at the steps followed in the PSO algorithm, one can observe that, at each
iteration cycle, one of the personal bests is qualified to become the global best of that iteration. The other personal bests are actually undesired local bests, which can only be saved
in the memory of the particles in order to guide them in their future trajectories. Therefore,
the idea of ignoring undesired personal best positions, as an alternate to the move-away
procedure in TS-PSO, seems to be reasonable. Hence, the following velocity function was
proposed in (Modiri, et. al., 2011b). There, the authors also changed the manner by which
the initial velocity was selected to a bi-state condition, as follows in (1.3).
vt = ωvt−1 + c2 η2 (gbestt−1 − xt−1 ).
j
j
v0 (i, j) = randomly selected to be equal to vminimum
or vmaximum
.
iǫ1, 2, ...m
jǫ1, 2, ...n
(1.3)
12
The reason behind the second modification can be found in nature. Bees, as an example,
start their initial flower-searching activities with a speed that is higher than that of the
subsequent search steps. In the conventional RPSO, ω is decreased from 0.9 to 0.4 during optimization. This way, the algorithm emulates the mentioned reducing search speed
concept. However, such an approach has shown no impact on BPSO (Jin, et. al., 2007),
therefore, ω is generally set to a constant value of 1 in the conventional BPSO. In (Modiri,
et. al., 2011b), however, this large initial velocity was introduced through (1.3), whereas in
j
j
the case of BPSO, vminimum
was set to 0 and vmaximum
was assumed to be 6. The answer to
the question ”why 6?” can be found in (Jin, 2007). More explanations are given in section
2.2.
Many articles in the literature, such as (Eberhart, et. al., 2001; Carlisle, et. al., 2002; Hoorfar, 2007; Jin, et. al., 2007; Camci, 2008), have investigated the best values for c1 and c2 .
According to the most recommended results, both c1 and c2 are either set to 1.49 or 2. Jin
(2007), however, has shown better convergence performance when c1 and c2 are both set to
2. Therefore, c1 = c2 = 2 is used in this study as well.
In the next step, a brief comparison of the conventional and modified algorithms when used in
optimizing a set of well-known benchmark fitness functions is reviewed. Rastigrin, Griewank
and Rosenbrock benchmark functions are studied in many articles in the literature, such as
in (Hoorfar, 2007; Nakano, et. al., 2007). These three functions possess a global minimum
that occurs at the origin of the solution space. The assessment results of the following four
methods are reviewed here. It is noteworthy that the initial random solutions should be kept
identical for all methods during each run in order to have reliable comparison results.
Method.1: c1 = c2 = 2, and v0 as shown in (1.1) (conventional method).
Method.2: c1 = c2 = 2, and v0 as shown in (1.3).
Method.3: c1 = 0, c2 = 2, and v0 as shown in (1.1).
Method.4: c1 = 0, c2 = 2, and v0 as shown in (1.3).
13
In (Modiri, et. al., 2011b), the convergence performances of the aforementioned methods
were studied in 500 iterations averaged over 1000 runs, and the results were presented in
terms of the average final value (AFV), and the average iteration number (AIN) required
to attain a given AFV. The average convergence trend over iterations, and the distribution
of the final values to which the algorithm has converged were also shown. A 10D scenario
(n=10) was analyzed with m= 5, 10, and 20 particles. Each parameter was allowed to vary
either over [-5,5], [-500,500], or [-50000,50000]. Optimization cost which was defined as the
time required for the algorithm to converge to an acceptable solution (predefined by a threshold) was claimed to be the highest priority in the study. Due to the fact that decreasing
the number of particles can shorten the optimization time considerably, no more than 2n
particles were employed. A short summary of the conclusions is depicted in this chapter.
For more details, readers are referred to chapter 2.
It was shown that, Methods 2, 3 and 4 outperformed the conventional one (Method. 1) in
terms of the convergence speed for Rastigrin function better than the two other functions. In
terms of the best final value, Rosenbrock function ended up with more improved results. The
very important point is that, the resulting error was only acceptable when the parameters
were allowed to vary in a wide variation span. Figure 1.1 depicts the fitness function trend
averaged over 1000 runs when n = m = 10. Evidently, the modified methods get far better
than the conventional one as the variation range becomes wider. The termination criterion
was set to the maximum iteration number of 500. Interestingly, for Griewank function, all
methods had similar performances. Let us, again, highlight an important fact at this point.
That is, the performances of the EAs are problem-dependent.
In terms of the computation complexity, it is obvious that Methods 3 and 4 are lower in
complexity due to elimination of a term from fitness function without any increase in the
required number of iteration cycles; however, more features are needed to be compared. In
Figure 1.2, the probability density functions of the final fitness values are plotted for Rastigrin. The results are achieved over 1000 runs. Each run is a 500-iteration procedure. In
14
the figure, MFFV denotes the maximum final fitness value. To better illustrate, the four
bars at 0.1MFFV show the probability associated to the final fitness value being less than or
equal to 0.1MFFV. Each bar represents one of the variants. Similarly, the bars at 0.2MFFV
show the probability of the final fitness value being less than or equal to 0.2MFFV and more
than 0.1MFFV. By studying the distribution of the final values to which the algorithms
converge, Modiri (2011) concluded that the probability of ending with lower final fitness
values increased in the modified methods as the variation span assigned to the parameters
increased. This means that the superiority of the convergence rate for the PSO variants is
more pronounced in the scenarios where the solution space is wider. The interested reader is
referred to (Modiri, et. al., 2011b), for more details. Similar trend is claimed to be observed
by studying the other two functions.
Moreover, in order to have a set of results comparable to that presented by Nakano (2007),
particle movements inside the solution space were traced in (Modiri, et. al., 2011b), and it
was shown that the modifications offer the particles a greater degree of freedom in the initial
stages of the exploration and faster convergence afterwards. Similar results were achieved
when the absorbing wall boundary condition was used.
The aforementioned versions of PSO (Method.1 to Method.4) are further addressed in pursuing case studies. It is worth reminding the reader that the main goal of this part of the
chapter is to give a picture of the flexibility of the PSO algorithm and show how it can be
redesigned according to the application. We are not trying to prove the viability of a single
variant of PSO for all applications. Interested readers are also referred to other variations
of PSO introduced in the literature, such as in (Ho, et. al., 2008; Gao, et. al., 2011; Luan,
et. al., 2012; Montes de Oca, et. al., 2009).
15
Figure 1.1. Fitness values of the four introduced methods averaged over one thousand runs
in a ten dimensional problem with ten particles for (a) Rastigrin, (b) Griewank and (c)
Rosenbrock functions. The parameter range is considered to be [-50000,50000].
Figure 1.2. Probability density functions of the final fitness values for the four introduced
variants considering one thousand runs in a ten dimensional problem for Rastigrin function
with parameter ranges of (a) [-50000,50000], (b) [-500,500] , and (c) [-5,5].
16
1.2
Particle swarm optimization-Reconfigurable radiators; Case study 1: PSO
in antenna beam forming
The previous section summarized the results achieved by some of the variations of PSO for
mathematical benchmark problems. Here, modified BPSO is tested in a more realistic and
demanding electromagnetic problem, namely, ’thinned array beam forming’, using the velocity function modifications described in the previous section and presented in (Modiri, et.
al., 2011b). The goal was to be able to block the radiation of an array antenna in a given
direction in an expeditious manner.
Thinned arrays are periodic antennas with a number of non-illuminating elements. Such
antennas are in demand for scenarios where software-defined pattern shaping is of interest;
see (Jin, et. al., 2007). The applications of this type of beam forming were described in the
Introduction of this chapter.
Forty-element and 200-element linear arrays, as well as 33 , 44 and 55 planar arrays, were
studied in the aforementioned paper and the results of this study are summarized in the
ensuing paragraphs. Details can be found in chapter 2.
BPSO, as it is obvious from its name, is quite attractive for problems with two-state parameters; see (Camci, 2008). However, in order to extend its utility to real-time smart
applications, again, modifications are required. The algorithm is set to minimize the side
lobe level (SLL) in the array broadside (as compared to the main-lobe strength). In other
words, the following fitness function is to be minimized:
f = max 20 log |arrayf actor|at the desired direction or directions
(1.4)
The array factor formula can be found in many references, such as Eq. (26) in (Jin, et.
al., 2007). AFV, AIN, the average convergence trend, and the distribution of the final values
averaged over more than 700 runs were studied in (Modiri, et. al., 2011b) for this problem.
17
Leaving the pure mathematical benchmark problems of previous section behind and tackling
the realistic electromagnetic problems, it is shown that the complexity of the problem as
well as the necessity of speeding up the design process increases.
Generally speaking, an increase in the number of particles increases the computation time
while reducing the probability of failure. Therefore, it is observed that a problem-dependent
trade-off is normally taken into account.
Interesting results are achieved when the previously mentioned four modifications are applied to BPSO. The performances of Method.2 and Method.4 in terms of AIN and AFV
were shown to be superior to that of Method 1 (see chapter 2). However, Method 3 ended
up with an inconsistent performance advantage over the conventional method. This modification (Method.3) was shown to even degrade the convergence performance. However, by
reshaping Method.3 to arrive at Method.4, improved results were observed.
The reader should note that, since the distinction between RPSO and BPSO is mainly
originated from the underlying quantization strategy (Jin, et. al., 2007), by changing the
quantization strategy, the results may also vary to some degree.
Furthermore, it is important to mention that, since in BPSO each parameter is either 1 or
0, the parameter range, which was an underscored factor in the previous section for RPSO,
is quite limited.
Figure 1.3 demonstrates the convergence trend in the case of 44 planar array when m = 2.
The dominance of Method.2 and Method.4 in terms of the convergence speed and final results, over the two other techniques are apparent. The radiation patterns are calculated in
various φ-planes. It is also shown in chapter 2 that a similar set of results emerge for the
case of the absorbing wall boundary condition.
An interesting comparison of the optimization speed is also demonstrated in chapter 2.
There, it is shown that, by adding two more particles in the 6-φ-plane scenario, the optimization time jumps from 2.8 hours, on a standard 2.83 GHz and 3.23GB personal computer,
18
Figure 1.3. Fitness value trends of the four introduced methods averaged over seven hundred
runs in a sixteen dimensional problem considering two particles. Optimization study is
performed in (a) six φ-planes, (b) eight φ-planes, and (c) ten φ-planes
to 5.25 hours. Therefore, the significance of a method, which is capable of achieving reliable
results expeditiously, becomes blaringly clear.
1.3
Particle swarm optimization-Reconfigurable radiators; Case study 2; PSO
in frequency selectivity
Yet as another demanding application, frequency selectivity of switchable microstrip antennas is studied in this section.
Microstrip antennas (MAs) are inexpensive and relatively easy to manufacture. They are
also conformal and flexible in shape. Therefore, the demand for MA applications has been
constantly growing. However, the design of MA structure using the available analytical
techniques becomes too complicated when one goes beyond the regularly shaped patches.
This restriction is imposed by the high computational complexity of irregular configurations.
In recent years, irregular structures are becoming more attractive due to their potential of
19
achieving large bandwidths or operating in a multi-band environment. However, existing
articles in the literature point to a weakness of the existing approaches; i.e., the lengthy
design and optimization processes, see (Balanis, 2008). It is interesting though that, EAs,
such as PSO, have also been used for the MA design, see (Jin, et. al., 2008) as an example.
As it was shown in the previous section, PSO is easy to implement and control, making it
exceptionally appealing for multi-dimensional electromagnetic problems, such as those introduced in (Jin, et. al., 2005; Jin, et. al., 2007; Camci, 2008). The modifications applied
to the velocity vector by Modiri, et. al. (2011) and explained in the previous sections are
used in this section, as well. Here, the results of the cooperation between modified BPSO
(implemented in MATLAB) and EM-SONNET, which is an MOM electromagnetic solver,
are reviewed with the purpose of underscoring the performance improvement of a PSO-based
optimization technique in problems where the real-time adjustment of the frequency response
of the antenna is of paramount significance.
In fact, this problem possesses a complexity higher than that of the previous case study.
Here the electromagnetic solver calculates the fitness function. To have a more reasonable
analysis, the comparison results of the convergence behavior for the conventional and the
modified algorithms are shown in terms of the average optimization trend. For a further
discussion of the performance metrics of this technique, such as the particle activity and
success rate, the reader is referred to chapter 2.
Switchable multi-segment antennas are one of the most versatile reconfigurable antennas due
to the fact that their frequency responses can be simply varied by connecting some of the
segments to the main antenna body and disconnecting the others. These types of antennas
are widely used where frequency-selectivity is required. Yet, the reliability of the switching
operation remains an issue.
Figure 1.4 depicts the 15-segment MA, etched over a 3mm duroid substrate, which was analyzed in (Modiri, et. al. 2011). The antenna feeding connection was performed through
20
the middle patch, marked by a dot in the figure. The optimization algorithm does not make
any changes to this middle segment. In this article, BPSO was employed to design irregular
structures out of both 15-segment and 20-segment MAs (Figure 1.4). In this example, each
segment was assumed to be a 66mm2 patch. The demonstrated results are obtained by averaging over 50 runs. A magnitude of -10 dB for the reflection coefficient, S11, at the design
frequency (or frequencies) of interest is assumed as the success criterion. Also, in order to
keep the optimization length reasonable for a real-time application, a maximum iteration
number of T=50 is imposed as the termination criterion.
It is noteworthy to mention that Jin (2005) has recommended a parallel strategy in order
to expedite the MA design procedure using PSO. There, FDTD solver was used along with
PSO. However, the parallel processing, although extremely effective in common applications,
may not be valid for most software-defined reconfigurable structures were portable platforms
are considered. The reason is that, parallel strategy requires large memory and processing
capacity, which are difficult conditions to satisfy for the case of small size portable devices,
which actually are the main focus of this chapter.
Both single-frequency and multiple-frequency resonances are studied in (Modiri, et.
al.,2011b) using Eq. (1.5) as the objective function.
f=
N
X
(S11 )atfi
(1.5)
i=1
where N depicts the number of resonant frequencies considered. It should be noted that,
although Eq. (1.5) seems to be simple and trivial, S11 (reflection coefficient) cannot even
be written in the form of closed formulae for irregular shaped patches. This renders this
problem mathematically intractable.
The frequency response and convergence trends of the conventional BPSO and the modified
one are shown in Figure 1.5 when considering single frequency resonance at f1 = 5.1GHz.
BPSO employs 7-particle swarm in this example. A close look at Figure 1.5 reveals the
21
Figure 1.4. The fifteen and twenty segment microstrip antennas used for simulation purpose
are shown in (a). The best design achieved by the conventional and modified methods for the
fifteen-segment scenario is shown in (b). (c) and (d) show the best designs of twenty-segment
antenna achieved by the conventional and modified methods, respectively.
convergence superiority of the modified BPSO over the conventional one. Regardless of the
termination point, the modified BPSO offers a gain of at least 1.21 dB over the conventional
one. More notably, it meets the design success condition of S11 =-10 dB faster than its
counterpart with an average speed improvement of 50%. This result is of great interest to
those who are engaged in the design of reconfigurable antennas with the computation time
as a major constraint.
The final irregular shaped structures associated with the best designs, achieved by the two
methods, are also shown in Figure 1.5. The discrete probability density functions associated
with the final fitness values, AFV, and AIN are also studied in (Modiri, et. al., 2011a). In
this study, it is shown that the modified methods presented here achieve higher success rate.
Furthermore, it was shown that the improvements stay valid for triple resonant design (4.8
GHz, 5 GHz and 5.2 GHz) as well.
Obviously, a triple frequency scenario presents higher levels of computation complexity. For
22
instance, looking back at Eq. (1.5), it is clear that in a multi-resonance scenario more care
should be taken in order to avoid the algorithm from getting locked in a cycle which reduces
S11 at one of the frequencies while leaving the other frequencies with unacceptable S11
values. As a solution to this problem, any S11 value of less than -15 dB is fixed at -15 dB in
(Modiri, et. al., 2011a). As a result, the swarm would stop reducing S11 at the frequencies
for which -15 dB has already been achieved. It is shown there that the modified algorithm
outperforms the conventional one in terms of the convergence speed, assuming a termination
criterion of T=50 iterations. The two algorithms converge to identical configurations for
this case. This configuration, along with the two other configurations, achieved in a singleresonance 20-segment MA, is shown in Figure 4b and 4c. For a more comprehensive study
of the optimization time, a run-time table was presented in (Modiri, et. al., 2011a), in
which the noticeable impact of the initial values on the optimization length was highlighted.
Chapter 2 presents this information.
1.4
Particle swarm optimization-Reconfigurable radiators; Case study 3: PSO
in bend effect compensation
In this section, one more application of PSO, which is an interesting antenna design problem, is investigated. Wearable antennas (WA’s), as the name implies, are intended to be a
part of everyday clothing. The earliest demand for WA’s was established by military with
the purpose of reducing the chance of the identification of a radioman within a squad. The
conventional military communications person often has to carry long antennas and such antennas are visible from a distance (Hall et al., 2006). Later on, WA’s received wide interest
for sports, emergency, space, and medical applications too. Among these, medical applications are the most demanding so far. As a critical components of body area networks
(BAN’s), WA’s need to maintain communications among in-body, on-body, and off-body
devices. Therefore, WA’s have been developed with a variety of frequency and radiation
23
specifications. Nevertheless, it has been shown that several factors can deteriorate the performance of a WA, and ultimately, create health hazards in telemedicine applications. Thus,
a serious need for real-time reconfigurable WA’s has emerged, especially for scenarios where
changing posture due to movement imposes serious performance degradation. According to
the application requirements, on-body textile antennas need to be flexible and conformal in
structure, light in weight, and reasonably small in size. Therefore, MA’s seem to be one of
the best candidates for such applications.
Considering BAN’s, two performance criteria are generally expected to be met:
1. The patient comfort should be satisfied, especially in long term monitoring situations,
2. The antenna should preserve an acceptable performance for various performancedegrading scenarios. Wrinkles and bends on the antenna surface or moisture absorbed
by substrate textile are some of such scenarios (Hall, et. al., 2006).
Textile materials, in general, have low dielectric constants (below 2), and, as a result, a
textile MA potentially has a relatively large bandwidth which can compensate for slight
variations in the resonant frequency. However, the environmental variations may easily go
beyond this limitation.
Here, the effect of one of these variations, i.e., bending, is studied. Other variations can
be treated in a similar way. Again, owing to the algorithm’s flexibility and high convergence
speed, modified BPSO is used here in order to reconfigure a 2.45GHz MA in a real time
manner. In (Modiri, et. al., 2011d), a patch antenna was proposed as the 2.45 GHz reconfigurable WA and was simulated in EM Sonnet. BPSO kernel was implemented in MATLAB.
At this point, let us have an in depth discussion of practical details of a few implemented reconfigurable WAs. In (Hall et al., 2006), a 56mm 51mm textile patch antenna was fabricated
on a 3mm fleece substrate with resonant frequency of 2.45 GHz. The ground plane size was
24
Figure 1.5. (a) Average fitness value trend and (b) the best optimization results achieved
by the conventional and the modified BPSO algorithms using seven-particle swarm.
considered to be 7671mm2 and the coaxial feeding point was located at the distance of 19 mm
from the edge. The same textile antenna was re-fabricated by Modiri et al., and measured
using Agilent E5071C VNA , see Figure 1.6. According to the measurements, it was claimed
that by creating bends of 45 and 90 degrees on the antenna structure frequency jumps as
large as 144 MHz and 250MHz, respectively, would occur in the frequency behavior of the
antenna (see section 2.4). It is worth noting that a structure modification (i.e., bending)
in only one dimension has resulted in such significant deviation in the frequency response
of the MA. In order to compensate for this undesired frequency deviation, parasitic narrow
patch elements were appended to the main antenna body. Once bending has occurred, some
25
Figure 1.6. Measurement arrangements for the wearable antenna in (a) flat, and (b) bent
modes. Switchable antenna with compensating parasitic element in bent mode is shown in
(c).
of these parasitic elements would be connected to the main antenna body via switches in
order to enlarge the actual length of the antenna, and thus, compensate for the shrinkage
which has occurred in the electrical length of the element.
Experimental verification is shown in chapter 2. Two scenarios were investigated in (Modiri,
et. al., 2011d). In one scenario, the reconfigurable antenna consisted of three switches and
a single parasitic element. While in the other one, eight switches were considered along
with three parasitic elements. Of course, the higher the number of switches and parasitic
elements, the better would be the optimization performance of the frequency response. Yet,
it is impractical to integrate a large number of switches in a wearable device.
The overall idea of this case study is quite similar to the previous one, although they
differ in details and application. In the current scenario, a monitoring system is required
to check the signal power at the desired resonant frequency by sampling the power period-
26
ically. As soon as the power falls below a predefined threshold (due to some environmental
variations), BPSO is invoked in order to find the best switching arrangement to retrieve the
original operation frequency. The objective function in these types of problems is typically
the reflection coefficient at the desired frequency, which, of course, should be minimized, see
Eq.(1.5).
Yet, as another solution to this problem, it seems feasible to generate a bank of predefined switching arrangements to compensate for a set of environmental variations. However,
in practice, it is quite possible that an unexpected variation, such as a twisting (multidimensional bend), can occur for which no solution is provided. Moreover, such a solution
can only be valid if it is customized to a single person due to the exclusive personal body
styles. Therefore, the first solution, i.e., real-time random decision making, was chosen by
the author.
To give the readers an idea of the final results achieved by the algorithm, Figure 1.7 shows
the retrieved frequency response in the case of 30 and 45 degree single-dimensional bends
when 8-switch, 3-element scenario is considered. Since, in practice, reflection coefficient
magnitude of -10 dB is acceptable in most antenna applications, a success threshold of -10
dB was imposed in this study. It was also shown therein, that, overall, the modified BPSO
outperformed the conventional one in maintaining the desired frequency performance. This
is a result similar to that of the case studies 1 and 2. More details, such as convergence
trends associated with the conventional and modified BPSOs, distribution curves related to
the final fitness values, and frequency response curves of the two studied scenarios, can be
found in the next chapter.
27
Figure 1.7. |S11 | measurement results of the flat and bent modes for the eight-switch, threeelement scenario.
1.5
Breast cancer detection using particle swarm optimization in MBMD
In the last part of this chapter, PSO algorithm is tested in a more complex and challenging problem where the makeup of an unknown multi-layer structure has to be estimated.
Gandhi (2010) and Yeung (2009) successfully used PSO to estimate the composition of a
multi-layer structure using simulations. The object under the test was exposed to electromagnetic radiation and the scattering field was processed. In these studies, the medium
under the test was assumed to either be lossless or low-loss in order to decrease the calculation complexity. However, the application of PSO in these types of problems is not limited
to low-loss structures. This concept can, in fact, be utilized in more demanding and, at
the same time, interesting fields, such as tumor detection. As shown by Golnabi (2011),
a typical MBMD system consists of an array of antennas, which encircle the breast tissue.
The antennas radiate the electromagnetic energy, one at a time, and the receivers gather the
28
scattered field. A set of measurements is performed at distinct frequencies and/or different
transmitter positions. The inputs to the algorithm are these measurement results. Then,
through a challenging inverse problem analysis, the algorithm has to get relatively close to
an acceptable guess for the tissue structure. Here, we particularly investigate the breast
cancer detection through MBMD. The makeup of the tissue layers can be estimated with
respect to their dielectric constants. Therefore, the optimization goal can be simplified in
words to ’estimating the dielectric constants of the tissue layers’.
Similar to many other cancer cases, it is well proven that the early detection of the breast
cancer drastically raises the survival rate of the patient. Moreover, the treatment and recovery becomes more tolerable, both physically and financially, when the tumor is detected
soon enough. Although, X-ray mammography is available as the most recommended breast
cancer detection technique, it has a dismal record of 20% detection failure, which cannot
be easily ignored (Lazebnik, et. al., 2007). In addition, X-ray is ionizing, rendering this
modality somewhat unsafe for scenarios where frequent checkups are required or highly recommended, see (Hassan, et. al., 2011; Nikolova, 2011).
There have been thorough attempts toward developing more convenient, non-invasive breast
cancer detection techniques that pose little risk to the patient when routine checkups are
performed. Ultrasound and MRI seem to be two promising examples. However, these two
techniques are either low in accuracy, in the case of ultrasound, or exceedingly expensive, in
the case of MRI, when compared with X-ray mammography. In other words, they cannot
compete with X-ray in cost/accuracy, especially when routine checkups come into the picture. That is why they are not used as widely as X-ray in clinical practice for breast cancer.
Moreover, remarkable initial studies have been published, introducing more innovative methods of cancer detection at microwave, near-infra-red, optical, and tera-hertz frequency bands,
see (Sabouni, 2010; Gamagami, 1997; Lazebnik, 2008; Khan, 2007; Jensen, 2011; Zhurbenko,
2010; Winters, 2009; Delbary, 2010; Zhurbenko, 2011; O’Halloran, 2009). Reviewing these
29
articles sheds light on the promise of MBMD as the diagnostic tool of choice for a noninvasive, safe, and accurate detection of breast cancer. The reason behind this fact is,
simply, the practical aspects of electromagnetic imaging techniques. For instance, one important factor is the detection of tumors deep within the tissue. It is well known that,
higher is the radiation frequency, smaller will be the penetration depth inside the human
body. Therefore, higher frequency radiation (optics and terahertz) can hardly reach subcutaneous tumors. Also, the standard sizes of the electrical components at microwave frequencies
are small enough to make a non-invasive, portable detector feasible. Yet as another fact,
microwave radiation is quite responsive to the liquid content of the tissue. This would be of
significant importance since the overall ratio of the blood flow in the breast tumor is between
4.7 and 5.5 times larger than that of the normal tissue (Dellile, et. al., 2002). As a result,
employing non-ionizing microwave radiation in cancer detection and treatment monitoring
has received a great deal of attention. A list of research studies in this area can be found in
(Lazebnik, et. al., 2007). However, a hybrid technique, may ultimately be shown to be the
most effective means of detecting breast cancers, see (Paulsen, et. al., 2005).
Reviewing the existing articles on the subject of breast cancer detection using microwave
radiation, one can identify the following key observations:
1. A lack of large-scale, in-vivo measurements for both validation and/or revision of the
theoretically proposed techniques.
2. Low success rates of malignancy detection in non-superficial areas of the breast, even
in the simulation stage, especially, when distinguishing between glandular tissue and
tumor becomes critical.
A closer examination of the above factors reveals the internal connectivity between them.
That is, in order to conduct extensive in-vivo measurement studies, one has to obtain approval from regulatory bodies, which rely heavily on theoretical viability of the proposed
30
techniques and their supporting studies.
In (Modiri, et. al., 2011c), it is argued that the inadequacy in modeling the tissue is the main
culprit behind one arriving at low success rate in detecting cancerous tissues. In particular,
non-negligible variations in the tissue characteristics from person to person, and even in a
single person at different times of the month or under diverse experimental conditions, must
be taken into account for the purpose of theoretical modeling. In one of the largest scale
studies, reported by Lazebnik (2007), 807 samples were investigated over the frequency band
of 0.5 GHz-20 GHz. There, it was practically demonstrated that the electrical properties of
the tissue extend over a wide range. Cho (2006) also illustrated that these electrical properties were significantly influenced by the measurement setup. Therefore, the authors, in
(Modiri, et. al., 2011c), proposed modeling permittivities as random variables. In addition,
PSO, as a random-based EA algorithm, was chosen to find the breast tissue composition
through solving the inverse scattering problem. It should be noted that dealing with inverse
problems with a large number of unknowns has its inherent difficulty of arriving at nonunique solutions. Hence, the unknowns have to be managed carefully.
In order to determine the solution technique, we need to model the measurement setup. It
should be noted that, typically, array radiators are used for cancer detection, see (Golnabi et
al., 2011), and the transmittance between two selected antennas is studied at a time. Therefore, the electromagnetic analysis can be simplified to the interaction of a narrow beam with
a multi-layer structure, and this can be further simplified to planar wave scattering analysis
in a lossy multi-layer structure (Orfanidis, 2008; Thakur, et. al., 2002).
As it is shown in Eq.(1.6), the accumulated error, considering both amplitudes and phases
of the reflection and transmittance coefficients, are utilized as the objective function (f) of
the search algorithm. Nm is the total number of measurements. R and T are the reflection
31
and transmittance coefficients, respectively.
f=
X
{||Ractual | − |Restimated || +
(1.6)
|phase(Ractual ) − phase(Restimated )| +
||Tactual | − |Testimated || +
|phase(Tactual ) − phase(Testimated )|};
where ’arg’ refers to the phase operator.
To illustrate how PSO can be used in such a problem, let us review the methodology and
summarize the results demonstrated in (Modiri, et. al., 2011c) in the next section. More
details are given in chapter 3.
1.5.1
Modeling and estimation methodology
The first step in solving this problem is to generate a model for the object under the test. The
breast tissue model was generated with respect to the breast physiology. The breast model
structure is composed of glandular tissue, fat, and skin. For the case of cancerous tissue,
tumor should be also added to the picture. As shown by Lazebnik (2007), the electrical properties of normal breast tissue are primarily dependent on its adipose content. Therefore, the
model shown in Figure 1.8 takes the adipose contents into account. According to the breast
physiology, various models of breast, in terms of the number of layers, can be suggested.
This, in fact, defines the accuracy and computation simplicity levels. It is also required to
assume a value as the total thickness of the tissue. Of course, this parameter is not a fixed
value for all subjects; nevertheless, 4.4 cm was used in (Modiri, et. al., 2011c). Since the
breast is typically in a encompassing radiator structure which would slightly squeeze the
tissue, 4.4 cm was determined to be reasonable. The microwave radiation was analyzed in
the frequency band of 1-2.25 GHz. The following two scenarios were studied:
32
1. Assuming there was no a priori knowledge available of the type and composition of the
tissue under the test.
2. Assuming there was some reasonable a priori knowledge of the tissue under the test.
Of course, the first scenario was the most difficult one in terms of the computation complexity.
It should be noted that, the complexity of the problem is not due to the fitness function
calculation. It is mainly due to the difficulty of solving the inverse problem in such a large
solution space.
Two PSO variants were proposed for the two desired scenarios. In both scenarios c1 and
c2 are assumed to be equal to 2. ω is set to 1 for discrete PSO and gradually decreasing
from 0.9 to 0.4 for the complex one. Here, it is noteworthy to emphasize again the problem
dependency of PSO variants. The variant introduced for the previous case studies was not
the best choice for this case study.
In scenario 1, due to the lack of a priori knowledge, the random variables representing the
dielectric constants were free to choose any values between the lowest possible and largest
possible constants related to the human body tissues. In this case, Complex number PSO
(CPSO) was used for which the variation range was relatively wide; CPSO is similar to RPSO,
except for dealing with complex numbers instead of real ones. In fact, by implementing this
scenario, it was shown that PSO could be utilized in estimation of the composition of any
tissue. This can be achieved if the penetration depth and receiver sensitivity satisfy the
detection requirements.
In the second scenario, it was known that the tissue under the test is the breast tissue.
There, at each iteration cycle, two optimization procedures were performed. First, for each
layer, a tissue type was randomly assigned out of a set of six tissues, including blood, skin,
fat-dominated tissue, glandular tissue, 31-84% fatty tissue, and malignant tissue (Lazebnik 2
et al.,2007). These are, in fact, the six tissue types which can exist in human breast. Each
33
of these six classes was represented by a random variable whose variation range was defined
according to the measurements reported in the literature.
In the second optimization step of scenario 2, after defining the arrangement of the tissues,
the exact value of the dielectric constant related to each tissue layer was chosen out of its
corresponding variation range, so that the optimum matching to the measured scattering
fields could be achieved. For this scenario, in order to better manage the problem, discrete
PSO (DPSO) was proposed. In principle, DPSO is similar to BPSO. The differences reside
in the following two features:
1. The number of data levels is two for BPSO, while it is generally more than two for
DPSO (six for the aforementioned DPSO).
2. The values which can be assumed at each level are restricted to 0 and 1 in BPSO,
while it is extended to a continuous variation span in the above DPSO.
For more detailed information, the interested reader is referred to chapter 3. The permittivity estimation results in the case of 7-layer modeling when DPSO is employed are
shown in Figure 1.8. A 3 mm thick tumor was added to the model in two distinct positions.
First, the tumor was added inside the fatty tissue, and then it was moved to the glandular
tissue. Obviously, due to similarity of the permittivity values of the glandular tissue and the
tumor, it is somewhat difficult for the swarm to make an accurate estimation for the second
case. Nonetheless, DPSO can successfully identify the abnormal heterogeneity in the tissue
characteristic in both cases. Figure 1.9 and Table 1.1 shows the comparison results for the
two cases of 5-layer and 7-layer tissue models along with the convergence trend. Figure 1.10
shows the estimated permittivities as compared to actual ones at the studied frequencies.
The variation of the permittivity as a function of frequency is clearly illustrated in this figure. In addition to many parameters existing in the solution domain, incident beam angle
can also be considered for the narrow beam radiation case. Since the number of unknowns
34
is relatively high in the inverse problem equations, adding more equations by adding more
incident angles seemed to be helpful; however, the author didn’t find this approach as a
reliable remedy. In fact, in many situations adding more equations leads the swarm to a
wrong direction, please see (Modiri, et. al., 2011c; Modiri, et. al., 2012).
To summarize, it is depicted that PSO-based algorithm proposed herein successfully identifies the contrast between the permittivities of the tissue layers with an average estimation
error below 10%. This accuracy is increased to an average error rate smaller than 5% in the
second scenario. Obviously, by increasing the optimization cost (for instance, increasing the
number of agents), the accuracy of the final responses is improved.
Table 1.1. Optimization cost
Layers
(Scenario)
Scenario 1
5-layers
Scenario 1
7-layers
Scenario 2
5-layers
Scenario 2
7-layers
Number of
agents
Number of
frequencies
Number of
incident
angles
Convergence
time
(min)
40
2
2
7.81
280
6
3
185.3
40
1
2
0.7
280
1
3
9.2
35
Figure 1.8. (a) Simplified two dimensional model of the breast tissue. (b) Actual permittivity
values and the estimated ones in the second scenario, assuming a 3 mm thick tumor in the
fatty area (top) and in the glandular area (bottom).
36
5−layers (Second Scenario)
5−layers (First Scenario)
7−layers (Second Scenario)
7−layers (First Scenario)
3
10
2
Fitness Function
10
1
10
0
10
First Scenario
−1
10
−2
10
0
Second Scenario
2000
4000
6000
Number of Iterations
8000
10000
Figure 1.9. Average fitness function trends for the two scenarios.
1.5.2
Implementation of microwave diagnosis tool
Although medical diagnosis using microwave technology has been under investigation for
almost a decade, there is no trace of a popular clinical device or an approved medical tool
under this technology. The reason resides in the accuracy, reliability, user-friendliness and
cost issues of the previous attempts. Other existing mass-screening diagnosis solutions suffer
from major deficiencies. MBMD has proved to be able to detect breast malignancy and bone
fractures, see (Hynes, 2012) and (Zhu, 2010) as two examples; however, for the case of breast
cancer, a low-cost, reliable MI platform for critical routine checkups is not available to the
public at this time. This serious issue became a motivation for the author and her supervisor
to study and introduce a preliminary implementable microwave radiating structure for breast
cancer detection. Encouraging simulation results, in terms of detection probability, were
demonstrated for the proposed structure. The simulation study was followed by preliminary
experimental results using an in-house manufactured breast phantom and cancerous tissue
Imaginary(dielectric constant) F/m
Real(dielectric constant) F/m
37
60
40
20
0
1
2
3
4
5
layer number
1
2
3
6
7
6
7
20
Actual @ 1GHz
Actual @ 1.25GHz
Actual @ 1.5GHz
Actual @ 1.75GHz
Actual @ 2GHz
Actual @ 2.25GHz
Estimated
15
10
5
0
4
5
layer number
Figure 1.10. Comparison of the actual values at different frequencies with the estimated
values in the first scenario for the 7-layer model.
in the form of a donated breast cancer tumor. Taking advantage of the results shown in
literature, an easily implementable structure which was, in fact, a crude model of a breast
examination device was designed. The difference between the scenario in which a tumor
existed inside the breast tissue and the scenario in which the breast tissue was healthy
were demonstrated. 3D simulations were performed using Ansoft HFSS which is a high
frequency electromagnetic (EM) solver based on the finite element method. To model the
human body, Ansoft digital phantom was used. This phantom had a fixed structure and
could not be used to model arbitrary body styles. For the experimental trials, a simple
prototype was implemented and the scattering parameters (SP) were measured. Provided
38
that such parameters can be measured readily using network analyzers, they serve as an
important tool for any noninvasive diagnostic modality. The measurement results of the
implemented radiator structure are reviewed in the ensuing sections. More details on the
proposed MBMD design and its associated measurements and simulations are presented in
chapter 3. The following section summarizes the authors studies published in (Modiri, et.
al., 2013a; 2013b; 2013c).
Our preliminary proposed architecture
The proposed device and its digitally simulated model are shown in Figures 1.11 and 3.15.
The hemisphere-shaped base structure was created by 3D printing using inflexible ABS
plastic and later covered by copper tape. The thickness of the plastic was 5 mm and it
had sixteen specially designed holes to hold the SMA connectors considered for feeding of
the antennas. The hemisphere structure was designed in SolidWorks and then printed using
Dimension Elite 3D printer. The hemisphere base had an inner radius of 68 mm. Our
preliminary microwave measurements were performed using ENA E5071C.
Due to the relatively irregular shape of the breast model, the size and shape of the radiator
structure were chosen by trial and error. The model in Ansoft for the female breast phantom,
as it is shown in Figure 3.15, was used for simulation and it consisted of glands and fat. Skin,
although very important, was not considered in the model. The glands were spread inside
the breast in a web-shaped random structure. In fact, this phantom represented a single
scenario in which the breast tissue was densely filled with glandular tissue. There were no
phantoms for other body styles available to us. For tumor model, the dielectric properties
were imported into HFSS. The Debye model corresponding to the dielectric constants of the
normal and cancer tissues are available in the literature.
Mechanism and analysis
We defined the following four metrics:
39
Figure 1.11. The preliminary radiator structure which includes a novel design of sixteen
antennae and a conductor cover.
X
Bent Dipole
Antennas
Y
Z
Figure 1.12. Digital breast phantom along with the proposed radiator structure.
11
|Snd
|
11
arg(Snd
)
=
16
X
||S jj
j=1
=
16
X
| arg(S jj
21
arg(Snd
)=
jj
− arg(Stumor
)|
jj
arg(Snormal )
normal )
j=1
21
|Snd
|=
jj
normal | − |Stumor ||
jj
|Snormal
|
15 X
16
ij
ij
X
||Snormal
| − |Stumor
||
ij
|Snormal |
i=1 j=i+1
15 X
16
ij
ij
X
| arg(Snormal
) − arg(Stumor
)|
ij
arg(Snormal )
i=1 j=i+1
(1.7)
(1.8)
(1.9)
(1.10)
40
jj
jj
are the S1 1 (rewhere the operator ’arg’ is the phase operator. S1,normal
and S1,tumor
flection) parameters for the j th terminal for the normal and cancerous cases, respectively.
Similarly, S2 ’s are the S21 (transmittance) parameters for the ith and j th terminals. Equations (1.7)-(1.10) depict how the measured data was analyzed using the scattering parameters
(SP). The main goal of the experiment was to isolate signatures due to the tumor. The signatures were detected as the result of comparing normal and cancerous tissues. Equations
(1.7)-(1.10), in fact, show the cumulative normalized difference between normal and cancer
tissues in terms of magnitude and phase for the reflection and transmittance coefficients at
a single frequency (continuous wave exposure). The magnitudes of the S parameters for
normal and cancerous breasts used in equations (1.7)-(1.10) were expressed in dB and the
phases were expressed in radian.
1.5.3
Preliminary results
After a number of trial attempts, the author was able to create reliable breast phantoms
following the recipes in the literature. There were different recipes available; however, one
was found to be more concise and precise (Hahn, 2012; Ostadrahimi, 2009). Creating tissue
phantoms which can model the body accurately was and is an important challenge in this
study. All tissue solutions are gelatin-type materials and degrade by time. Building the
complete phantom requires careful temperature control. Fortunately, we received a tumor
tissue from a breast cancer patient to use in our phantom. This was, in fact, a major step
toward a realistic study of the effectiveness of the proposed structure. We performed all our
measurements, summarized herein, within 48 hours of the completion of the creation of the
phantoms. When not used, the phantoms were kept in a refrigerator, as it was recommended.
Here, the results from two breast phantoms are reviewed; a normal breast phantom and a
cancer breast phantom. The cancer breast phantom contained the donated tumor tissue.
The two phantoms were created in exactly same way, at the same time and in exact same
41
containers. An attempt was done to make the two phantoms identical in any way other
than the existence of the tumor. However, operator mistakes cannot be prevented easily and
they add some levels of uncertainty to the solution. The tumor tissue was placed inside the
glandular tissue. The phantoms had a base diameter of 12 cm and height of 4.5 cm. The
tumor tissue donated to us was a relatively thin piece, the thickness of which was not more
than 3 mm; however, it had an irregular spread shape with the largest dimension of 1 cm.
The phantom and the radiator were placed on a bag of ice water during the measurements
for two reasons; first, for mimicking the chest wall muscle backing the breast tissue in real
scenarios, and second, for keeping the phantom cool throughout the measurements. This
was not indeed the most accurate way to mimic the chest wall muscle, yet it was our best
available option. Since the radiator structure was entirely covering the phantom, we were less
concerned about the phantom dehydration during the measurement. Flexible RF cables were
used to connect the ENA arms to the SMA connectors. Having sixteen radiating antennas
and using a two port ENA, the two set of measurements using the two previously described
phantoms took almost an work entire day performed by two people.
Due to the nature of our measurement technique, we were able to measure the reflection
coefficient for each antenna fifteen times and the transmittance coefficient between each two
antennas two times for each phantom. This was the only way to have less concern about the
reliability of our measurements as we had only one chance to go through the measurement
steps due to phantom degradation.
The antennas were originally designed to resonate at 1.2 GHz; however, as expected, the
experimental setup, the environmental factors, and the presence of the phantom led to a
slightly different resonating frequency. The impact of environment is another important
issue when studying for such diagnostic modalities.
The other important point was that, the sixteen antennas were cut by hand out of copper
tape and then attached to the ABS structure’s inner body (see Figure 1.11), and it was well
42
600
500
Star: Frequency step of 42.5MHz
Circle: Frequency Step of 85MHz
Cross: Frequency Step of 127.5MHz
Square: Single frequency of 1.354GHz
|S11nd|
400
300
200
100
0
0
0.5
1
1.5
2
2.5
Frequency (up to which the results are added) (Hz)
3
9
x 10
11
Figure 1.13. Cumulative magnitude difference of reflection coefficient (|Snd
|) for singlefrequency and multi-frequency studies.
expected that there would be slight difference between their performances. Yet, since all the
measurements were performed with the same radiating structure, the results were dependable
for a preliminary study. It was observed that the optimum frequency was 1.354 GHz instead
of 1.2 GHz. The effect of this deviation on the ultimate results was negligible since the new
resonance frequency was still inside the frequency range which was recommended for MI
measurements in the literature.
Figures 1.13 and 1.14 demonstrate the cumulative normalized difference between normal
and cancer tissues in terms of magnitude for the SP. Using ENA settings, we were able to
gather the data with steps of 42.5 MHz; however, in order to show the significant advantage
of multi-frequency measurement over a single-frequency one, the results for multi-frequency
studies using frequency steps of 42.5, 85, and 127.5 MHz are shown in Figures 1.13 and 1.14.
43
600
Star: Frequency step of 42.5MHz
Circle: Frequency Step of 85MHz
Cross: Frequency Step of 127.5MHz
500 Square: Single frequency of 1.354GHz
|S21nd|
400
300
200
100
0
0
0.5
1
1.5
2
2.5
Frequency (up to which the results are added) (Hz)
3
9
x 10
21
Figure 1.14. Cumulative magnitude difference of transmittance coefficient (|Snd
|) for singlefrequency and multi-frequency studies.
The results for a single-frequency measurement at 1.354 GHz are shown, as well. In
multi-frequency analysis, the cumulative differences achieved at all considered measurement
frequencies were simply added up. It is evident that the tumor signatures are noticeable in all
the figures. Phase responses are shown in chapter 3. In Figures 1.13 and 1.14, as expected,
the slope of the magnitude signature decreases as the measurement frequency gets far from
the optimum resonance frequency of the antennas. At 1.354 GHz, the values achieved for
equations (1.7)-(1.10) are 33.7610, 19.8374, 3.6856 and 69.0095, respectively. The results
were quite encouraging since these signatures were easily detectable using a common ENA.
However, these were the results obtained from a single set of measurement trial and needed
to be verified in several cases for the sake of reliability and further improvement. Chapter 3
reviews the details of this study.
CHAPTER 2
PROPOSED PSO AND ITS APPLICATIONS IN RECONFIGURABLE
ANTENNA DESIGN
2.1
Introduction
As it was mentioned in the previous chapter, a set of electromagnetic problems with known
solutions were chosen to test and validate PSO algorithm prior to introducing the complex
and unknown objective problem of MBMD. Chapter one presented a summary of these
problems and how PSO was customized to tackle them. This chapter, however, brings the
details of those studies to the forefront. The papers which were published by the author and
her supervisor on this subject are the building blocks of this chapter.
2.2
PSO in thinned array beam forming1
Here, PSO variants are used to block the radiation pattern a thinned antenna array in a
certain direction. To recall, some modifications in the velocity calculation of the Particle
Swarm Optimization (PSO) algorithm were introduced in section 1.1. The suggested modifications aimed to arrive at a more straightforward and robust search procedure as compared
to the conventional method. Here those modifications are assessed in more details.
Two main factors, i.e., personal best influence and initial velocity values, are evaluated. It is
shown in this section that in problems with wide-range parameters, the effect of personal best
locations is intrinsically encompassed by that of global best locations, thereby allowing for
1
c 2011 IEEE. Adapted, with permission, from Modiri,A. and Kiasaleh, K., Modification of Real-Number
and Binary PSO Algorithms for Accelerated Convergence, IEEE Transactions on Antennas and Propagation,
Jan./2011 for section 2.2 .
44
45
further simplification of the PSO algorithm by eliminating the factor which accounts for the
personal best solutions in the velocity calculation. This simplification expedites the convergence procedure in real PSO. It is also shown that the initial velocity values can be modified
in order to enhance performance in terms of achieving better solution when compared with
the existing algorithms, particularly in binary PSO. In order to validate the viability of the
proposed procedure, the performances of the real-number and binary PSO algorithms with
different velocity calculations are assessed in 1000-run sets, and pros and cons are studied.
In particular, the performance of the proposed velocity function shown in Eq.(1.3), when
used to design software defined thinned array antennas, is shown to be superior to that of
the conventional velocity function shown in equations (1.1) and (1.2).
The viability of Eq.(1.3) can even be proved mathematically by evaluating the average position of particles in each iteration cycle. Let x(i,j)t and xv(i,j)t denote the position and
velocity of the ith particle in the j th dimension during the tth iteration, respectively. In PSO
algorithm, this position is achieved by
x(i,j)t = x(i,j)t−1 + v(i,j)t .
(2.1)
Using the conventional velocity function Eq.(1.1), the average particle velocities are given
by
vt = ωvt−1 + c1 (
pbestt−1
2
−
gbestt−1
xt−1
xt−1
) + c2 (
−
).
2
2
2
(2.2)
Where pbestt and gbestt denote the average personal best position vector of all existing
particles and the global best position vector in the tt h iteration, respectively. xt and vt are
the position and velocity vectors averaged over i. Substituting c1 = c2 = 2 in Eq.(2.1) and
using Eq.(2.2), the average position vector is simplified to
xt = ωvt−1 + pbestt−1 + gbestt−1 − xt−1 .
(2.3)
46
If the algorithm moves the particles in the correct direction, xt−1 should approach pbestt−1 .
Therefore, the average position of the particles in a successful iteration cycle in which the
majority of particles have moved to positions with better fitness values (compared to previous
iteration) can be approximated as Eq.(2.4).
xt = ωvt−1 + gbestt−1 .
(2.4)
One can observe that the average position of particles in Eq.(2.4) may also be achieved
by using the proposed velocity function, given by Eq.(1.3) in above procedure.
2.2.1
Evaluation of the proposed velocity function in RPSO
In order to have comparable results, the benchmark fitness functions used in (Hoorfar, 2007;
Nakano, 2007), shown in equations (2.5), (2.6) and (2.7), are used. To elaborate, these three
functions all possess a global known minimum that occurs at the origin of the n-dimensional
solution space. So the goal of optimization is to approach this zero value by moving m
particles in an n-dimensional solution space. Rastigrin function:
f (x) =
n
X
i=1
(x2i − 10 cos(2π xi + 10)).
(2.5)
Griewank Function:
n
n
1 X 2 Y
xi
f (x) =
xi −
cos( √ ) + 1.
4000 i=1
i
i=1
(2.6)
Rosenbrock function:
f (x) =
n
X
i=1
(100(xi+1 − x2i )2 + (xi − 1)2 ).
(2.7)
Where x = [x1 , x2 , , xn ].
To assess the impact of proposed modifications separately, the results of the following four
47
methods are investigated (the initial solutions are identical for all methods during each run
throughout this paper):
• Method1: c1 = c2 = 2, and the initial velocity is given by Eq.(1.1).
• Method2: c1 = c2 = 2, and the initial velocity is given by Eq.(1.3).
• Method3: c1 = 0, c2 = 2, and the initial velocity is given by Eq.(1.1).
• Method4: c1 = 0, c2 = 2, and the initial velocity is given by Eq.(1.3).
In order to have comprehensive statistical study, the convergence behaviors in different cases
are observed in 500 iterations during 1000 runs, and the average results associated with each
benchmark function are depicted in terms of the achieved average final value (AFV) and the
average iteration number (AIN) required to attain a given AFV. The results are averaged
over a set of runs. The number of sample runs in the set increases by 20, i.e. the results of
20, 40 1000 sample runs are averaged and plotted. In this manner, in addition to achieving
consistent statistical results, we are also able to measure the number of samples required for
attaining reliable results.
A 10D scenario (n=10) is assumed with particle numbers of 5, 10, and 20. The parameters
can vary in the ranges of [-5,5], [-500,500], and [-50000,50000]. Although a higher number of
particles are generally considered in RPSO, two objectives are obtained here by these selections. First, the (m,n) values are chosen so that they match the parameters for the ensuing
binary PSO study, allowing for a comparative study. Second, since speed of convergence is a
priority in this study, key solutions for accelerating the convergence while preserving PSO’s
intrinsic robustness are sought. In real cases, it is almost impossible to reduce the number of
dimensions in order to speed up the solution. However, decreasing the number of particles
can drastically shorten the optimization time. Thus, we do not step beyond m=2n particles
in the first step of our study. AIN/AFV results are shown in Figures 2.1-2.4.
48
Figure 2.1. AIN behaviour of the four methods over gradually increasing sample runs in a
10D problem with 5, 10 and 20 particles for (a) Rastigrin, (b) Griewank and (c) Rosenbrock
functions and parameter range of [-500,500].
From Figures 2.1-2.4, two interesting trends are discovered. The first one is the high similarity between the results of the three modified methods, particularly in terms of convergence
speed. The second common trend is that the modified methods mostly expedite the convergence by almost 18% at the expense of higher errors for lower parameter ranges. However, as
the parameter range increases, the modified methods begin to slow down, while getting much
lower errors with respect to Method1. For Rastigrin and Griewank, this transition happens
somewhere between 500 and 50000, whereas for Rosenbrock it occurs somewhere between
5 and 500; i.e, in addition to exhibiting encouraging convergence behavior, the modified
methods outperform the conventional one, both in speed and error rate, provided that the
proper parameter ranges are selected. For Griewank, 50000 is such a range.
49
In order to cope with the realistic computation limitations of a standard 2.83 GHz and
3.23GB RAM PC running MATLAB, the procedure is performed for each function separately and the resulting figures are grouped together as in Figures 2.1-2.4. For the sake of
clarity, the figure in the right column of Figure 2.4(c) as well as some other figures which
will appear later in this section are shown in logarithmic scale.
Figure 2.2. AFV behaviour of the four methods over gradually increasing sample runs in a
10D problem with 5, 10 and 20 particles for (a) Rastigrin, (b) Griewank and (c) Rosenbrock
functions and parameter range of [-500,500].
To summarize the results, the modified methods outperform the conventional one in
terms of convergence speed for Rastigrin function better than the two others, and in terms
50
of the best final value for Rosenbrock function. However, the cost of final value error is only
acceptable for high parameter ranges.
Figure 2.3. AIN behavior of the four methods over gradually increasing sample runs in a
10D problem with 10 particles for (a) Rastigrin, (b) Griewank and (c) Rosenbrock functions
and parameter range of [-5,5], and [-50000,50000].
Figures 2.1-2.4 reveal the AFV and the required AIN, but they do not explain the gradual
convergence behavior, which is of great importance in this study. Figure 2.5 demonstrates the
fitness value variation during optimization process, averaged over 1000 runs for 10D problem
with 10 particles. It is obvious that modified methods are superior for the typical range
50000 with termination criterion of 500 iterations. For the case of Griewank, all methods
51
Figure 2.4. AFV behavior of the four methods over gradually increasing sample runs in a
10D problem with 10 particles for (a) Rastigrin, (b) Griewank and (c) Rosenbrock functions
and parameter range of [-5,5], and [-50000,50000].
perform almost similarly.
Since average fitness trends may not be adequate for presenting the success rate, in Figure
1.2 the probability density function of the final fitness value in 1000 runs for 500 iteration
scenario was plotted for Rastigrin function. It was shown that the probability of ending
with smaller fitness values increased in modified methods as the parameter range increased.
Similar trend was observed by studying other functions. For the sake of conciseness, we have
omitted those results.
52
Figure 2.5. Fitness value of the four methods averaged over 1000 runs in a 10D problem
with 10 particles for (a) Rastigrin, (b) Griewank and (c) Rosenbrock functions and parameter
range of [-50000,50000].
In addition to convergence performance, the behavior of particles has also been studied
by Nakano (2007) yet as another performance criterion. Figure 2.6 depicts how the average
position of a sample particle in a sample dimension varies. The results are averaged over
1000 runs for Rastigrin function assuming n = 10, m = 20, and parameter range of 500. In
order to have a better view, two areas in Figure 2.6(a) are zoomed out in figures (b) and
(c). It is noteworthy that the modifications offer the particles a slightly greater degree of
freedom in the initial phase of the exploration, but faster convergence in later iterations.
53
Similar trend is observed by studying other sample particles, other dimensions, and other
functions. Again for the sake of brevity, those results are not included.
Figure 2.6. (a) Average sample particle position in an arbitrary dimension for the four methods over 1000 runs with 10 particles in a 10D problem for Rastigrin function with parameter
range of [-500,500], (b) zoom of 1-100 iterations, and (c) zoom of 200-300 iterations.
A similar procedure is repeated using absorbing wall boundary condition for a 10D problem with 5, 10 and 20 particles. Figure 2.7 shows the convergence behavior for Rosenbrock
function in the same 10D problem with 5 particles. It is apparent that the results are similar
to those of the invisible wall. Figures related to the other cases and functions are eliminated
since they also show analogous outcomes. Since it was concluded from previous study that
54
Figure 2.7. Fitness value of the four methods averaged over 500 runs in a 10D problem with
5 particles for Rosenbrock function with parameter range of [-50000,50000] using absorbing
boundary condition.
500 runs are enough for collecting reliable average results, the sample number is reduced to
500 runs.
Finally, to provide a comparable analysis with studies available in the literature, the
parameters of the search are chosen similar to those presented in (Hoorfar, 2007; Nakano,
2007). It is assumed that PSO has a population size of m=30 particles, and the fitness
functions have three parameters (n=3) ranging in [−50000, 50000]. The termination criterion is defined by a maximum iteration number of 500. Figure 2.8 depicts the convergence
behavior averaged over 500 runs. It is observed that the modified methods yield much better performance in terms of the convergence speed and final fitness value for Rastigrin and
Rosenbrock, but for Griewank, the results of all methods are similar. In comparison, TSPSO algorithm requires more than 5000 iterations to achieve similar results for parameters
with shorter range ([-5,5]) (Hoorfar, 2007).
It was shown in this section that the two modifications proposed for PSO yield similar
improvements in the RPSO algorithm behavior. The modifications generally speed up the
55
Figure 2.8. Fitness value of the four methods averaged over 500 runs with 30 particles
for (a) Rastigrin, (b) Griewank and (c) Rosenbrock functions with parameter range of [50000,50000].
algorithm. As the parameter range increases, the superiority of the proposed modifications
improves greatly both in convergence speed and final value, particularly for Rastigrin and
Rosenbrock functions. It is also shown that run number of less than hundreds is not sufficient
for making statistical decision.
2.2.2
Evaluation of the proposed velocity function in thinned array design
In this section, we assess the performance of the proposed algorithm in BPSO scenario, which
has been focus of some recent studies (Camci, 2008). BPSO is highly useful in problems
with two-state parameters, such as irregular shaped planar antenna design and array antenna
design (Jin, 2007), but to extend its appliance to smart type of applications requiring fast
resulting, it is not appropriate yet. This paper studies thinned antenna arrays which are
periodic arrays with a number of non-illuminating elements. In addition to general array
56
applications, these types of antennas can be used where software-defined beam-forming or
pattern-shaping is required by using the currents of elements as tuning factors.
In this section, we first study 2n-element linear half-wavelength thinned antenna array design,
using BPSO, assuming c1 = c2 = 2. It has been proven for BPSO that the gradual decrease
of ω has no effect on performance, thus, as recommended in literature, we assume ω to be
constantly equal to ”1” (Jin, 2007).
The optimization goal is to minimize the side lobe level (SLL) in the antenna’s broadside.
The lobes are assessed in ±90 degrees angular distance from main lobe center. The fitness
function, shown in Eq.(2.8), is minimized.
f = max 20 log |array f actor|
(2.8)
The array factor formula can be found in many references such as equation (26) in (Jin,
2007). Assuming the antenna in xy plane, in this paper, the SLL is assessed in the plane
including the array itself and broadside (θ = 0) for linear array, but for planar arrays a
number of φ-planes are considered.
In order to have consistent study, first, the convergence behaviors in cases similar to those
encountered in RPSO are observed in 1000 runs, and the average results associated with
each case are gathered in two figures depicting AIN and AFV values. Then, planar array
antenna cases are studied.
As the complexity of fitness function evaluation increases, the necessity of achieving better
results in a fewer number of iterations becomes more obvious. In addition, as the number of particles reduces, the calculation time reduces significantly, although the chance of
convergence to appropriate results also drops. Thus, a problem-dependent trade-off should
be considered. Since this study considers large number of runs, the dimensionality of the
problems are selected according to address memory and time limitations.
57
Figure 2.9. Symmetric half-wavelength thinned linear array.
2n-element symmetric linear array
The linear arrays are assumed to be symmetric arrays with 2n elements, as shown in Figure
2.9.
A 10D scenario (n=10) with particle numbers of 5, 10 and 20 are considered and AIN
and AFV curves are shown in Figures 2.10-2.11. The performance of Method2 and Method4
in terms of AIN and AFV for BPSO (similar to the RPSO case) is superior to Method1.
However, although Methods2 and Method3 for RPSO yielded superior performance as compared to the conventional method, only Method2 yielded superior performance consistently
for BPSO case, while Method3 offered inconsistent performance advantage over the conventional method in our studies. The major reason for this distinction between RPSO and
BPSO is due to insertion of the two-level (binary) quantization (Jin, 2007). In fact, c1 = 0
(Method3) may even degrade the convergence performance unless it is used together with
proposed initial velocity (as in Method4). The figures clearly show that the convergence
speed (AIN) is significantly improved by Method2 and Method4, whereas the AFV gain is
just slightly enhanced.
Also, it is important to mention that since in BPSO each parameter can either be 1 or 0,
parameter range will not be a considerable factor.
58
Figure 2.10. AIN behaviour of the four methods over gradually increasing sample runs in a
10D problem with 5, 10 and 20 particles.
Figure 2.11. AFV behaviour of the four methods over gradually increasing sample runs in a
10D problem with 5, 10 and 20 particles.
Non-symmetric planar array
Similar study is repeated for the more complicated case of planar half-wavelength, nonsymmetric thinned array antennas (Figure 2.12). A 44 (n=16) problem is first considered.
In order to shorten the total run time, 100 iterations are performed in 700 runs, and the
59
Figure 2.12. Non-symmetric thinned planar antenna array.
swarm size is limited to two particles. The number of runs is reduced to 700 from 1000
according to the study results in previous section, showing reliable run number. The pattern
is assessed in 6, 8 and 10 φ-planes (φ=30:30:180, φ=0:20:180, and φ=0:15:180, respectively).
The superiority of Method2 and Method4 both in terms of convergence speed and final result
is evident in Figure 1.3 which is not repeated here.
Similarly, 33 , 44 and 55 planar arrays are studied with 3, 4 and 5 particles, respectively.
This time, the pattern is assessed in no more than 6 φ-planes (φ=30:30:180), 200 iterations
are performed in each of 700 runs. In order to alter the fitness function, a success boundary
of -20dB is added to the fitness function introduced in 2.9, resulting in the following fitness
function:
f = max 20 log |array f actor| + 20
(2.9)
Figures 2.13-2.15 show the AIN/AFV, density, and average fitness trend for the 33 array,
respectively. The results confirm the previous conclusions. Method2 and Method4 are
superior to the conventional method and Method3, both in terms of convergence speed
and final value. Figure 2.14 shows that the probability of achieving a smaller final fitness
values is higher for Method2 and Method4 as compared to other methods. An important
point underscored by Figure 2.15 is that wherever the optimization procedure is terminated,
60
Figure 2.13. AIN and AFV behaviour of the four methods averaged over gradually increasing
sample runs in a 9D problem with 3 particles.
Method2 and Method4 outperform the other two techniques. Similar trend is observed by
studying other cases, but, for the sake of conciseness, the results are not included here.
Finally, as in RPSO, absorbing-wall scenario is investigated, and similar outcomes are
achieved. In Figure 2.16, the optimization results associated with the 33 array with three
particles averaged over 500 runs, using absorbing boundary condition, are depicted. It is
apparent that the conclusions of invisible boundary condition can be extended to absorbing
wall case.
The overall results confirm that the proposed methods (Method2 and Method4) yield
superior performance for BPSO.
Finally, to underscore the time-consuming nature of the optimization process, 2.8, 3.4,
and 5.5 hours were required for the 700 runs of simple 44 array with only 2 particles for
pattern assessment in 6, 8 and 10 φ-planes, respectively, and angular resolution of ∆θ=0.167
degrees. To underscore the computing requirement, it is notable that adding two more
particles to 6 φ-plane case converts 2.8 hours of computing on a standard 2.83 GHz and
61
Figure 2.14. Probability density function of the final fitness value for four methods, with
700 runs in a 9D problem with 3 particles in 6 φ-planes.
Figure 2.15. Fitness value trend of the four methods averaged over 700 runs in a 9D problem
with 3 particles in 6 φ-planes.
3.23GB RAM PC to 5.25 hours. Assuming a realistic software-defined thinned array scenario,
a substantially smaller number of runs will be required. Thus, significance of a method or
62
Figure 2.16. Fitness value trend of the four methods averaged over 500 runs in a 9D problem
with 3 particles in 6 φ-planes using absorbing wall boundary condition.
methods with achieved satisfactory results in a smaller number of iteration becomes quite
evident.
2.3
PSO in antenna frequency selectivity2
In this section, several irregularly shaped microstrip antenna structures are designed using
the new version of Binary Particle Swarm Optimization (BPSO) algorithm explained previously, particularly aimed for software-defined radio (SDR) applications. The optimization
results are compared using both the modified and the conventional BPSO algorithms. Pros
and cons are studied in terms of optimization length, convergence speed and final design
conformability to desired objectives. It is depicted that the modified BPSO achieves the
design criterion considerably faster than the conventional one, at the cost of slightly limiting
2
c 2011 IEEE. Adapted, with permission, from Modiri,A. and Kiasaleh, K., Efficient Design of Microstrip
Antennas for SDR Applications Using Modified PSO Algorithm, IEEE Transactions on Magnetics, May/2011
for section 2.3 .
63
particle exploration ability. This section, in fact, presents the details of the study summarized in section 1.3.
The demand for microstrip antenna (MA) application has been continuously increasing owing to its well known unique properties. Although there are analytic models available for
the design of MA structures, these models are merely proper for simple regularly shaped
patches. This limitation is imposed due to the high computational complexity of irregular
structures. In recent years, however, irregular structures are gaining popularity due to their
ability to achieve large bandwidth or multi-band operation. Nonetheless, available studies
have shown lengthy optimization procedures for such designs. The PSO’s simplicity, ease of
implementation, and flexibility make it extremely appealing for multi-dimensional electromagnetic designs (Jin, 2007; Jin, 2005; Camci, 2008).
Modifications in PSO velocity vector, proposed by the authors in (Modiri, et. el., 2011b)
and explained previously are applied to MA design herein. In (Modiri, et. el., 2011b), the
problem-dependent characteristic of PSO was underscored which is a critical feature required
to be considered while customizing the algorithm to fit different problems.
As it was mentioned in section 2.3, BPSO (implemented in MATLAB) is used along with
EM-SONNET, which is an electromagnetic simulator based on the method of moments
(MoM), to design irregularly-shaped MA structures. The main goal is to depict the performance enhancement of a PSO-based optimization method in optimizing antenna design for
software-defined radio (SDR) application where prompt frequency response characteristic of
the antenna is of paramount importance. The convergence behavior of the conventional and
the proposed methods are compared in terms of the average optimization behavior, particle
activity and success rate.
The uniqueness of the study demonstrated in this paper resides in analyzing the proposed
algorithm in an actual real time electromagnetic design, instead of the benchmark math
problems of 2.5-2.7. Utilizing both MATLAB (as the optimizer tool) and SONNET (as the
electromagnetic analyzer tool), the optimization time lengths are also investigated.
64
2.3.1
Switchable antenna design using modified BPSO
In this section, the performance of the proposed algorithm is evaluated in several MA design scenarios. BPSO, in general, is greatly advantageous in optimization problems with
two-state parameters; however, it is not appropriate for real-time applications in its conventional format. The design of MAs consisting of several switchable segments are studied here.
Connecting or disconnecting any number of the segments to the main antenna body results
in distinct frequency behaviour. These types of antennas can be used where frequencyselectivity is desired. This study focuses on reconfiguring the antenna structure by defining
different frequency behaviour objectives (in SDR scenarios). The issues related to the switching system and switch types are beyond the scope of the paper and are not pursued here.
Figure 2.17 shows the structure of a 15-segment MA on 3mm duroid substrate. The middle
patch contains the feed and will not be affected by the algorithm. BPSO is used to design
several irregularly shaped structures using 14 segments. The feeding position on the middle
patch is deviated from the patch center to better accord irregular structures. In order to
remove a segment, BPSO assigns 0’s for both the segment length and width, resulting in the
disconnection of the patch from the main antenna body in SONNET simulation. In addition
to 15-segment structures, 20-segment ones have also been designed.
15-Segment single-frequency design
The final results of a single frequency optimization problem for a 15-segment antenna was
presented in section 1.3. The S11 curves related to the best designs achieved by the two
methods with termination criterion of T=50 iterations were shown in Figure 1.5. The irregular shaped structures designed by each method were also added to the figure.
Figure 2.18 demonstrates the discrete probability density function of the final fitness values,
as a measure of success. In this figure, MFFV stands for the maximum final fitness value
achieved in the 50 runs. The optimization goal is to minimize the fitness function. Thus, if
65
Figure 2.17. 15-segment reconfigurable microstrip antenna structure.
the probability of ending with lower final fitness values is higher for one method, the success
rate will also be higher for that method. It is apparent that the proposed method shows
higher success rate.
In order to further complete the comparison study, the achieved average final value (AFV) is
shown in Figure 2.19. The results are averaged over an increasing number of runs. The number of runs is increased by 2; i.e., the averaged results are obtained over 2, 4 50 runs. In this
manner, in addition to performing more consistent study, we are also able to recognize the
number of sample runs required for attaining reliable results. Converging to a smooth mode
in Figure 2.19 confirms that 50 runs are acceptable for observing the convergence behavior
of AFV. Of course, the more runs, the more reliable statistical results will be. However,
since runs can be quite time-consuming (as will be seen later), 50 runs are selected herein.
In order to observe the particle trajectory, the optimization procedure with a single particle
was studied in (Janson, 2007). For the same reason, Figure 2.20 depicts the average particle
trajectory in single-particle scenario for the fitness function given by Eq.(1.5) in one randomly selected dimension (out of 14 available ones). Similar overall exploration activities
are observed for the two algorithms in this figure. It is depicted that in the proposed algorithm, the particle starts with slightly higher activity due to the initial velocity definition of
66
Figure 2.18. Probability density function of the final fitness value for the conventional and
the proposed BPSO algorithms using 7-particle swarm.
Eq.(1.3), but slows down faster, and consequently, converges faster due to the elimination of
the personal best term. Same trends are observed in other dimensions which are not shown
here.
15-Segment triple-frequency design
As the second goal, in order to achieve wider bandwidth, a design optimization at three
frequencies of f1=4.8GHz, f2=5GHz and f3=5.2GHz is studied with a 4-particle swarm.
This time the fitness function of Eq.(1.5) has three frequency terms; i.e., N=3 (triple-term).
In this case, it is necessary to prohibit the optimization from getting locked in a loop, which
decreases S11 at one of the frequencies while leaving the other frequencies with high S11
values. Accordingly, a condition has been added to the code which substitutes any S11 value
of less than -15dB with -15dB. Consequently, the swarm will not attempt to improve S11
values at frequencies for which -15dB threshold has been achieved.
Figure 2.21 depicts the convergence behavior of the two methods averaged over R=50 runs.
67
Figure 2.19. Final fitness value averaged over increasing number of runs for the conventional
and the proposed BPSO algorithms using 7-particle swarm.
The superiority, in terms of convergence, of the proposed method is again confirmed. The
proposed method gains at least 2.32dB when compared with the conventional method. It
is noteworthy that the proposed algorithm achieves -30dB at the 4t h iteration, whereas the
conventional one requires 8 iterations to achieve the same. However, at the 8t h iteration,
where both algorithms have satisfied f −30dB, the proposed algorithm outperforms the
conventional one by 5.47dB.
The optimization resolution is assumed to be 66 mm2 . Therefore, it is not possible to
further refine the results of triple-frequency designs by either of the algorithms, unless better
resolution is considered at the cost of higher optimization length, which is not studied here.
In Figure 1.5(a), the best design achieved by the two methods with termination criterion of
T=50 iterations is shown. This time, both methods have designed the same irregular shaped
structure, shown in the figure, as their best design (in terms of minimum final fitness value)
among 50 runs. However, the proposed algorithm achieves the desired objective faster, as it
can also be observed from the average trend shown in Figure 2.21.
68
Figure 2.20. Particle position averaged over 50 runs in an arbitrary dimension for the conventional and the proposed BPSO algorithms using 7-particle swarm.
Similar results are achieved from 7-particle scenario which is not demonstrated here for the
sake of conciseness.
20-Segment single-frequency design
The same single resonance optimization of Eq.(1.5) with N=1 and f1=5.1GHz is performed
for a 20-segment structure. Five more segments are added in a row on top of the structure
shown in Figure 2.17, to convert the 15-segment antenna to a 20-segment one. Thus, the
problem dimensionality increases to 19 from 14. Results similar to those of the previous
subsections are observed using 2, 3 and 6 particles, which are not shown here due to limited
space. In Figure 1.5(b) and (c), the best irregular shaped designs achieved by the two
methods, for the 6-particle scenario, are demonstrated.
69
Figure 2.21. Fitness value trend averaged over 50 runs for the conventional and the proposed
BPSO algorithms, using 4-particle swarm.
2.3.2
Optimization length comparison summary
In previous parts, the optimization length dependency on the fitness function complexity,
termination criteria and particle number was briefly addressed in several MA designs, particularly aiming at real-time applications of BPSO. It is shown that the optimization length
is almost proportional to the number of particles. Thus, decreasing the swarm population is
desired, but the possibility of the optimization goal being satisfied should also be taken into
account. This possibility is highly dependent on the problem itself.
In order to quantify optimization length, Table 2.1 depicts the required run-time in several
design scenarios. From the exhibited 50-run results, one can arrive at the conclusion that
the proposed method achieves an optimization time that is at least 40% shorter than the
conventional method. To elaborate, in Table 2.1, a time period of 9.5min is estimated for
a 25-iteration run in a 2-particle, single-term fitness function scenario in the case of the 20segment design. However, on the average, 12 and 6 iterations are enough for the conventional
70
and the proposed methods, respectively, to meet the success criterion of -10 dB. Thus, the
real design times are roughly 4.75min and 2.37min for the conventional and the proposed
methods, respectively.
Table 2.1. Required optimization time for a single 25-iteration run in minutes.
Single-Term
fitness
function
(N=1)
Triple-Term
fitness
function
(N=3)
Number
of
particles
m=2
m=4
m=6
m=8
m=2
m=4
m=6
m=8
Run Time for
15-segment
Scenario (min)
5
8.5
18
27
5.2
12.5
18.8
29
Run Time for
20-segment
Scenario (min)
5.5
9.5
19.5
26
9.5
16
20
31
Two important points have been considered while preparing data for Table 2.1:
1. The convergence length is affected by the initial values. Therefore, to make the evaluation consistent, same initial values are used for similar cases.
2. For the problems considered in this paper, on the average, the convergence occurs
before the 25t h iteration for both methods. Thus, the iteration number is limited to
25.
It is observed in Table 2.1 that, if the number of particles is increased by α, the run time
increases by β which falls in the range [0.6α, 1.2α]. In addition, moving from 15-segment
design to 20-segment design, one certainly observes an increase in the optimization length,
but the time increase is not easily predictable, since the convergence time is dependent on
the number of particles. The same situation is encountered when moving from single-term
to triple-term functions.
71
Finally, it should be noted that simulators with more efficient processing management can be
customized to be used in practical SDR applications in order to guarantee lower optimization
lengths as compare to those achieved by MATLAB/SONNET arrangement.
2.4
PSO in wearable antenna frequency selectivity3
This section introduces the details of the study which was summarized in section 1.4. This
study aims at the need for real time reconfiguration of the wearable antennas (WA’s) as a
result of being positioned on an evermoving mass (human body) with changing postures and
exclusive electrical specifications. Owing to the algorithm’s flexibility and high convergence
speed, modified Binary Particle Swarm Optimization (BPSO) is utilized for the real time
reconfiguration of a proposed wearable microstrip antenna (MA) consisting of switchable
parasitic elements. The viability of the modifications is validated in terms of convergence
speed and ultimate fitness value for a proposed simple microstrip antenna designed to maintain resonating at 2.45GHz, ISM band, in different situations.
In addition to eliminating clumsy devices that can tangle in surroundings, the incorporation
of the antenna into the clothing has the added benefit of minimum discomfort in long-term
usage. This feature is particularly critical in biomedical remote monitoring applications,
where bio signatures should be tracked continuously. In telecare procedures that last for
months or years, the significance of the patient comfort increases even more.
In addition to patient comfort, WA’s should maintain acceptable performance during monitoring period. It has been shown in literature that various factors, such as wrinkles and
bends of the antenna surface or moisture absorbed by substrate textile, degrade the WA
3
c 2011 IEEE. Adapted, with permission, from Modiri,A. and Kiasaleh, K., Real time reconfiguration of
wearable antennas, Proceedings of IEEE Topical conference on Biomedical Wireless Technologies, Networks
and Sensing Systems (BioWireleSS), Jan./2011 for section 2.4 .
72
performance Hall et al., 2006). These incidents are easily caused by different body movements, poses, environmental alternations, and accidents. It should be noted that the telecare
devices are even used by children who don’t realize the safety rules of the device. Therefore,
real time self-tuning feature of the antenna can largely decrease the health risks created by
these types of variations causing antenna malfunctioning.
2.5
Modeling antenna bend effect
In (Hall et al., 2006), a 56x51mm2 textile patch antenna is fabricated on a 3mm fleece
substrate to resonate at 2.45GHz, the ground plane size is mentioned to be 76x71mm2 and
the patch is fed by coaxial line at distance of 19mm from the edge. As it is shown in Figure
1.6, the antenna was refabricated and its frequency behavior was tested using Agilent E5071C
vector network analyzer (VNA) and Agilent 53708 coax cable. VNA calibration was done by
Agilent 85033E 3.5mm calibration kit. The resonance frequency of the fabricated antenna
is lower than 2.45GHz (2.155GHz ), which is the result of using different textile from the one
mentioned in (? ). The dielectric constant of the textile used here is larger than 1.1 which is
mentioned in (Hall et al., 2006). Figure 2.23 depicts the measurement results related to the
effect of antenna bending on its frequency behavior. It is observed that bends of 45 and 90
degrees result in frequency shifts of 144MHz and 250MHz. It is noteworthy that only due to
a one-directional bend, the antenna completely repels the original frequency band for which
it was designed. This experiment simply confirms the need for antenna reconfiguration.
In order to compensate for the frequency hop resulted from antenna bending, parasitic narrow
patch elements are proposed to be added to the main antenna body as it is shown in Figure
2.24. In the case of the antenna bending, these parasitic patches should be connected to the
main antenna body in order to increase the actual length of the antenna, and consequently,
compensate for the decrease in electrical length of the antenna.This method has been verified
by experiment. Figure 2.25 depicts that, the three switches shown in Figure 2.24(a) are able
73
to return the resonant frequency back to the original one. In fact, one parasitic element and
three switches, as shown in Figure 2.24(a), can not support more than five cases (one flat
and four bend cases).
Of course, it is not straightforward to simulate a variable bend by Sonnet or any other
electromagnetic simulation 2D or 2.5D tool. However, it is necessary to input a variation
for the optimizer to imitate the practical case. In reality, a bend or a twist causes power
drop at the desired frequency. This effect is recognized by power sampler and then the
optimizer is enabled. Thus, for the sake of creating a test environment, in this paper, the
mechanical variation is modeled by an electrical variation which is decided to be dielectric
constant variation in the substrate. For sure, this doesn’t mean that a bend can be easily and
completely modeled by permittivity change. Nevertheless, it was shown in Figure 2.23 that
bending the antenna results in its resonating at a higher frequency which means that the
electrical length of the antenna has decreased. Since the power at the desired frequency is
the only required metric, the bending phenomenon can be imitated by changing the antenna
substrate by a lower permittivity material.
Re-designing and optimizing the same antenna in Sonnet to resonate at 2.45GHz, patch
lengths of 53mm and 44mm are achieved for substrate permittivity values of 1.1 and 1.7,
respectively. The optimized patch width is 49mm and the feeding point distance from the
patch edge is 14.5mm. The permittivity of the original substrate of the antenna is selected
to be 1.7. Four parasitic elements and eight switches are assumed as it is shown in Figure
2.24(b).
PSO algorithm is used to decide which one/ones of the switches should be on in order to shift
the resonant frequency of the antenna back to 2.45GHz. Of course, the higher the number of
switches is, the better the frequency response becomes. However, it is not practically reliable
to incorporate large number of switches in a wearable device. Therefore, we have assumed
eight switches in this paper. Each switch is modeled by a 2x1mm2 conductive tape in its on
mode.
74
(b)
(a)
(c)
Figure 2.22. Measurement arrangement for the antenna in (a)flat,and (b)bent modes.
(c)Switchable antenna with compensating parasitic element in fixed bent mode.
0
−5
|S11|[dB]
−10
−15
Flat (OFF−OFF−OFF)
45deg (OFF−OFF−OFF)
90deg (OFF−OFF−OFF)
−20
−25
−30
−35
2
2.1
2.2
2.3
2.4
2.5
2.6
Frequency(GHz)
2.7
2.8
2.9
Figure 2.23. |S11 | measurement results for the flat and bent cases.
Here, an eight-dimensional problem is solved by BPSO in which the swarm population
is assumed to be 4. It was shown before that the optimization length is almost directly
75
Parasitic elements
3mm
2mm
Switch
S1
S2
S3
1mm
9mm
S1
S3
S2
S4
1mm
11mm
44mm
56mm
49mm
S6
19mm
51mm
(a)
S5
14.5mm
S8
S7
Switch
(b)
Feeding point
Figure 2.24. Reconfigurable antenna with (a)three and (b) eight switches.
proportional to the swarm population. Thus, decreasing the swarm population speeds up
the convergence trend, but the optimization goal may not be satisfied with low number of
individuals. Therefore, swarm size is highly dependent on the problem itself. The optimization goal is to minimize the magnitude of the reflection coefficient of the antenna at the
feeding point, at 2.45GHz.In order to compare the convergence performance of the proposed
BPSO with the conventional BPSO, three different cases are optimized using both methods.
Since high optimization time cost is not acceptable in real time applications, the termination
criterion is set to be the maximum iteration number of 50. It will become clear in the next
section that 50 iterations are more than enough for this problem. In order to present more
consistent results, 20 runs have been executed for each of the three cases and both average
and individual results are depicted.
76
0
−5
|S11|[dB]
−10
−15
−20
−25
−30
−35
−40
1.5
20deg (OFF−ON−OFF)
45deg (ON−OFF−ON)
70deg (ON−ON−OFF)
90deg (ON−ON−ON)
2
Frequency (GHz)
2.5
3
Figure 2.25. |S11 | measurement results for different switch arrangements compensating for
different bends
2.6
Results
Modeling the effect of different bend angles with different dielectric constants of the antenna
substrate, the optimization algorithm decides what the arrangement of the ON/OFF switching modes should be for the eight switches of Figure 2.24(b) in order to retain 2.45GHz as
the resonant frequency of the antenna. Figure 2.26 shows the average convergence behavior
of both optimization algorithms in three different cases of ǫr = 1.1, ǫr = 1.4, and ǫr = 1.7,
respectively. It is observed that the algorithm proposed in Eq.(1.3) outperforms the conventional one in the two first cases by 8.5dB and 3dB, respectively. In the third case, both
algorithms have similar behavior and their difference is limited to tenths of dB.
It is important to mention that, an |S11 | (reflection coefficient magnitude) of -10dB is practically acceptable in most applications. Therefore, a termination criterion of -10dB had to
be added to the algorithm. However, since the convergence behaviors of the two algorithms
77
were supposed to be compared, this criterion was not utilized in this paper. Nonetheless,
Figure 2.26 depicts that the proposed algorithm meets the success criterion much faster.
Figure 2.27 shows the final fitness values achieved by each algorithm after 50 iterations in
all 20 runs. It is observed that the proposed algorithm converges to lower values with higher
probability. Studying Figure 2.26 and Figure 2.27, one can conclude that the modification
of the initial velocity has offered higher chance to the swarm particles to start the optimization procedure from local solutions close to the best solution. On the other hand, omitting
the personal best term speeds up the convergence trend by slightly limiting the exploration
ability of the swarm.
Although both algorithms eventually converge to almost the same fitness values, it is noteworthy that the proposed algorithm satisfies the design goal in less number of iterations,
and therefore, it is more suitable for real-time applications. The best designs (in terms of
low final fitness value) achieved by the two algorithms for the eight switches shown in Figure
2.24(b) are [on on on on on on on on], [on on off on off on off on], and [on on off off on off
on off] for the three permittivity cases of 1.1, 1.4, and 1.7, respectively.
The time period required for each iteration varies by permittivity value and the arrangement
of the switches. However, in this problem, an average of 33sec of computing is required for
each iteration on a standard 2.83GHz and 3.23GB RAM PC. In fact, simulators with more
efficient processing management than that of Sonnet-Matlab arrangement can be customized
for practical real time applications in order to guarantee faster computations. Nonetheless,
the comparison results related to the optimization algorithms are valid regardless of the
computation speed of the computer.
In fact, Figure 1.7 in chapter one depicts the measurement results of the antenna structure
shown in Figure 2.24(b)in a number of flat and bent cases. It is observed that, with all
switches in off mode, the antenna resonates at 2.42GHz in its flat mode. Since the antenna
length (44mm) should almost be equal to half wavelength at its resonant frequency, it can
78
be concluded that the substrate permittivity is almost 1.98. It is observed that bending the
antenna by 30deg and 45deg shifts the resonant frequency to 2.55GHz and 2.63GHz, respectively. With respect to the known relation between permittivity and resonant frequency,
the 2.63GHz resonance can be modeled by a permittivity value of almost 1.7. Therefore,
the switch arrangement achieved for permittivity of 1.7 in previous paragraph is used to
compensate for the bend effect. It is observed that 45deg shift effect can be successfully
recovered by the predicted switch arrangement but this arrangement can not compensate for
30deg bend properly. It is also shown in Figure 1.7 that if the same switch arrangement is
used while the antenna is in its flat mode, the resonant frequency jumps down to 2.37GHz,
as expected.
Fitness Value(dB)
Fitness Value(dB)
Fitness Value(dB)
79
Conventional BPSO
Proposed BPSO
0
−5
−10
−15
−20
0
10
20
30
Iteration Number
40
50
(a)
10
20
30
Iteration Number
40
50
(b)
10
20
30
Iteration Number
40
50
(c)
−5
−10
−15
−20
−25
0
−18.5
−19
−19.5
−20
−20.5
0
Figure 2.26. Average fitness value trend over 20 runs for (a)ǫr = 1.1, (b)ǫr = 1.4, and
(c)ǫr = 1.7
80
Conventional BPSO
Proposed BPSO
0
−10
Final Fitness Value (dB)
−20
−30
0
5
10
Run Number
15
20
(a)
5
10
Run Number
15
20
(b)
5
10
Run Number
15
20
(c)
0
−10
−20
−30
0
−20
−20.5
−21
0
Figure 2.27. The final fitness value of each run for (a)ǫr = 1.1, (b)ǫr = 1.4, and (c)ǫr = 1.7
CHAPTER 3
PROPOSED MBMD ARRANGEMENT FOR BREAST CANCER
DETECTION
This chapter reviews the author’s studies on MBMD technology and describes the novel
MBMD technique which was proposed by the author and her supervisor. It is worthy of
acclaim and emphasis that since microwave radiation is non-ionizing and microwave technology is relatively cheap and reliable, MBMD is a promising modality for development of
self-/physician-assisted screening tools in the case of breast cancer detection where routine
checkups are required. However, MBMD is in its early stages of development and many
unknowns and ambiguities pop up when one tries to approach any of its various aspects.
The author has studied both hardware and software aspects of the problem. The ultimate
goal of the study has been to introduce a preliminary model of a self examination breast
monitoring tool. The proceeding sections present the challenges faced and accomplishments
gained in the course of this investigation.
This chapter starts with the PSO-based algorithm proposed to solve the inverse problem
for MBMD technique and continues with explaining the proposed MBMD radiator structure
and its simulation and measurement results. A summary of this study was given in section
1.5. Here, more details are presented.
81
82
3.1
PSO in solving the inverse problem1
Basically, in MBMD, the scattered/transmitted fields from/through an inhomogeneous structure at microwave frequency band are collected and analyzed in order to define the composition of the object under the test. The dielectric constants of the contents are often calculated
as the indicators of the material type. A comprehensive review of microwave diagnosis applications are found in (Hassan, 2011; Nikilova, 2011; Golnabi, 2011). In these studies, MBMD
is used to create an image of the human body by highlighting the contrast between dielectric
constants of different tissues.
MBMD techniques deal with inverse electromagnetic problems, for which a variety of algorithms have been proposed and studied in the literature (Golnabi, 2011). It is quite evident
that the algorithm needs to be relatively fast and reasonably accurate for biomedical diagnostics. However, such complex inverse problems potentially end up with non-unique results.
For that reason, global optimization algorithms prove to be more appropriate computational
techniques as compared to the analytical optimization methods. There are some articles in
literature which use evolutionary algorithms for non-biomedical problems, see (Cui, 2005)
and (Genovesi, 2007). These problems are considerably simpler that the MBMD problem
due to dealing with low loss or lossless materials.
When using evolutionary algorithms, the main challenge is to analyze and classify the results
obtained by a large number of agents at each iteration, and to lead the algorithm in the correct path accordingly, in order to ultimately achieve the best estimation. That is the correct
composition of the tissue under the test. A burden in this study is the penetration depth
which is restricted by the high absorption rate of the electromagnetic energy by body tissue
at frequencies of interest to the MBMD studies. Yet, some parts of human body, such as
1
c 2012 IEEE. Adapted, with permission, from Modiri, A. and Kiasaleh, K., A novel discrete particle
swarm optimization algorithm for estimating dielectric constants of tissue, Proceedings of 2012 Annual
International Conference of the IEEE Engineering in Medicine and Biology Society (EMBC), Sep./2012 for
section 3.1 .
83
breast, stay suitable candidates for MBMD, firstly because they are not consisted of muscle
or bone.
It is well known that global optimization algorithms are inherently problem-dependent and
need to be customized according to the problem introduced to them. As it was reported
earlier in this manuscript, several variants of PSO were developed for different electromagnetic problems. Here, the PSO variant proposed for MBMD estimates the permittivity
arrangements of lossy multi-layer structures of body tissue models. The impact of various
parameters, namely, the MBMD frequency, the immersion medium, the number of agents,
the smoothing coefficient, and the maximum velocity are studied on the estimation performance, in terms of the maximum estimation error. It is demonstrated that by choosing the
parameters correctly, one can expeditiously achieve estimation results with a maximum error
less that 10%.
In (Modiri, et. al., 2011c), it was demonstrated that PSO variants are able to successfully
act as detection algorithms in a complex MBMD problem. There, two proposed versions of
PSO were used based on complex number and discrete PSO. Here an even improved PSO
variant is introduced based on (Modiri, et. al., 2012). Two main factors that make this
study distinct from the other ones available in literature are
• The new versions of PSO introduced,
• The measurement-based exact loss factors used (see (Lazebnik, 2007)).
The loss factors have generally been ignored or over simplified in order to decrease the complexity of the problem in other studies (Gandhi, 2010; Yeung, 2009).
3.1.1
Problem scope and estimation algorithm
The proposed DPSO algorithm finds the tissue-types and their corresponding dielectric constants in a hierarchical manner (see Figure 3.1). The makeup of each tissue layer is first
84
selected from a set of six possible tissue groups, namely, (t1: skin), (t2: adipose dominant
tissue), (t3: 30%-80% fatty tissue), (t4: glandular tissue), (t5: blood) and (t6: malignant
tissue), see (Lazebnik, 2007) and (Clegg, 2010). For the sake of conciseness, the tissue-types
will be called with their t-numbers. These six groups are, in fact, the tissue-types one can
expect to find inside the breast. After tissue-type selection, the best complex values of the
dielectric constants are found inside the variation spans assigned to those tissues in the literature, (Lazebnik 2 , 2007). In other words, here, the inherent variations of the dielectric
constants of the tissues are taken into account. These variations are created by differences
which exist either between different people or even in a single person, under different conditions. Error threshold is considered to be 10% as it is shown in Figure 3.1. This means that,
if all the estimated dielectric constants have an error of lower than 10%, then the second
optimization cycle is activated. It is noteworthy that the threshold of 10% does not address
a perfect estimation, however it is acceptable in MBMD. The reason for this claim is that
the chance of detecting the tissue-type incorrectly has been shown to be very low at this
threshold. In addition, 10% variation can even happen due to inherent inaccuracies in the
process.
It was explained in chapter one that the popular PSO velocity function is used for binary
PSO, as well. The only requirement is a mapping function which maps the real values to
zeros and ones (see equation 12 in (Jin, 2007)). Likewise, for DPSO, a similar mapping function is introduced in (Modiri, et. al., 2012) to address the six required levels. The mapping
function is shown in Eq.(3.1).
S = 0.5 + NT /(1 + exp(−ν × sc))
(3.1)
where ’sc’ is the smoothing coefficient and NT is the number of tissue types which is
equal to six for the breast tissue considered in this study. ν is the velocity and is a real
number. Figure 3.2 shows the mapping function using different smoothing coefficients. It
85
Optimization1:
Estimate the tissue−type
for each layer
No
Error < Threshold?
Yes
Optimization2:
Estimate the complex
dielectric constants
Figure 3.1. The block diagram of the proposed PSO-based optimization procedure consisting
of two customized optimization cycles.
should be noted that, in order to have a correct mapping, the maximum allowed velocity
should be selected with respect to ’sc’. Any velocity value mapped inside [a − 0.5, a + 0.5)
in Figure 3.2 is interpreted as ’a’ in order to create discrete values.
Equation (3.2) shows the fitness function that is minimized by DPSO. ’R’ and ’T’ depict
the reflectance and transmittance coefficients.
f =
X
{||Ractual | − |Restimated || +
|arg(Ractual ) − arg(Restimated )| +
||Tactual | − |Testimated || +
| arg(Tactual ) − arg(Testimated )|};
(3.2)
86
6.5
5.5
S
4.5
Vmax=1.5 & sc=5
Vmax=6 & sc=2
Vmax=12 & sc=0.5
Vmax=9 & sc=1
3.5
2.5
1.5
0.5
−15
−10
−5
0
Velocity
5
10
15
Figure 3.2. Mapping function with different smoothing coefficients.
where arg () denotes the phase of the enclosed complex number. The normal incidence
of plane wave to multi-layer structure is considered to solve the forward problem (Orfanidis,
2008). Here, both magnitude and phase errors are taken into account in Eq.(3.2), as such
quantities bear distinct information relevant to the optimization process (Orfanidis, 2008).
An immersion medium is considered to surround the tissue, as it is recommended in many
other research articles to improve MBMD detection performance (Nikolova, 2011; Gonabi,
2011). The immersion medium is assumed to be similar in properties to the fatty tissue (t2).
Two types of controlling parameters are involved in this problem:
• MBMD parameters
• DPSO parameters
MBMD parameters are the immersion medium, tissue thickness and the radiation frequency.
The controlling gears of DPSO are the maximum allowed velocity (Vmax), which is related
to the smoothing coefficient in the mapping function, and the number of agents. The effects
87
of these parameters are studied in the following section in terms of the estimation error in 100
independent runs per analysis. Each run is limited to 1000 iterations for the first optimization
cycle shown in Figure 3.1 and 100 iterations for the second one. These restrictions were
imposed by the knowledge gained from trials and errors for the sake of optimizing both the
final error and the computation time.
To be able to compare the results with those presented in (Modiri, et. al., 2011c), the tissue
is modeled as a 5-layer structure (t1, t2, t4, t2, t1) for which it is shown later that 40-agent
swarm (m=40) is fairly strong to gain an accuracy better than 10%. Reviewing the model,
one can easily recognize that this is a model of normal breast tissue. The thicknesses of the
tissues are t1:2mm, t2:15mm, and t4:10mm. The total thickness is 4.4cm, which represents
a good model for the breast tissue when the tissue is squeezed for MBMD measurement.
Although other frequencies are also studied, the reference frequency in this paper is 1GHz,
which has been proved to be appropriate for MBMD (Hassan, 2011; Nikolova, 2011).
3.1.2
Parameter variation study
MBMD parameters
As the first MBMD parameter, the immersion medium is studied. This medium acts as
matching interface for the fields to enter the body tissue with lower reflections. Figure 3.3
shows the effect of using different types of immersion media on the detection error in 100
runs. It should be noted that, unlike previous studies associated with MBMD immersion
medium, here, the impact of this intermediate medium on the optimization trend of the
search algorithm is highlighted. The dielectric constants of air, water and fat at 1GHz are
assumed to be 1, 78.2 − 3.796j and 4.79 − 0.8j, respectively. As it is evident in Figure
3.3, the probability of achieving a final estimation with error higher than 10% increases to
6% in the case of water. It is interesting that this increase in error happens for imaginary
part of the dielectric constant which has lower significance in identifying the tissue type.
Maximum Error
Percentage
88
15
Maximum Error
Percentage
Fat Imaginary
10
5
0
10
20
30
40
50
60
Number of Runs
70
80
90
100
Air Imaginary
15
Air Real
10
5
0
Maximum Error
Percentage
Fat Real
10
20
30
40
50
60
Number of Runs
70
80
90
100
Water Real
20
Water Imaginary
10
0
0
10
20
30
40
50
60
Number of Runs
70
80
90
100
Figure 3.3. The effect of immersion medium on detection error for a 44mm-thick tissue at 1
GHz. Vmax, sc and m are considered to be 1.5, 5 and 40, respectively.
However, the mean error percentage for all three cases remains between 7% and 9%. It is
worth mentioning that for the case of fat, the maximum required iteration number is 191,
which is almost 20% of the maximum iteration number of 1000 (see Table 3.1). This study
simply confirms that oily and fatty substances are preferred for MBMD.
Figure 3.4 shows the estimation error at two other frequencies, 500 MHz, and 5 GHz. 5
GHz case, apparently, has the highest probability of error while 1 GHz (Figure 3.3) has the
lowest probability of error. It is interesting that the standard deviation of the results achieved
from 5 GHz study is more than four times that of 1 GHz study. The mean error percentages,
however, for all the cases remain between 7.5% and 12% (see Table 3.1). Although 500
MHz radiation has higher chance of penetration than its 1 GHz counterpart, the relative
thicknesses of the tissue-layers also comea into picture and introduces a new trend.
Maximum Error
Percentage
Maximum Error
Percentage
89
500MHz Real
20
500MHz Imaginary
10
0
0
10
20
30
40
50
60
Number of Runs
40
70
80
90
100
5GHz Real
5GHz Imaginary
20
0
0
10
20
30
40
50
60
Number of Runs
70
80
90
100
Maximum Error
Percentage
Figure 3.4. Estimation error at some popular MBMD frequencies for a 44mm-thick tissue
(V max = 1.5, sc = 5, m = 40). Immersion medium is considered to be fat.
20
59mm Real
15
59mm Imaginary
10
5
0
0
10
20
30
40
50
60
Number of Runs
70
80
90
100
Figure 3.5. The error for thicker tissue(f = 1GHz, V max = 1.5, sc = 5, m = 40).
In practice, the tissue thickness is defined by the person’s body style (fat, skinny, etc.).
Here, we repeat the study for another case in which the thicknesses of the t2 and t4 tissues
are increased by 5 mm. The total tissue thickness is 59 mm in this case. As shown in
Figure 3.5, the probability of achieving an estimation with an error level higher than 10%
increases to 3% when considering fat as the immersion medium. Nonetheless, the mean error
percentage for all the cases stays in the range of 7.5-8.5%.
DPSO parameters
MBMD parameters studied in previous section are very important; however, they are not
fully under the control of the MBMD operator. Optimization algorithm, on the other hand,
90
can be controlled with less limitations. In this section, the impacts of some of the optimization parameters on the estimation process are studied.
It should be noted that, correctly mapping the real velocity values to discrete values is quite
challenging. The first step is to define the smoothing coefficient of the mapping function.
By decreasing sc, introduced in 3.1, the probability of achieving errors higher than 10%
increases. Further increasing of sc above 5 will not result in any further improvement either.
Therefore, here we demonstrate our results mostly considering sc =5. The second issue is
the maximum velocity. In discrete versions of PSO, due to intrinsic limitations imposed by
mapping function, V max does not have the same direct impact on the performance as it has
in the real number PSO case. Reviewing Figure 3.2, one can simply conclude that V max
should be selected with respect to sc, not only to cover all six levels, but also to preserve a
smooth variation. As shown in Figure 3.6 and Figure 3.3, the average errors for sc = 0.5, 1,
2, 5 and 10 are 8.5, 8.1, 7.8, 8.1 and 7.7, respectively. Lower V max offers higher estimation
accuracy; however, it also increases the required number of iterations in order to converge.
On the other hand, higher velocities decrease the chance of finding the solution. sc = 5 and
V max = 1.5 are the best parameter set for keeping the error below 10% in all 100 runs for
this problem.
Finally, the number of agents is studied. It was shown in chapter 2 that, although having
large number of agents improves the search activity, it also increases the optimization cost in
terms of computation resources and optimization length. Therefore, it is always desired to
find an optimum agent-number. As it is shown in Figure 3.7, lowering the number of agents
from 40 to 30 increases both the probability of achieving higher errors than the desired
10% and the average maximum error. Furthermore, increasing the number of agents to 50
decreases the average error from 8% in the case of 40 agents to 7.8%, which is not a significant
improvement.
Maximum Error Maximum Error
Percentage
Percentage
Maximum Error Maximum Error
Percentage
Percentage
91
40
sc=0.5 − Vmax=12 − Real
sc=0.5 − Vmax=12 − Imaginary
20
0
0
10
20
30
40
50
60
Number of Runs
70
80
90
100
sc=1 − Vmax=9 − Real
sc=1 − Vmax=9 −Imaginary
20
10
0
0
10
20
30
40
50
60
Number of Runs
70
80
90
100
sc=2 − Vmax=6 − Real
sc=2 − Vmax=6 − Imaginary
20
10
0
0
10
40
20
30
40
50
60
Number of Runs
70
80
90
100
70
80
90
100
sc=10 − Vmax=0.65 − Real
sc=10 − Vmax=0.65 − Imaginary
20
0
0
10
20
30
40
50
60
Number of Runs
Maximum Error
Percentage
20
Maximum Error
Percentage
Figure 3.6. Estimation error for a 44mm-thick tissue at 1GHz when assuming different
maximum velocities and smoothing coefficients. 40 agents are considered.
15
30 agents − Real
30 agents − Imaginary
10
0
0
10
20
30
40
50
60
Number of Runs
70
80
90
100
50 agents − Real
50 agents − Imaginary
10
5
0
10
20
30
40
50
60
Number of Runs
70
80
90
100
Figure 3.7. Estimation error for a 44mm-thick tissue at 1GHz when assuming different
number of agents. V max is 1.5 and sc is 5
Table 3.1 summarizes the optimization length for some of the cases studied. Comparing
these results with those shown in (Modiri, et. al., 2011c), one can simply conclude that the
92
Table 3.1. Optimization length comparison
Medium
air
water
fat
fat
fat
fat
fat
1
2
3
F(GHz)
1
1
1
5
0.5
1
1
m1
40
40
40
40
40
30
50
max(iter 2 )
33
1000
191
1000
1000
1000
190
mean(iter)
222
158
50
454
370
93
47
P 3 (high error)
0
6
0
40
26
0
0
Number of agents
Required number of iterations for lower than 10% error
Probability of achieving higher than 10% error
new version of DPSO introduced herein has significantly enhanced the detection performance
by decreasing the estimation time by 85%.
3.2
MBMD proposed radiator design2
A new MBMD structure proposed by the author and her supervisor is studied in this section (Modiri, et. al., 2013a). The novelty of this design resides in introducing an easily
implementable structure for in-vivo, non-invasive, breast monitoring with great potential of
supporting self-examination and promising detection performance. MBMD has been widely
studied for cancer detection recently. In general, either an array or a scanning antenna is
used in MBMD. Employing an array has the advantage of avoiding the mechanical issues
of a scanning antenna. Moreover, a scanning antenna is not a practical arrangement for
self-examination purposes. However, designing the array still remains an important subject
in MBMD technology.
2
c 2013 IEEE. Adapted, with permission, from Modiri,A. and Kiasaleh, K., Characterizing a proposed
sixteen-element array antenna designed for microwave imaging of breast cancer, Proceedings of IEEE Topical
conference on Biomedical Wireless Technologies, Networks and Sensing Systems (BioWireleSS), Jan./2013
for section 3.2 .
93
The deficiencies of X-ray, such as relatively large false negative rates and ionizing property,
have made scientists seek an alternative modality, especially for the situations where successive imaging tests are required (Nikolova, 2011; Hassan 2011; Fear, 2003). Breast cancer
monitoring tests fit in this very scenario; hence, MBMD has been receiving a great deal of
interest and attention in the case of breast cancer (Paulsen, 2005; Fear, 2002; Irishina, 2009).
In the ensuing parts of this dissertation, the development stages of the radiator structure
are explained first in simulation and then in practice. The 3D simulations are performed
in Ansoft HFSS. Since the array had to be characterized in an environment similar to the
actual measurement, Ansoft digital female phantom is used for the majority of simulation
studies. The Ansoft digital phantom is a reliable model for microwave studies as it provides
a relatively accurate model of the frequency dependent glandular and fatty tissues in the
breast. For measurement studies, however, the radiator structure is built and breast tissue
phantoms are created. A donated tumor tissue was used as the target to be detected.
3.2.1
The array structure
As it is shown in Figure 3.8, the antenna array consists of sixteen identical bent dipoles,
separated from each other by 22.5 degrees. These antennas are located on the perimeter of a
circle which goes around the breast tissue. The size of the circle is dependent upon the size
of the breast. The phantom breast size is considered here in order to define the diameter of
this circle (100 mm). To better clarify the dimension and the shape of the antennas in a 2D
space, by fixing the feeding point at the center of the coordinate system (0,0), the top most
and the bottom most edges of the antenna arms will be located at (8.5mm,27.5mm) and
(19 mm, 20.5mm), respectively. The radius of the antenna wire is assumed to be 0.4mm.
This antenna is designed to resonate at 1.2GHz in the presence of the tissue. The spaces in
between the antennas and the tissue are filled with a fat-type material as immersion medium
(Golnabi, 2011). The dielectric constant of fat is used for the immersion medium. This layer
94
Figure 3.8. The Ansoft digital phantom breast tissue and the antenna array.
of fat even covers the antennas. The thickness of this fat layer is assumed to be 17 mm. The
fat medium builds a hemisphere with the radius of 67 mm, which covers the antennas and
the breast tissue, see Figure 3.8. Due to the relatively irregular shape of the breast tissue,
there is no systematic way to calculate these dimensions; therefore, they are achieved by
trial and error.
3.2.2
Analysis results
In practice, the elements of the array are excited one at a time while the other antenna
elements act as receivers (Sill, 2005). Figure 3.9 shows the reflection coefficient for five
elements of the array. Parameter t in Figure 3.9 denotes the angular location of each antenna.
95
Figure 3.9. The reflection coefficient of the five elements of the array.
The sixteen antennas have almost similar resonance behavior. The curves related to the other
eleven elements of the array are not shown in Figure 3.9 in order to keep the figure easily
readable. However, for these remaining eleven elements, the reflection coefficient magnitudes
(—S11—) are shown only at 1.2GHz which is the desired radiating frequency. The straight
dipole structure is bent and optimized in order to present a structure that almost follows
natural body curves and is also smaller in size when compared to the conventional straight
dipoles.
Figures 3.10 and 3.11 show the near field radiation patterns of the antenna at two different
distances (10 mm and 50 mm). In HFSS, two spheres with the aforementioned radii are
created around the radiating antenna and the fields are calculated on those spheres and
plotted as it is seen in these figures. It should be noted that in these types of applications,
near field study is always necessary. Figure 3.10 shows the total Electric field pattern and
96
Figure 3.10 shows the polarization-based patterns. Two important points are observed in
the figures:
• A relatively large portion of the power is reflected back from the tissue and scattered
in the surrounding space. Although the immersion medium is present, the major part
of electromagnetic energy is not able to bypass the material discontinuity.
• As expected, the largest component of the electric field (Ey) is the polarization, which
better complies with the orientation of the dipole. Ex and Ez are lower in intensity by
almost 6 dB and 3dB, respectively. However, the radiation at different polarizations,
due to both the bent structure of the antenna and scattering phenomenon, sets the
stage for detecting polarization dependent malignancies.
To have a more comprehensive study, the electric field distributions, inside and outside
the tissue, are shown in Figure 3.12 from two different views. This figure actually gives an
idea to the interested reader of how far the radiation from the sixteen antennas can penetrate
inside the tissue, what the electric field distribution is, and how much energy is scattered in
the surrounding environment.
The values shown in the color bar of Figure 3.12 are for the case of having an excitation
of 1W; however, they can be scaled to any desired intensities. To better illustrate, Figure
3.12c shows the field distribution in the fatty tissue and glandular tissue separately. The
bent antenna shown in the figure is the transmitter.
Finally, Figure 3.13 shows the transmission coefficient when one of the antennas is transmitting and the other fifteen are receiving. Many parameters, mainly, the multipath effect,
surface waves traveling on the skin, absorbance and scattering phenomena create such an
|S21| distribution. Although transmittance is not the only detection factor considered and
reflection also plays an important role, |S21|′s do not seem to be large enough to support
a reliable detection. Therefore, in the pursuing section, the radiator structure is modified
toward a better performance.
97
Figure 3.10. The radiation pattern of the radiator when only one of the antennas is radiating
in the plane of θ = 90o . Total electric field is shown for the two desired scenarios.
3.3
MBMD proposed radiator design with conductor cover3
As it was shown in the previous section, the transmittance coefficient needs to be enhanced
in the proposed radiator. There are a few articles that introduce a conductor enclosure
in order to improve the detection sensitivity of non-invasive MBMD. In these articles, the
enclosure encompasses both the entire human body and the radiating setup (Gilmore, 2008;
Mojabi, 2010). In other words, these designs require that the patient or the tissue under the
test is placed in a box similar to a Faraday cage in order to enhance the detection chance.
Here, it is shown that a conductor cover, or dome, added to the proposed radiator design
3
c 2013 IEEE. Adapted, with permission, from Modiri, A. and Kiasaleh, K., Non-invasive Microwave
Breast cancer detection - A Comparison Study, will appear in proceedings of 2013 IEEE Global Humanitarian
Technology Conference (GHTC), Oct./2013 for section 3.3 .
98
100
200
300
400
500
600
100
200
300
400
500
600
700
Figure 3.11. The radiation pattern of the radiator when only one of the antennas is radiating
in the plane of θ = 90o . Electric field components are shown separately.
ameliorates the performance of the proposed MBMD structure noticeably. The proposed
conductor cover does not need to encompass the entire breast (which is impossible for an
in-vivo measurement). Instead, it is designed to cover the frontal section of the breast. This
way, a preliminary model of a portable breast examination device with a great potential as
a self-examination tool is introduced. With the aid of simulation, the difference between
the scenario in which a tumor exists inside the breast tissue and the scenario in which the
breast tissue is healthy are demonstrated for the two cases of having the conductor dome
in the measurement setup and when it is absent. The tumor detection sensitivity, as the
performance indicator, is defined as the cumulative difference between the received electric
field intensities of the normal and cancerous tissues at different measurement points. The
phase sensitivity is also calculated in a similar way. Each simulation is conducted when only
99
Figure 3.12. The electric field distribution (a) inside the tissue in xy plane, (b) outside the
tissue in xy plane and (c) inside the tissue in zy plane when the dome is not present.
one of the antennas is set to be transmitting. Then, the same procedure is repeated for the
other elements of the array, allowing for each element of array to act as a transmitter at
some point in the experiment.
It should be noted that similar field measurements, both magnitude and phase, have been
100
Figure 3.13. The transmittance coefficient of the array elements.
used in the past for cancer detection purposes, see (Klemm, 2010; Dun, 2003). This contribution follows on a similar path. As the main goal of this section, however, the performance
improvement using the proposed radiator structure along with the conductor dome is underscored. It is also shown that the detection chance is affected by the polarization of the
electric field (Cho, 2006; Arunachalam, 2008; Woten 2008). We observe a correlation between the detection parameters and the polarization of the transmitted wave in this study
which is similar to those of the previous studies.
It is noteworthy that, due to the high memory requirement of the simulation, a workstation
with very high RAM is employed.
The simulations are demonstrated at 1.2 GHz. This frequency is within the frequency range
which allows one to achieve the required resolution and the sufficient penetration depth for
breast cancer detection, see (Golnabi, 2011).
101
3.3.1
Proposed structure and 3D model
The structure of the proposed radiator is shown in Figures. 3.14 and 3.15 along with the
Ansoft female body and breast model. The radiator consists of an array of sixteen dipole
antennas inside the proposed conductor dome. In order to create the dome model in HFSS,
a sphere was plotted and cut at the borders where it was intersecting the phantom body
exterior. The radius of the sphere was assumed to be 67 mm. The following rules are followed
to achieve a better implementable design:
• (1) The dome has to be as small as possible and yet capable of encompassing the breast
tissue and the 16 antennas completely.
• (2) An immersion medium is assumed to be separating the antennas and the tissue
from each other.
The center of the dome sphere is assumed to be on the X axis, shown in Figure 3.15. One
of the issues faced in this study is the large computational complexity. Some solutions are
needed to make the simulation feasible and still achieve a reliable results. To this end, the
following simplifications are considered:
• (1) As it is shown in Figure 3.14, the female human body phantom is cut horizontally at
two planes and only the breasts and the tissues surrounding them or very close to them
are kept in the model. Since human body tissue significantly absorbs the microwave
radiation, the effect of this modification is negligible. The dimensions of the model
and the surrounding boundary are shown in Figure 3.14. Due to the HFSS modeling
rule, the enclosure boundary is set to be more than λ/4, where λ refers to the free
space wavelength at the desired frequency of 1.2 GHz. This implies that, there exists
a gap of at least λ/4 between the model of the body and the simulation boundaries
(see Figure 3.14).
102
• (2) Instead of letting all the sixteen antennas be present in the model at the same
time and then letting them radiate one at a time, the simulation process is split into
sixteen steps. At each step, only one of the antennas is present, and the electric field
is simulated on 101 points on the perimeter of a circle surrounding the breast tissue.
This circle is called field-measurement circle. The electric field plots are achieved by
creating field reports in HFSS. Optimetrics analysis is performed in HFSS for the
sixteen steps. This second simplification reduces memory resource requirement by a
sizable margin. However, the author was able to keep all the antennas in the simulation
in the studies which will be reported in the later sections. This opportunity was created
after upgrading both the computer and HFSS version.
As it is shown in Figure 3.15, the feeding points of the sixteen antennas are located on the
perimeter of a circle at steps of 22.5 degrees. It should be noted that, this circle is not
identical to the field-measurement circle; however, the two circles are located close to each
other. The reason for choosing two distinct circles instead of one was to avoid very high
field intensities at the vicinity of the antenna feeding points. The field-measurement circle
is shown in Figure 3.15. It should be noted that the field-measurement circle encompasses
the breast tissue without touching it or intersecting it. Therefore, it is a good representative
for data reading points in a realistic scenario. The radius of the field-measurement circle is
50 mm.
The breast model in Ansoft female body phantom, as it is shown in Figure 3.14, consists
of glands and fat. The glands are spread inside the breast in a web-shaped random structure.
In fact, this phantom represents a worst case scenario in which the breast tissue is densely
filled with glandular tissue. In the next part, a variety of tumors with different shapes and
sizes are introduced to the model and the analysis results of the previously explained 16-step
simulation procedure is demonstrated for the two following scenarios:
1. When the outer boundary of the dome is perfect electric conductor,
103
1200mm
2200mm
1815mm
190mm
560mm
180mm
190mm
Figure 3.14. Ansoft female body phantom and our separated section of it for the breast
cancer study.
2. When the outer boundary of the dome is air.
For the tumor, Eq.(3.3) is imported into HFSS in order to model its frequency dependent
dielectric constant. The Debye model corresponding to the dielectric constants of the normal
and cancer tissues are available in (Lazebnik, 2007).
ǫT umor = 7.44048 + 48.5595/(1 + 4.72414 × 10−021 × f 2 )
(3.3)
Although the sixteen antennas are identical, the relatively irregular shape of the breast
phantom creates some discrepancy in their performances in terms of reflection coefficient.
104
X
Bent Dipole
Antennas
Y
Z
Figure 3.15. Digital breast phantom along with the proposed radiator structure.
3.3.2
Detection mechanism and analysis
In order to compare the two cases of having and not having the conductor dome, the field
strengths and phases are found in simulation, and the following two equations are computed:
i
|End
|
i
arg(End
)
=
=
N
X
||E ij
j=1
ij
normal | − |Etumor ||
ij
|Enormal
|
N
X
| arg(E ij
ij
− arg(Etumor
)|
ij
arg(Enormal )
normal )
j=1
(3.4)
(3.5)
In the above two equations, The E-field magnitudes are in dB and phases are in radians.
The operator ’arg’ is the phase operator and ’i’ refers to any of the coordinate axes x, y or
z. The summations in equations (3.4) and (3.5) are over the receiving points N. In fact, the
detection probability increases by increasing N. In our simulations, the results are calculated
over 101 equally spaced points on the perimeter of the field-measurement circle shown in
Figure 3.15; however, considering the ease of implementation, it is more reasonable to use
the existing fifteen antennas as the receiving ports (one antenna transmitting and fifteen antennas receiving). The results for both studies (101 and 16 ports) are shown in the ensuing
paragraphs. In order to overcome the impact of noise and other factors causing fluctuations
in the received field strength, a threshold dependent summation has been plotted; i.e., a
105
threshold is introduced so that only the normalized difference values above that threshold
are added up. This way, one can set measurement sensitivity on a case by case basis in order
to yield the best performance.
In the proposed algorithm, a decision in favor of the presence of an anomaly is made if either
of the two measurement results obtained using equations (3.4) and (3.5) show that the field
differences in the summation exceed the threshold. It should be mentioned that, in order to
model the cancerous tissue, a tumor is added to the normal tissue model. Here, the detection hinges upon changes in the electric field measurements at the receiving antennas in the
presence and absence of the cancer tissue. In other words, using this method, an attempt is
made to establish the ”normal” response when the cancer is absent and compare the cancer
case to that of the normal case. In practice, the normal scenario may be established by
repeated measurements of a healthy individual. This information may then be stored and
maintained by the individual under investigation. However, if the person starts using this
device when the cancer is in its initial stages, the changes due to cancer growth are going
to be detected. It should be noted that, this change-based study is done at the absence of
the PSO-based algorithm explained previously, just to emphasize the effect of modifying the
radiator structure by adding the conductor dome.
The simulations’ results for the following tumors are summarized herein:
• (1) Spherical tumor with a diameter of D=10mm.
• (2) Spherical tumors with diameters of D=5mm.
• (3) Spherical tumors with diameters of D=1mm.
• (4) Cylindrical tumor with a diameter of D=10mm and height of h=10mm.
• (5) Gland-shaped tumor.
106
Results
Figure 3.16 shows the locations of the tumors (discussed above) inside the breast phantom
from two different views (front and top). An attempt has been made to introduce a variety
of shapes and locations in order to have a comprehensive study. In addition to the tumor
cases mentioned in the previous section, a number of other cases involving tumors with
different shapes and sizes with varying locations have been studied; however, for the sake of
conciseness, and also due to the similarity of the results, one set of results, which represent
the overall performance of the proposed structure, is chosen carefully.
As it is shown in Figure 3.17, for each tumor scenario, there are sixteen simulation results
showing the normalized differences between the normal and cancerous cases on the fieldmeasurement circle. Figure 3.17 shows the results for the magnitude of the electric field
in the x direction, i.e., |Ex |. Nonetheless, similar results are achieved for magnitudes and
phases at the other two coordinates, which means 96 analysis results for each case. Then,
equations (3.4) and (3.5) are calculated for different threshold values. In Figure 3.17, the x
axis of the sixteen plots shows the distance from a reference point on the field-measurement
circle. Since the perimeter of the field-measurement circle is 314mm, the last x-axis value is
314. It should be noted that in some of the plots in Figure 3.17, the dashed curves are very
close to the x-axis (almost zero).
The thresholds start with zero and do not exceed the peak level of the magnitude of the
field for each data set. As it is depicted in Figure 3.16 and other figures of this part, at each
step, two scenarios are simulated. In one scenario. the proposed dome structure exists and
in the other one it is absent. The curves related to the latter scenario are shown in dashed
lines and solid lines are chosen for the former scenario. Figures 3.18 to 3.26 show the 3.4
and 3.5 plots for the tumor scenarios shown in Figure 3.16 when only sixteen receivers are
considered on the field-measurement circle.
107
Figure 3.16. Tumors’ shapes and locations inside the breast.
Examining Figures 3.18 to 3.26, one can easily conclude that the detection chance is
noticeably higher when the proposed structure is used with the conductor dome. Although
the results are highly polarization dependent, the aforementioned conclusion is still valid. It
should be noted that the dipole antennas are mainly oriented in the x direction. Since the
dipoles have bent shape, they are not in fact parallel to the x axis. The main conclusion
of the analysis of both x and y axes is that the conductor dome improves the possibility
of detecting the presence of an anomaly. However, for many z direction results adding the
conductor dome decreases the detection chance. Nonetheless, it should be noted that y
direction improvement is much larger than the z direction degradation.
Figure 3.18 is related to a spherical tumor with a diameter of 10 mm at distance of 15 mm
from the skin surface. This location is referred to as a superficial tumor location. Figures
3.19 and 3.20 show the results for the same tumor when it is pushed back inside the breast
tissue by 2.5 cm and 3.5 cm on y direction, respectively. In Figure 3.20, the new tumor
location is closer to the chest wall by 10 mm. In Figures 3.21 and 3.22 the tumor diameter
shrinks to 5 mm and 1 mm, respectively. The centers of both tumors are placed at a distance
of 15 mm from the skin surface. The D=1 mm tumor is studied at the depth of 30 mm from
the skin surface in Figure 3.23. To better underscore the polarization dependency, superficial
Gland−shape Tumor at Location1
TX Antenna#1
15
10
Normalized |Ex|
Difference
0.4
0.2
0
0
100
200
3
300
0
0
400
100
0.2
TX Antenna #3
200
300
400
300
400
300
400
300
400
300
400
300
400
300
400
300
400
TX Antenna#4
2
0.1
1
0
0
Normalized |Ex|
Difference
TX Antenna #2
0.6
Without dome
With dome
5
Normalized |Ex|
Difference
Normalized |Ex|
Difference
108
100
200
1.5
300
0
0
400
TX Antenna#5
1
1
0.5
0.5
0
0
100
15
200
300
100
200
TX Antenna#7
200
TX Antenna#6
0
0
400
10
200
TX Antenna#8
100
5
0
0
100
200
Distance(mm)
1.5
Normalized |Ex|
Difference
Normalized |Ex|
Difference
100
1.5
300
400
0
0
100
2
TX Antenna#9
200
Distance(mm)
TX Antenna#10
1
1
0.5
0
0
100
200
200
300
400
100
Normalized |Ex|
Difference
200
300
400
0
0
TX Antenna#13
4
2
0.5
100
80
200
100
1.5
1
0
0
200
TX Antenna#12
1
6
Normalized |Ex|
Difference
100
2
TX Antenna#11
100
0
0
0
0
300
400
0
0
TX Antenna#14
100
4
TX Antenna#15
200
200
TX Antenna#16
60
40
2
20
0
0
100
200
Distance(mm)
300
400
0
0
100
200
Distance(mm)
Figure 3.17. The normalized differences of |Ex | for the sixteen transmitting antennas.
109
and deep cylindrical tumors are also analyzed in Figures 3.24 and 3.25. For the latter case,
the tumor is pushed back by 35 mm in the y direction shown in Figure 3.15. Finally, a
superficial tumor with the exact shape of a gland, placed very close to an actual gland, is
investigated in Figure 3.26.
Studying the aforementioned figures, one can easily realize the signature degradation in
the case of deeper tumors. Cylindrical tumor reveals this fact in a more obvious manner.
Although detectable signatures still exist in the case of deep tumors, the detection probability
reduces significantly. Also, looking at the tumors with the same shape, as the size of the
tumor decreases, the overall detectable signatures get weaker, as one would expect. One
other conclusion is that, although the gland-shape tumor is larger than the other tumors
considered here, the signature for this type of tumor is not better than those of other types
with smaller size. This observation can be attributed to the similarity of the electrical
properties of the glandular tissue to those of the tumor. The aforementioned results are all
y
expected; yet, what is not expected is the relatively large values of |End
| for some scenarios.
The reason for this phenomenon is the radiation nulls created in the setup in which the
conductor dome is considered. To further illustrate, when using the conductor dome, the
radiated electric field pattern shows nulls at several locations. By adding the tumor, due to
scattering, some low levels of radiation appears in those locations. This creates a relatively
large signature for those cases.
Obviously, as the number of the receiving points increases, the detection probability will
increase accordingly. Thus, in the next part, the results for the two of the aforementioned
scenarios are studied when the number of receiving points are increased to 101 points. It
should be mentioned that this is not a feasible study; however, it sheds light to few more
dark corners of MBMD technique studied here. It is clear from Figures 3.27 and 3.28 that by
increasing the number of receivers, the signatures become more pronounced; however, having
such a high number of receivers can create implementation issues for a self-examination
device.
15
10
With dome
5
Without dome
0
0
2
4
6
Summation Threshold
8
With dome
200
100
Without dome
100
200
300
Summation Threshold
0
0
15
10
Without dome
5
0
0
1
With dome
2
3
4
5
Summation Threshold
Cumulative Ex
Normalized Phase Difference
20
Cumulative Ey
Normalized Phase Difference
25
Cumulative Ez
Normalized Phase Difference
Tumor Diameter=1cm
Location1
Magnitude
300
Cumulative |Ez|
Normalized Difference
Cumulative |Ey|
Normalized Difference
Cumulative |Ex|
Normalized Difference
110
8
Tumor Diameter=1cm
Location1
Phase
6
4
2
0
0
With dome
Without dome
0.5
1
1.5
Summation Threshold
2
4
3
2
With dome
1
0
0
Without dome
0.5
1
1.5
2
Summation Threshold
6
4
2
0
0
Without dome
With dome
0.5
1
1.5
Summation Threshold
2
i
i
Figure 3.18. |End
| and arg(End
) for the superficial spherical tumor with diameter of 10mm.
The summation is over sixteen receiving points.
3.4
Experimental results4
Finally, the performance of the proposed breast cancer detection tool based on microwaves
is studied through experiment. Single-frequency and multi-frequency detection strategies
are investigated. The normal magnitude and phase differences of the scattering parameters
4
c 2013 IEEE. Adapted, with permission, from Modiri, A. and Kiasaleh, K., Experimental Results
for a Novel Microwave Radiator Structure Targeting Non-invasive Breast Cancer Detection, will appear in
proceedings of 2013 IEEE Global Humanitarian Technology Conference (GHTC), Oct./2013 for section 3.4
.
10
5
0
0
With dome
Without dome
0.5
1
1.5
Summation Threshold
2
25
20
15
With dome
10
5
0
0
Without dome
5
10
15
Summation Threshold
15
10
Without dome
5
0
0
With dome
0.2
0.4
0.6
0.8
Summation Threshold
1
Cumulative Ex
Normalized Phase Difference
15
Cumulative Ey
Normalized Phase Difference
20
Tumor Diameter=1cm
Location2
Magnitude
Cumulative Ez
Normalized Phase Difference
Cumulative |Ez|
Normalized Difference
Cumulative |Ey|
Normalized Difference
Cumulative |Ex|
Normalized Difference
111
6
Tumor Diameter=1cm
Location2
Phase
4
2
0
0
Without dome
With dome
0.5
1
1.5
2
Summation Threshold
4
3
2
With dome
1
0
0
Without dome
0.5
1
1.5
Summation Threshold
2
6
4
Without dome
2
0
0
With dome
0.5
1
1.5
Summation Threshold
2
i
i
Figure 3.19. |End
| and arg(End
) for the deep spherical tumor with diameter of 10mm. The
summation is over sixteen receiving points.
between normal tissue and cancerous ones are used as preliminary indicators. It is shown
that this practically implementable and yet relatively simple structure clearly highlights the
existence of a deep tumor via a set of in-vivo non-invasive measurements.
Earlier quite encouraging simulation results of using the proposed tool as a tumor detector were explained. Here, the measurement results of an implemented radiator structure
are demonstrated which in fact, validate the previous simulation results. The implemented
structure is improved in terms of antenna design as compared to the previous ones intro-
10
Without dome
5
0
0
With dome
2
4
6
Summation Threshold
8
20
15
10
With dome
5
0
0
Without dome
2
4
6
Summation Threshold
15
10
5
Without dome
0
0
With dome
0.5
1
1.5
2
Summation Threshold
2.5
Cumulative Ex
Normalized Phase Difference
15
Cumulative Ey
Normalized Phase Difference
20
Tumor Diameter=1cm
Location3
Magnitude
Cumulative Ez
Normalized Phase Difference
Cumulative |Ez|
Normalized Difference
Cumulative |Ey|
Normalized Difference
Cumulative |Ex|
Normalized Difference
112
6
Tumor Diameter=1cm
Location3
Phase
4
2
0
0
Without dome
With dome
0.5
1
1.5
2
Summation Threshold
4
3
2
With dome
1
0
0
Without dome
0.5
1
1.5
2
Summation Threshold
6
4
2
0
0
Without dome
With dome
0.5
1
1.5
Summation Threshold
2
i
i
Figure 3.20. |End
| and arg(End
) for the deep spherical tumor with diameter of 10mm close
to the chest wall. The summation is over sixteen receiving points.
duced in (Modiri, et. al., 2013a).
It is worthy of claim that the ultimate goal of this study is that the patients be able to use
the device themselves with no need to any physical presence in a clinic. The device needs to
be used periodically for routine checkups.
Here, the experimental results of a set of preliminary tests of the implemented device are
demonstrated. The implemented device was shown in Fig.1.11 of chapter 1. The hemisphere
shaped base structure is created by 3D printing. For the sake of realistic experiment, breast
5
0
0
With dome
Without dome
1
2
3
4
5
Summation Threshold
With dome
150
100
50
0
0
Without dome
50
100
150
200
Summation Threshold
10
5
Without dome
0
0
With dome
0.5
1
Summation Threshold
1.5
Cumulative Ex
Normalized Phase Difference
10
Cumulative Ey
Normalized Phase Difference
15
Cumulative Ez
Normalized Phase Difference
20
Tumor Diameter=5mm
Location1
Magnitude
200
Cumulative |Ez|
Normalized Difference
Cumulative |Ey|
Normalized Difference
Cumulative |Ex|
Normalized Difference
113
5
Tumor Diameter=5mm
Location1
Phase
4
3
2
With dome
1
0
0
Without dome
0.5
1
1.5
2
Summation Threshold
4
3
2
With dome
1
0
0
Without dome
0.5
1
1.5
2
Summation Threshold
4
3
2
Without dome
1
0
0
With dome
0.5
1
1.5
2
Summation Threshold
i
i
Figure 3.21. |End
| and arg(End
) for the superficial spherical tumor with diameter of 5mm.
The summation is over sixteen receiving points.
tissue phantoms are built. The very useful recipes introduced in (Hahn, 2012) and (Ostadrahimi, 2009) are followed for this purpose. The measurement results are gathered as
.S2P files from ENA and analyzed in MATLAB.
15
10
With dome
5
0
0
Without dome
1
2
Summation Threshold
3
25
20
15
10
With dome
5
0
0
Without dome
2
4
6
8
Summation Threshold
8
6
4
Without dome
2
0
0
With dome
0.2
0.4
0.6
0.8
Summation Threshold
1
Cumulative Ex
Normalized Phase Difference
20
Cumulative Ey
Normalized Phase Difference
Tumor Diameter=1mm
Location1
Magnitude
Cumulative Ez
Normalized Phase Difference
25
Cumulative |Ez|
Normalized Difference
Cumulative |Ey|
Normalized Difference
Cumulative |Ex|
Normalized Difference
114
5
Tumor Diameter=1mm
Location1
Phase
4
3
2
With dome
1
0
0
Without dome
0.5
1
1.5
2
Summation Threshold
4
3
With dome
2
1
0
0
Without dome
0.5
1
1.5
2
Summation Threshold
4
3
Without dome
2
1
0
0
With dome
0.5
1
1.5
Summation Threshold
2
i
i
Figure 3.22. |End
| and arg(End
) for the superficial spherical tumor with diameter of 1mm.
The summation is over sixteen receiving points.
3.4.1
Measurement setup
After a number of trial attempts, reliable breast phantoms are created. Although there are
some other phantom recipes in the literature, as well, the above mentioned ones are found
to be more concise and precise. Fortunately, a donated tumor tissue from a breast cancer
patient has become available to the author to use in phantom studies. The recipe authors
have mentioned that the phantom doesn’t follow the behavior of the real breast tissue if it
is not used in almost five days after being built. Thus, all the measurements summarized
20
Tumor Diameter=0.1cm
Location2
Magnitude
15
10
5
0
0
With dome
Without dome
1
2
3
Summation Threshold
4
30
20
10
0
0
With dome
Without dome
5
10
Summation Threshold
15
15
10
5
0
0
Without dome
With dome
0.5
1
Summation Threshold
1.5
Cumulative Ez
Cumulative Ey
Cumulative Ex
Normalized Phase Difference Normalized Phase Difference Normalized Phase Difference
Cumulative |Ez|
Normalized Difference
Cumulative |Ey|
Normalized Difference
Cumulative |Ex|
Normalized Difference
115
6
Tumor Diameter=0.1cm
Location2
Phase
4
With dome
2
0
0
Without dome
0.5
1
1.5
2
Summation Threshold
4
3
With dome
2
1
0
0
Without dome
0.5
1
1.5
2
Summation Threshold
8
6
Without dome
4
2
0
0
With dome
0.5
1
1.5
Summation Threshold
2
i
i
Figure 3.23. |End
| and arg(End
) for the deep spherical tumor with diameter of 1mm. The
summation is over sixteen receiving points.
herein are performed within 48 hours of the completion of the creation of the phantoms.
During the measurements, the phantoms rest on the bags of ice water.
In our studies here, we show the results of using two breast phantoms; a normal breast
phantom and a cancer breast phantom. The cancer breast phantom contains the donated
tumor tissue. As it is shown in Figure 3.29, the tumor tissue is placed inside the glandular
tissue. First, the fatty tissue solution is created and poured inside a container which is
covered by aluminium sheet. Using aluminium sheet makes it easier to later separate the
phantom from the container. A beaker filled with ice water is placed in the container while
pouring the fatty tissue solution, so that it preserves the required space for glandular tissue.
4
Cumulative Ex
Normalized Phase Difference
40
Cumulative Ey
Normalized Phase Difference
60
Tumor Diameter=1cm
Height=5mm
Location1
Magnitude
1
Cumulative Ez
Normalized Phase Difference
Cumulative |Ex|
Normalized Difference
30
Cumulative |Ey|
Normalized Difference
116
20
10
With dome
Without dome
0
0
40
1
2
3
Summation Threshold
With dome
Cumulative |Ez|
Normalized Difference
20
0
0
Without dome
10
20
30
Summation Threshold
8
6
4
Without dome
2
0
0
With dome
0.2
0.4
0.6
0.8
Summation Threshold
3
Tumor Diameter=1cm
Height=5mm
Location1
Phase
2
1
0
0
Without dome
With dome
0.2
0.4
0.6
Summation Threshold
0.8
3
2
With dome
1
Without dome
0
0
0.5
1
1.5
Summation Threshold
2
4
3
2
With dome
1
0
0
Without dome
0.5
1
1.5
Summation Threshold
2
i
i
Figure 3.24. |End
| and arg(End
) for the superficial cylindrical tumor. The summation is over
sixteen receiving points.
After keeping the container inside the refrigerator for almost an hour, the fatty tissue becomes
solid. Next, the ice water is replaced with hot water to make it easier to remove the beaker.
Then, the glandular tissue solution is poured in the empty space left behind by removing
the beaker. The tumor is added during this step. At the final step, the outer surface of the
phantom is covered with skin solution. All tissue solutions are gelatine type materials and
building the complete phantom requires careful temperature control. In order to remember
which phantom contains the tumor, some dye powder is spread at one point of the surface
10
5
0
0
Without dome
With dome
1
2
3
Summation Threshold
4
20
15
10
With dome
5
0
0
Without dome
2
4
6
Summation Threshold
8
6
4
Without dome
2
With dome
0
0
0.2
0.4
0.6
0.8
Summation Threshold
1
Cumulative Ex
Normalized Phase Difference
15
Cumulative Ey
Normalized Phase Difference
Tumor Diameter=1cm
Height=5mm
Location2
Magnitude
Cumulative Ez
Normalized Phase Difference
20
Cumulative |Ez|
Normalized Difference
Cumulative |Ey|
Normalized Difference
Cumulative |Ex|
Normalized Difference
117
4
Tumor Diameter=1cm
Height=5mm
Location2
Phase
3
2
With dome
1
0
0
Without dome
0.5
1
1.5
Summation Threshold
2
3
2
With dome
1
0
0
Without dome
0.5
1
1.5
Summation Threshold
2
4
3
2
Without dome
1
0
0
With dome
0.5
1
1.5
Summation Threshold
i
i
Figure 3.25. |End
| and arg(End
) for the deep cylindrical tumor. The summation is over
sixteen receiving points.
of the phantom which has the tumor inside (see Figure 3.29). The phantoms have a base
diameter of 12 cm and height of 4.5 cm. The tumor tissue is a relatively thin piece, the
thickness of which is not more than 3 mm; however, it has an irregular spread shape with
the largest dimension of 1 cm.
Measurement setup is shown in Figure 3.30. Flexible RF cables are used to connect the
ENA arms to the SMA connectors on the radiator. It is noteworthy to mentions that while
5
0
0
With dome
Without dome
0.5
1
1.5
2
Summation Threshold
40
30
With dome
20
10
0
0
Without dome
5
10
15
20
Summation Threshold
25
15
10
Without dome
5
0
0
With dome
1
2
3
4
Summation Threshold
5
Cumulative Ex
Normalized Phase Difference
10
Cumulative Ey
Normalized Phase Difference
15
Gland−shape Tumor
Close to a Superficial Gland
Magnitude
Cumulative Ez
Normalized Phase Difference
Cumulative |Ez|
Normalized Difference
Cumulative |Ey|
Normalized Difference
Cumulative |Ex|
Normalized Difference
118
4
Gland−shape Tumor
Close to a Superficial Gland
Phase
3
2
With dome
1
0
0
Without dome
0.5
1
1.5
Summation Threshold
2
4
3
2
With dome
1
0
0
Without dome
0.5
1
1.5
Summation Threshold
2
2
1.5
1
With dome
0.5
0
0
Without dome
0.2
0.4
0.6
Summation Threshold
0.8
i
i
Figure 3.26. |End
| and arg(End
) for the superficial gland-shaped tumor. The summation is
over sixteen receiving points.
measuring the parameters for two ports, the other fourteen SMA connectors are left as open
circuits in this experiment.
3.4.2
Results and analysis
Equations (1.7)-(1.10) shown in chapter one show the measured data is analyzed. The main
goal of the experiment is to find the signatures in the microwave radiation parameters due
to the tumor. The signatures are detected as the result of comparing normal and cancerous
tissues. Equations (1.7)-(1.10), in fact, show the cumulative normalized difference between
500
Tumor Diameter=1cm
Location1
Magnitude
400
300
200
100
0
0
With dome
Without dome
50
100
Summation Threshold
150
500
400
300
With dome
200
100
0
0
Without dome
100
200
300
Summation Threshold
100
50
With dome
Without dome
0
0
5
10
Summation Threshold
15
Cumulative Ez
Cumulative Ey
Cumulative Ex
Normalized Phase Difference Normalized Phase Difference Normalized Phase Difference
Cumulative |Ez|
Normalized Difference
Cumulative |Ey|
Normalized Difference
Cumulative |Ex|
Normalized Difference
119
50
Tumor Diameter=1cm
Location1
Phase
40
30
Without dome
20
10
0
0
With dome
0.5
1
1.5
Summation Threshold
2
20
15
10
5
0
0
With dome
Without dome
0.5
1
1.5
Summation Threshold
2
30
20
Without dome
10
With dome
0
0
0.5
1
1.5
Summation Threshold
2
i
i
Figure 3.27. |End
| and arg(End
) for the superficial spherical tumor with diameter of 10mm.
The summation is over 101 receiving points.
normal and cancer tissues in terms of magnitude and phase for the reflection and transmittance coefficients at a single frequency. where the operator ’arg’ is the phase operator. The
magnitudes of the S parameters are in dB and the phases are in radian. In order to remove
the undesired spikes due to zero phase values, all the phases are summed with 2pi.
The antennas are originally designed to resonate at 1.2 GHz; however, as expected, the
experimental setup, the environmental factors, the radiator creation procedure and the presence of the phantom lead to a slightly different resonating frequency. Therefore, the analysis
presented herein is performed in the new optimum resonance frequency. Figure 3.31 shows
200
With dome
100
0
0
Without dome
50
100
150
Summation Threshold
Cumulative |Ez|
Normalized Difference
100
50
0
0
With dome
Without dome
10
20
30
40
Summation Threshold
80
60
40
20
0
0
With dome
Without dome
5
10
Summation Threshold
15
Cumulative Ex
Normalized Phase Difference
300
Cumulative Ey
Normalized Phase Difference
150
Tumor Diameter=1cm
Height=5mm
Location1
Magnitude
Cumulative Ez
Normalized Phase Difference
Cumulative |Ex|
Normalized Difference
400
Cumulative |Ey|
Normalized Difference
120
30
20
Tumor Diameter=1cm
Height=5mm
Location1
Phase
Without dome
10
With dome
0
0
0.5
1
1.5
Summation Threshold
2
15
10
5
0
0
With dome
Without dome
0.5
1
1.5
Summation Threshold
2
30
20
10
With dome
Without dome
0
0
0.5
1
1.5
Summation Threshold
2
i
i
Figure 3.28. |End
| and arg(End
) for the superficial cylindrical tumor. The summation is over
101 receiving points.
the |S11 | for the sixteen antennas at the presence of the normal and cancer phantoms (two
curves per antenna). It is observed that the optimum frequency is 1.354 GHz instead of 1.2
GHz. The effect of this deviation on the results is negligible since the resonance frequency
is still inside the frequency range which is recommended for MBMD measurements.
Figures 1.13 and 1.14, in chapter one, demonstrated the cumulative normalized difference
between normal and cancer tissues in terms of magnitude for the reflection and transmittance
coefficients in the measurements. Figures 3.32 and 3.33 show the phase results. Using ENA
settings, it is possible to gather the data with minimum steps of 42.5 MHz; however, in order
121
Glandular Tissue
Fatty Tissue
First layer
Ice
Tumor Sign
Fatty Tissue, Second layer
Figure 3.29. The creation process of the two breast phantoms: one presenting normal tissue
and the other one having a tumor tissue inside it.
to show the significant advantage of multi-frequency measurement over a single-frequency
one, the results for multi-frequency studies using frequency steps of 42.5, 85, and 127.5 MHz
are shown in all four figures. The results for a single-frequency measurement at 1.354 GHz
are shown, as well. In multi-frequency analysis, the cumulative differences achieved at all
considered measurement frequencies are added up. It is evident that the tumor signatures are
noticeable in all the figures. The phase signatures in Figures 3.32 and 3.33 keep increasing
with frequency in a step-shape manner with almost equal jumps at each step. That can be
explained by the intrinsic periodical behavior of the phase.
3.4.3
Comparing with a competing MBMD design
In this section, the measurement results for the proposed device is compared with the measurements reported by another group working on MBMD in the Duke University (Stang,
122
2008). It is obvious that, in general, conducting a fair comparison of the studies conducted
by two separate groups with distinct set of constraints is not easy; however, here, an attempt
has been made to give the reader some idea of these two devices. It is critical to first mention
the differences between two studies. They are:
• The tumor considered in the Duke University study is a spherical one with the diameter
of 9 mm placed inside a phantom with a base diameter of 9.5 cm and a height of 5.5
cm. In our study, however, the tumor was a real breast tumor and had a flat irregular
shape with a maximum thickness of 3 mm and the largest dimension smaller than 1
cm. Overall, one can conclude that our study is using a smaller tumor, which is indeed
more difficult to detect by palpation or by MBMD. In our study, the tumor is placed
in a phantom with a base diameter of 12 cm and a height of 4.5cm.
• Both tumors were placed in glandular tissue; however, in our study the tumor is placed
in a deeper location (2 cm deeper).
• Due to the antenna designs, this study is performed at 1.2GHz and the Duke University
one is done at 2.7GHz.
• The Duke University study targets a bulky clinical device while this study targets a
self examination (small form factor) tool; therefore, they are more open to adding
complexity. A key difference between the two design is the number of antennas; the
Duke University device has 36 bowtie antennas, whereas this design has 16 bent dipole
antennas (See Figure 3.34).
As it is shown in Figure 3.35, the device introduced here shows signatures which are at
least 15 dB stronger than those shown by the Duke University group when comparing the
average values. It is also superior by at least 24 dB when comparing the peak values.
123
It is, hence, worthy of acclaim that, since the tumor scenario considered here presents
a more challenging detection problem as compared to that offered by the Duke University
study, the above results suggest that the proposed device provides much stronger signatures overall as compared to those offered by the Duke University platform when identical
situations are encountered.
3.4.4
Calibration
The user interface dome structure should be attached to the user’s body tightly (it will
ultimately have a bra-shaped design), so that any unwanted effect from the operator misplacement is removed. However, to take care of inevitable errors caused by an inexperienced
operator, the device needs to be calibrated before starting any rounds of measurements.
Calibration will remove the impact of minute misplacement effects or similar environmental
phenomena. As an example, Figure 3.36 shows the signatures created by a 7 mm movement
of the tissue as compared to the signatures created by a deep tumor which is 5 mm in radius.
The normalized scattering parameters are calculated (see 1.9). Figure 3.37 shows the HFSS
models of the aforementioned movement and tumor. HFSS data is analyzed in Matlab to
create the figures. This is a simple example of misplacement which is modeled in order to
show how calibration works. In this study, first the transmittance magnitude signatures
caused by tumor are extracted. Then, removing the tumor, the effect (signature) caused
by a 7 mm movement (misplacement) is calculated. The signature created when these two
phenomena happen simultaneously are examined as well. Finally, the answer to this question
is investigated: Are we able to extract merely the tumor signature if we have had the chance
to know the movement signature by means of calibration? Here, it is shown that the answer
is ”Yes”. As it is clear in the figures, the tumor signatures are retrievable after removing the
movement signatures. Figure 3.36 only focuses on the magnitude of the transmittance coefficient when antenna 1 is transmitting at 4 selected frequencies of 0.8GHz, 1.2GHz, 1.6GHz
124
and 2.4GHz. In fact, there is a large amount of data to analyze, since reflection coefficients,
phase values, and the results for the cases when the other fifteen antennas are radiating are
not shown here for the sake of conciseness. The presented figures, however, clearly show the
principle idea.
In order to calibrate the device for eliminating the movement effects, the transmittance coefficients between the adjacent antennas are used. The reason for choosing this technique is
that, the amount of energy received by the antenna positioned right next to the transmitting
antenna stays almost the same at the presence and absence of the tumor. Due to the reflection from body surface and creation of surface waves, the wave mainly travels around, and
not through, the tissue in order to get to the adjacent antenna. Also, this adjacent-antenna
energy transfer is affected by the entire tissue placement due to the change in the reflection
coefficient. This fact has been shown in Figure 3.38 for the scenario shown in Figure 3.37.
Consequently, adjacent-antenna transmittance coefficients are appropriate parameters for
device calibration. The simulation observations validate this claim specially at or close to
the resonance frequency of the antennas.
125
Figure 3.30. The measurement setup using an ENA.
126
Figure 3.31. The measured |S11 | of the sixteen antennas in two cases of having the normal
phantom under the test and having the cancer phantom under the test.
2000
1800
1600
Star: Frequency step of 42.5MHz
Circle: Frequency Step of 85MHz
Cross: Frequency Step of 127.5MHz
Square: Single frequency of 1.354GHz
arg(S11nd)
1400
1200
1000
800
600
400
200
0
0
0.5
1
1.5
2
2.5
Frequency (up to which the results are added) (Hz)
3
9
x 10
11
Figure 3.32. Cumulative phase difference of reflection coefficient (arg(Snd
)) for singlefrequency and multi-frequency studies.
127
3500
3000
Star: Frequency step of 42.5MHz
Circle: Frequency Step of 85MHz
Cross: Frequency Step of 127.5MHz
Square: Single frequency of 1.354GHz
arg(S21nd)
2500
2000
1500
1000
500
0
0
0.5
1
1.5
2
2.5
Frequency (up to which the results are added) (Hz)
3
9
x 10
21
Figure 3.33. Cumulative phase difference of transmittance coefficient (arg(Snd
)) for singlefrequency and multi-frequency studies.
128
Figure 3.34. The MBMD design from Duke University and their phantom.
129
Figure 3.35. Comparing the phantom measurement results of this study with that of the
Duke University.
130
Figure 3.36. Comparing signatures achieved from tumor with movement indications at four
different frequencies.
Figure 3.37. Digital breast phantom modeling misplacement.
131
Figure 3.38. Tumor signature and movement effect on adjacent antennas.
CHAPTER 4
CONCLUSIONS AND FUTURE RESEARCH DIRECTIONS
Microwave-base medical diagnosis is in its early stages of development and many unknowns
and ambiguities reside in this field of research. The author has studied both hardware and
software aspects of the problem and the preliminary model of a self examination breast
monitoring tool was introduced and tested on phantoms. Yet, there is long way to having a
clinically approved device. The following items show the possible future research studies in
this field:
- Since it is desirable to configure the final design in the form of a wearable device, an
extensive study is required to develop textile radiator structures. This study needs to fully
analyze and compare different applicable materials for embedding the radiator as well as
various conformal antenna designs. It should also cover the impacts of human body on
the device and suggest solutions to avoid damages (perspiration is an example). Simulation
models are necessary to be developed for full characterization. Measurement trials will follow
the simulations for validation. Since the device is desired to be a wearable one (similar to
sport bras), it will need to be fabricated and tested in different sizes.
- A comprehensive study is required on designing the measurement unit. The ultimate goal
is to have a screening device that is easily readable not only by a physician, but also by the
user herself. There have been new technologies which can be of significant utility in this
regard. Copper Mountain Technologies, for instance, has recently introduced a miniature
Vector Network Analyzer unit. Designing a Graphical User Interface (GUI) will be the next
step. Throughout this part of study, although focusing on user-friendliness, an effort should
be done to reduce the impact of the operator or environment on final results.
132
133
- Different types of radiation modalities, namely, continuous-wave, pulsed, and continuousmodulated wave, are needed to be compared both at system and performance levels. There
is no such study in the literature at this times. Yet, the impact is predicted to be remarkable.
- Another suggested study is the optimization of the radiation power and exposure duration
with respect to the required detection accuracy. The results of such a study will have
nontrivial impacts on the final configuration of the design of a wearable device. - One very
important step is to investigate the normal changes of the breast tissue from time to time,
in order to train the device not to detect them as abnormalities. The changes in the breast
tissue throughout menstrual cycles are required to be well defined. Also, the changes due
to losing/gaining weight should be taken into account. - Although it was shown that PSO,
in its various versions, can successfully handle the desired problem, there are a handful of
algorithms in the literature which can be used for this end; however, discovering which ones
are the best options demands an extensive comparative study. This step can eventually make
a huge impact on the detection speed and resource consumption.
- Since the initial studies cannot involve human subjects, the other important part of this
project is to study and develop body tissue phantoms that mimic human body close enough to
support the primary experiments. Existing studies in the literature are based on simplified
models. It is noteworthy that any imperfection in this part of the project is indeed an
impediment to the reliability and precision of the final product. Chemistry and biology
skills come into the picture for this section. - An extensive statistical study is required once
a prototype is fabricated in order to collect feedback from groups of future users regarding
possible improvements in the design. It is also important to study how women feel about
such a technology and what they need to be able to operate the device comfortably.
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VITA
Arezoo Modiri received her BS degree in 2001 from University of Tehran, Iran, and her MS
degree in 2005 from Iran University of Science and Technology, both in Electrical Engineering.
She entered the Electrical Engineering doctoral program at The University of Texas at Dallas
in 2009. During her PhD which was focused in RF and Microwave, she had the opportunity to
experience part time internships in The University of Texas Southwestern Medical Center, as
well as AWR Corporation. Her research areas of interest are microwave diagnostic techniques,
antennas, evolutionary algorithms, RF and microwave engineering and electromagnetics.
Publications
Journals:
• A. Modiri and K. Kiasaleh, ”Radiation and Scattering Analysis of a Proposed Microwave Radiator Targeting Tissue Malignancy Detection, Submitted to IEEE transactions on microwave theory and techniques”, August 2013.
• D. Cao, A. Modiri, G. Sureka, and K. Kiasaleh, ”DSP Implementation of the Particle
Swarm and Genetic Algorithms for Real-Time Design of Thinned Array Antennas,”
IEEE Antennas and Wireless Propagation Letters, vol.11, no., pp.1170-1173, 2012.
• A. Modiri and K. Kiasaleh, ”Efficient Design of Microstrip Antennas for SDR Applications Using Modified PSO Algorithm”, IEEE Transactions on Magnetics, vol.47, no.5,
pp.1278-1281, May 2011.
• A. Modiri and K. Kiasaleh, ”Modification of Real-Number and Binary PSO Algorithms
for Accelerated Convergence,” Antennas and Propagation, IEEE Transactions on ,
vol.59, no.1, pp.214-224, Jan. 2011.
Conferences:
• A. Modiri and K. Kiasaleh, ”Non-invasive Microwave Breast Cancer Detection - A
Comparison Study”; Accepted to be presented in IEEE Global Humanitarian Technology Conference, GHTC 2013.
• A. Modiri and K. Kiasaleh, ”Experimental Results for a Novel Microwave Radiator
Structure Targeting Non-invasive Breast Cancer Detection”; Accepted to be presented
in IEEE Global Humanitarian Technology Conference, GHTC 2013.
• A. Modiri, K. Kiasaleh and S. Chandrahas, ”Characterizing a Proposed SixteenElement Array Antenna Designed for Microwave Imaging of Breast Cancer”, IEEE
Topical Conference on Biomedical Wireless Technologies, Networks and Sensing Systems, BioWireleSS 2013, RWW 2013, Austin, TX, Jan. 20-23, 2013.
• J. Singh, A. Modiri, and K. Kiasaleh, ”Novel UWB Hybrid Dipole Antenna with QuasiIsotropic Radiation Pattern”, IEEE Radio and Wireless Symposium, RWS 2013, RWW
2013, Austin, TX, Jan. 20-23, 2013.
• A. Modiri and K. Kiasaleh, ”A Novel Discrete Particle Swarm Optimization Algorithm
for Estimating Dielectric Constants of Tissue”, 34th Annual International Conference
of the IEEE Engineering in Medicine and Biology Society ,EMBC2012, San Diego, 28
Aug.-1 Sep. 2012.
• A. Modiri, K. Kiasaleh, ”Improvements on Active Microwave Radiometry for Breast
Cancer Detection”, Metroplex Day, Annual Conference on Research Opportunities,
Arlington, TX, 3 Feb. 2012.
• A. Modiri and K. Kiasaleh, ”Permittivity Estimation for Breast Cancer Detection
Using Particle Swarm Optimization Algorithm”, 33rd Annual International Conference
of the IEEE Engineering in Medicine and Biology Society ,EMBC2011, Boston, 31
Aug.-3 Sep. 2011.
• A. Modiri and K. Kiasaleh, ”Real Time Reconfiguration of Wearable Antennas”, IEEE
Topical Conference on Biomedical Wireless Technologies, Networks and Sensing Systems, BioWireleSS 2011, RWW 2011, Phoenix, AZ, 16 - 20 Jan. 2011.
• A. Modiri and K. Kiasaleh,”Efficient Design of Microstrip Antennas for SDR Applications Using Modified PSO Algorithm”, 14th Biennial IEEE Conference on Electromagnetic Field Computation (CEFC), Chicago, IL, 9-12 May 2010.
Book Chapter:
• Book Title: ”Swarm Intelligence for Electric and Electronic Engineering”
Chapter Title: ”Particle Swarm Optimization Algorithm in Electromagnetics- Case
Studies: Reconfigurable Radiators and Cancer Detection”
Chapter Authors: A. Modiri, K.Kiasaleh
Book Editors: Girolamo Fornarelli, Luciano Mescia
published late in December 2012 in the United States of America by Engineering
Science Reference (an imprint of IGI Global).
Pending Patent:
• K. Kiasaleh; A. Modiri; Systems And Methods For Detecting Breast Cancer, filed with
the USPTO on 3/15/2013 and has been given the serial number 61/790,988.
Technology Disclosure:
• J. Singh; A. Modiri; K. Kiasaleh; Hybrid Dipole Antenna, Tech ID:13-022.
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