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Computer Aided Surgery
4:65?76 (1999)
Biomedical Paper
Fluoroscopy as an Imaging Means for ComputerAssisted Surgical Navigation
R. Hofstetter,
Dipl.-Ing.,
M. Slomczykowski, M.D., Ph.D., M. Sati, Ph.D.,
and L.-P. Nolte, Ph.D.-Ing.
Maurice E. Mu?ller Institute for Biomechanics, University of Bern, Bern, Switzerland
ABSTRACT Objective: Intraoperative fluoroscopy is a valuable tool for visualizing underlying
bone and surgical tool positions in orthopedic procedures. Disadvantages of this technology include
the need for continued radiation exposure for visual control, and cumbersome means of alignment.
The purpose of this article was to highlight a new concept for a computer-assisted freehand
navigation system that uses single intraoperatively acquired fluoroscopic images as a basis for
real-time navigation of surgical tools.
Materials and Methods: Optoelectronic markers are placed on surgical tools, a patient
reference, and the fluoroscope to track their position in space. Projection properties of the fluoroscope are acquired through an initial precalibration procedure using a tracked radiopaque phantom
grid. Corrections are applied to compensate for both the fluoroscope?s image intensifier distortions
and the mechanical bending of the C-arm frame. This enables real-time simulation of surgical tool
positions simultaneously in several single-shot fluoroscopic images. In addition, through optoelectronically tracked digitization of a target viewpoint, the fluoroscope can be numerically aligned at
precise angles relative to the patient without any X-ray exposure.
Results: This article shows the feasibility of this technology through its use in cadaver trials
to perform the difficult task of distal locking of femoral nails. Comp Aid Surg 4:65?76 (1999). �99
Wiley-Liss, Inc.
Key words: C-arm, fluoroscopic image registration, computer-integrated surgery, distal locking,
femoral nail
INTRODUCTION
Mobile fluoroscopic devices (C-arms) are an integral part of the standard equipment used in orthopedic surgery to provide real-time feedback of bone
and surgical tool positions. A constant or pulsed
mode allows control of surgical actions, and single
buffered images can be used for diagnosis and
verification. Although C-arms provide very valuable in situ image information, their use results in
exposure of the patient and operating room staff to
radiation. This is especially true when constant and
higher-frequency pulsed modes are used for longer
periods of time while the surgeon is operating close
to the field of view. Since fluoroscopy is a twodimensional (2-D) imaging technique, it cannot
provide depth information within a single image.
This makes the control of complex three-dimensional (3-D) manipulations difficult and requires
repeated use of the fluoroscope at different viewing
angles.
Currently available computer-assisted ortho-
Received October 23, 1998; accepted May 3, 1999.
Address correspondence/reprint requests to: R. Hofstetter, Maurice E. Mu?ller Institute for Biomechanics, University of Bern,
Murtenstrasse 35, CH-3010 Bern, Switzerland. E-mail: robert@mem.unibe.ch.
�99 Wiley-Liss, Inc.
66
Hofstetter et al.: Fluoroscopy for Surgical Navigation
pedic surgery systems are generally based on 3-D
image data sets that are acquired preoperatively in
a computed tomography (CT) scanner. Multiple
views of the imaged anatomy are then generated
and brought to the computer screen where the positions of surgical tools are represented.1,2 These
systems provide the missing link between the anatomy and medical images by visualizing the positions of surgical tools relative to the image data,
and they have proven to be very useful for certain
orthopedic interventions such as pedicle screw insertion. However, for other surgeries, e.g., in the
field of osteosynthesis, it is often not possible to
justify acquisition of a CT data set because of both
cost and time constraints.
This is especially true for distal locking of the
unreamed femoral nail, one of the most difficult
steps in the process of femoral fracture reduction.
During this closed procedure, the C-arm is presently the only aid to guiding a surgical drill through
the locking holes of the intramedullarly placed nail.
In the first step of the conventional approach, an
accurate C-arm alignment has to be achieved to
obtain a view through the locking holes, reproducing them as perfect circles. This trial-and-error
method requires patience and sound experience in
C-arm handling. Next, a special radiolucent drill is
guided under constant C-arm control through the
bone and the locking holes. Note that here the
surgeon is performing a 3-D surgical action using
only one 2-D view, which often leads to errors.
Furthermore, these two steps are responsible for a
high percentage of the total X-ray exposure during
this procedure.
This has led us to integrate the C-arm into a
computer-assisted surgery system and to provide
surgical navigation based on intraoperatively acquired single-shot images. Two different features
required for the described locking procedure have
been included: navigation of surgical tools in Carm images, and navigation of the C-arm to obtain
optimal views of the anatomy.
The concept required the development of an
accurate method for automatic registration of fluoroscopic images which did not interfere with routine operation procedures. This article describes the
general methods of this technology and demonstrates its feasibility for use in locking femoral
nails.
passive robotic manipulator was used to measure
the positions of surgical tools and a phantom for
C-arm image registration. For a project involving
total hip replacement,4 the use of registered C-arm
images has been proposed for coregistering CTimage data or 3-D models to a surgical robot.
Hamadeh et al.5,6 and Weese et al.7 used
several images of a calibrated C-arm for coregistration of spinal CT data. Brack et al.8 used a
calibrated C-arm to guide saw cuts for knee arthroplasty surgery; Brandt et al.9 proposed the use of
intraoperatively acquired and registered C-arm images to guide active or semiactive surgical robots in
positioning orthopedic implants. Viant et al.10 recently published a method based on biplane-registered C-arm images for determining the trajectories
of distal locking screws through a femoral nail. In
all of these papers, C-arm calibration is performed
separately for each intraoperative image by placing
a calibration phantom in the field of view. We have
chosen an approach involving C-arm precalibration
that avoids the use of calibration phantoms in every
image and is thus advantageous for routine clinical
use. A first prototype using our concept was briefly
described in 1997.11
Because the C-arm?s X-ray projection can be
modeled as an optical camera system (extrinsic
calibration), our work has been inspired by the
calibration of video cameras, especially in industrial and robotic applications where much work has
been done.12 Martins introduced an extrinsic biplane calibration method using coplanar-placed
calibration markers and studied its accuracy.13 A
detailed mathematical approach for calibrating a
video camera using a pinhole camera model was
given by Gembran et al.,14 and a more complicated
method using B-spline instead of linear approximation was proposed by Champleboux and colleagues.15 Accurate calibration of the C-arm requires consideration of distortions caused by the
image intensifier process that can be separately
modeled (intrinsic calibration). In earlier published
reports,16 ?18 pincushion distortions were handled
through a few parameters of a physical model and
methods for correction were proposed. Mansbach19
developed a method to calibrate an optical system
without distinguishing between intrinsic and extrinsic distortions.
Previous Work
An optoelectronic position sensor (Optotrak 3020;
Northern Digital, Waterloo, Ontario, Canada) is
used to track the position of surgical tools, a patient
reference, and the image intensifier of the C-arm
The first attempts to use fluoroscopy in a system for
image-guided surgery were made in 1989, when the
technique was applied to kidney stone removal.3 A
MATERIALS AND METHODS
Hofstetter et al.: Fluoroscopy for Surgical Navigation
67
system (P-COS). The transformation matrix TT,P,
transforming coordinates from the T-COS to the
P-COS, is provided in real time by the position
sensor; then, vP is transformed into the C-arm image intensifier coordinate system (A-COS). The
transformation matrix TP,A describes the position
of the C-arm image intensifier relative to the patient
at the time of image acquisition. The position of the
C-arm must be acquired directly after its trigger
button has been pressed. Note that every C-arm
image loaded onto the computer requires a separate
TP,A. Finally vA is transformed to vI located in the
2-D, pixel-based C-arm image coordinate system
(I-COS).
Fig. 1. System setup with components and their associated local coordinate systems (COSs). Top left: C-arm and
the image intensifier. Top middle: Position sensor. Top
right: Example with a surgical instrument. Bottom left:
Patient representing the surgical object. Bottom right: Carm image as displayed on the monitor; this is a 2-D COS.
within the region of the operating table. Each component is equipped with infrared (IR) light-emitting
diode (LED) markers which define local coordinate
systems (COS) (Fig. 1). The position sensor supplies data needed to perform coordinate transformations between these local coordinate systems.
This data is passed on to a workstation computer
(Ultra 1; Sun Microsystems, Mountain View, CA).
An off-the-shelf video framegrabber board (SLICVideo; Osprey Systems, Cary, NC) allows loading
of gray-scale images from the video buffer of the
C-arm (BV22; Philips, Hamburg, Germany) to the
workstation.
Our objective was to arrive at a system that
works as follows: The surgeon acquires a number
of single C-arm views from different orientations
that are all displayed on the workstation screen. A
computer-generated projection of a surgical tool is
then displayed on each image. This is equivalent to
its representation under conventional constant fluoroscopic control. An update rate of 10 Hz enables
real-time navigation in up to four C-arm images
simultaneously.
The underlying computations are based on a
chain of transformations as given in Equation 1. Let
the tip of a drill bit be described by a 3-D vector vT.
This vector is constant in its local tool coordinate
system (T-COS; note that the subscripts indicate
the COS in which the vectors are reported). Considering the position of the surgical object, which
can either be the patient or an implant, vT is first
transformed to vP in the patient/implant coordinate
vT f vP f vA f vI
with
v P 5 v T z T T,P, v A 5 v P z T P,A, v I 5 v A z T A,I
(1)
The associated transformation TA,I that models the actual X-ray projection is based on a cone
beam projection model as shown in Figure 2. It was
used by Mansbach19 to calibrate video cameras for
robotic applications. Let the center of the X-ray
source be represented by a 3-D vector, fA (focal
point). Two vectors, rA and cA, that point along the
rising row and column coordinates of the digital
image define the orientation of the image plane
onto which the C-arm image is projected. The 2-D
vector pI, present in the I-COS, represents the
piercing point of an image plane normal through fA.
To transform a 3-D coordinate vA into a 2-D
coordinate vI, a unit vector sA is first calculated
Fig. 2. Linear cone beam projection model as used for
extrinsic C-arm calibration. The X-ray is emitted at location
fA and projects a point vA, representing a tool tip, onto the
image plane as vI. pI is defined by the point where rays pass
directly normal through the plane.
68
Hofstetter et al.: Fluoroscopy for Surgical Navigation
Fig. 3. Setup for the extrinsic C-arm calibration. A plate containing steel spheres is imaged in a proximal and a distal position
with respect to the C-arm image intensifier. The 3-D centers of the spheres vAProx, vADist are determined through optoelectronic
tracking.
(Equation 2) that points in the direction of the
appropriate X-ray beam.
sA 5
vA 2 fA
uv A 2 f Au
(2)
In a second step, sA is projected onto the
image plane through two scalar products (Equation
3). The result is a 2-D coordinate, which is added to
the piercing point pI.
vIrow 5 sA z rA 1 prow
vIcol 5 sA z cA 1 pcol
pI 5
S pp D, v 5S vv D
row
col
Irow
I
(3)
Icol
The vectors fA, rA, cA, and pI are projection
parameters and describe the properties of the Carm. While the vectors rA and cA are assumed to
remain constant in the A-COS for the lifetime of
the C-arm, we have found that the X-ray source
position fA and the related piercing point pI change
on most C-arm models owing to mechanic deformations of the C-arm frame. However, all projection parameters are determined in an initial extrinsic C-arm calibration procedure, which is repeated
periodically when required. The method used to
compensate for the C-arm frame deformations is
described below.
Extrinsic Calibration
The calibration is based on a plate containing radiopaque spherical markers arranged in a rectangular grid. This plate is imaged twice by the C-arm at
a proximal and distal position relative to the image
intensifier.
To obtain the 3-D positions of each sphere,
the calibration plate is instrumented with LED
markers defining a local COS (C-COS). Three
reference indents, iC1, iC2, and iC3, locally define
the position of the rectangular grid (Fig. 3). The
coordinates of these points are digitized with
respect to the C-COS using an optoelectronically
tracked pointer. After these incision points have
been transformed from the C-COS into the ACOS, the coordinates of all sphere centers are
generated. From here on, we will refer to the 3-D
sphere centers of the proximal (vAProx) and the
distal (vADist) calibration image in the A-COS as
the original coordinates. The corresponding image coordinates vIProx and vIDist (I-COS) of the
projections of the spheres are obtained through
image analysis (Fig. 4). By sorting according to
their positions, every image coordinate can be
assigned to a corresponding original coordinate.
For each calibration image, a 3 3 3 transformation matrix AProx, ADist is set up. This allows
transformation of a 2-D image coordinate vI into
3-D original coordinates vAProx and vADist, located in the plane of the calibration plate using
the following equations:
Hofstetter et al.: Fluoroscopy for Surgical Navigation
Fig. 4. C-arm image of the calibration plane in proximal
position. The 2-D center coordinates of the sphere markers
are automatically detected through image analysis and
marked by a white cross.
v AProx 5 A Prox z
v ADist 5 A Dist ?
S D
S D
v IProxx
v IProxy ,
1
v IDistx
v IDisty
1
(4)
AProx is calculated by setting up an overdetermined system of equations using all vAProx and
vIProx of a calibration image. A solution with a
least-squares error is found using the matrix
pseudoinverse:
A Prox 5 V AProx z V TIProx z (V IProx z V TIProx)21
(5)
where the matrix VAProx contains all column vectors vAProx and the matrix VIProx all column vectors
vIProx with the z-components set to 1. The same
method is used to calculate ADist.
By means of Equation 4, it is then possible to
find the corresponding coordinates vAProx and
vADist for any vI. The lines formed by joining each
vAProx, vADist represent the X-ray beams belonging
to each image coordinate vI (Fig. 3). All X-ray
beams ideally intersect at the focal point of the
system?in this case, the center of the X-ray source
fA. This point is found by minimizing its distance to
all lines of sight corresponding to the spheres of the
proximal calibration image using a least-squares
error minimization algorithm.14
The remaining projection parameters, rA, cA
69
and pI, are determined by setting up a system of
equations based on Equations 2 and 3, with all pairs
of image and original coordinates for both calibration images. A matrix pseudoinverse technique is
used to solve this overdetermined system of equations. A detailed description of the mathematics can
be found in Gembran et al.14
The errors included during this precalibration
step have a direct and systematic influence on the
navigation error. Errors are introduced through the
two optoelectronic-based coordinate transformations, which are required to measure the 3-D centers of the spheres in the two calibration-plate positions. The tracking system provides coordinate
data with a total RMS error of about 0.2 mm per
LED marker.20 Roughly estimated for the use of
optimized LED marker shields on both the image
intensifier and calibration plate, this introduces a
mean error of 0.4 mm for navigation within the
volume between proximal and distal calibration
plates. Additional errors are caused by the image
analysis algorithm detecting the centers of the
spheres in the calibration images. However, it was
found through informal testing that, owing to the
high number of markers, this error is negligible.
Overall accuracy analysis is described later in System Accuracy Study.
C-Arm Frame Deformations
Because of the significant weight of the image
intensifier and the X-ray source unit, the C-arm
frame is subject to variable states of stress according to the C-arm?s orientation. This may cause
significant frame deformations when the C-arm is
moved. With one of the C-arm models available for
our studies, we experimentally measured a maximum movement of fA to be 9.8 mm relative to the
A-COS. The C-arm frame must therefore be con-
Fig. 5. Position-sensing concept with corresponding coordinate systems to compensate for errors related to C-arm
frame deformations.
70
Hofstetter et al.: Fluoroscopy for Surgical Navigation
fS in the S-COS, where it is assumed to remain
constant.
? With the C-arm in a position (a,b), fS is
transformed back from the S-COS into the
A-COS using a transformation matrix provided by the position sensor.
Fig. 6.
tool.
Weight-based optoelectronic gravity measurement
sidered to act as a nonrigid body with changing
projection parameters fA and pI.
To avoid related inaccuracies, these deformations are measured using the optoelectronic position sensor. A set of LED markers is attached to the
X-ray source chassis of the C-arm (Fig. 5) to define
a local coordinate system S-COS. Our strategy is to
store deformations preoperatively over all possible
C-arm orientations in a lookup table (deformation
calibration). During the surgical intervention, when
the S-COS markers are not visible for the position
sensor, the stored deformation parameters corresponding to a given C-arm position are used to
update the extrinsic projection parameters fA and
pI. In a preliminary study with two different C-arm
models, we found that deformations of the frame
caused by changes in its position are within the
elastic range of the material, and mechanical parts
such as bearings add no significant hysteresis. We
therefore assume that the C-arm frame deformation
is a repeatable function of its orientation relative to
gravity.
The spatial orientation of the C-arm relative
to gravity can be expressed by two angles, a and b
(Fig. 5). An optoelectronic weight-based gravity
measurement tool has been designed to provide the
system with a vertical gravity vector gA as a reference orientation (Fig. 6). a and b can be calculated
from the orientation of this gravity vector relative
to the image intensifier.
The corresponding focal point position
fA(a,b) is measured in the following way:
?
At the time of extrinsic calibration, the initial
value of fA is transformed from the A-COS to
To generate the lookup table, the C-arm is
moved in 10� intervals to all orientations required
in an operation, and values of a, b, and fA(a,b) are
stored.
When an image is acquired during an operation, the C-arm orientation angles as and bs are
measured. Four entries closest to as and bs are then
identified in the lookup table. A bilinear interpolation between the related four values of fA(a,b) is
used to compute the focal point fA(as,bs) valid for
the currently acquired image.
To recalculate the piercing point pI for the
new value of fA(as,bs), Equations 2 and 3 are
solved for pI with vI 5 (0,0). vA is calculated using
Equation 4 for the proximal calibration image. We
assume that AProx remains constant for all possible
fA(a,b) because the proximal calibration plate is
placed directly on the image intensifier input phosphor.
Efforts were made to keep errors originating
from this method of C-arm deformation compensation at a low level. These errors are mainly
caused by the two involved optoelectronic-based
coordinate transformations between the A-COS
and the S-COS, in conjunction with the large distance of the associated marker shields. We optimized the size and position of these marker shields
and experimentally isolated the related errors. We
therefore mounted both marker shields at the ends
of a nondeforming metal bar at a distance equal to
their positions on the C-arm. Ideally, the tracking
system should provide a constant position of fA for
every orientation of this rigid body in space. With
optimized marker shields, we obtained a maximum
variation for the position of fA of 0.8 mm.21
Image Intensifier Distortions
Video-fluoroscopic images often have distortions
that originate from the image intensification process. Two main sources for these distortions have
been distinguished:
?
Stationary pin-cushion distortions arise from
both the spherical shape of the image intensifier input phosphor, onto which the X-rays are
projected, and the optical characteristics of the
video camera.
Hofstetter et al.: Fluoroscopy for Surgical Navigation
Fig. 7. Triangle-based displacement interpolation for distortion correction. Correction vector from pixel inside of top
left triangle obtained from linear interpolation of vertex
corrections.
?
Distortions of more arbitrary shape that depend on the position of the image intensifier
relative to the earth?s magnetic field and other
external magnetic fields are caused by the
electron optics of the photo multiplier.17
Our approach is to undistort the image so that
it fits into the linear projection model. The stationary component of the distortion is removed in an
intrinsic precalibration procedure using a local linear distortion correction method.
The centers of the markers in the proximal
extrinsic calibration image represent actual local
values of the distortions. Reference values for an
undistorted image are calculated by transforming
the 3-D coordinates of the spheres forming the grid
into the C-arm image (I-COS), using the projection
algorithm described above (Eqs. 2 and 3) with the
parameters obtained during the extrinsic calibration. This guarantees an exact adjustment of the
distortion correction to the extrinsic calibration. For
every marker, a displacement vector is calculated
that points from a position in a distorted image to a
corresponding position in an undistorted image.
A bilinear local interpolation is performed for
every pixel of the image to identify its displacement. The grid of distorted markers is therefore
divided into triangles as shown in Figure 7. The
displacement of every pixel within a triangle is
71
interpolated using displacement values of the markers at the three edges and is then rounded to a
nearest integer neighbor. For reasons relating to
computing speed, the displacement of all C-arm
image pixels is precalculated and stored in a table,
and this is applied to every image obtained during
an operation. Remaining gaps are finally filled using linear gray-value interpolation.
When applying this method, errors are caused
by the fact that nonlinear distortions are compensated for using linear interpolation. To ascertain
this error experimentally, we imaged a phantom
with 30 steel spheres (2 mm in diameter) arranged
in a straight line, and applied the described distortion correction algorithm. Image analysis software
was then used to determine the centers of the
spheres in the corrected image. An ideal straight
line was obtained from these data using a linear
regression algorithm. The mean deviation of the
preserved coordinates from this ideal line was 1.2
pixels, with a standard deviation of 1.0 pixel for a
total of five trials.
Further errors originate from distortions
caused by external magnetic fields, which cannot
be compensated for by precalibration. They appear
once the C-arm is moved to positions differing
from that at the time of calibration, however, the
C-arm under investigation here was not very sensitive to magnetic field distortions. The effect of
this error on overall system accuracy is reported in
System Accuracy Study.
C-Arm Alignment
A major difficulty in handling a C-arm during an
operation is ensuring its accurate alignment relative
to the patient?s anatomy to obtain the desired view.
In current clinical practice, this is accomplished by
repeatedly acquiring verification images or by using the constant imaging mode. To reduce the resultant radiation exposure, we propose a computerbased C-arm alignment method.
The basic concept is to define the desired
C-arm orientation with respect to a patient reference using a pointing device. Defined points are
represented by a graphical object that is rendered
through the cone beam projection model onto a
virtual C-arm image. This preview mode allows
C-arm alignment without radiation exposure.
For example, two C-arm views must be well
aligned when preparing the holes to lock an unreamed femoral nail. One view must be oriented
exactly through the locking holes so that they appear as ideal circles, while the second view has to
be aligned perpendicular to this hole. In our system,
72
Hofstetter et al.: Fluoroscopy for Surgical Navigation
in which the surgeon can control the position of the
active surgical tool. A zoom on two of these images
can be seen in Figure 9, showing the pseudo 3-D
navigation obtained through simultaneous multiplanar views. Note that the drill is represented as a
correspondingly thick line, and that an optional
?trajectory line? (dotted line) helps surgeons predict where the drill will pass with the current orientation. The system can be easily configured to
visualize any defined landmark (Fig. 8) and can be
used to navigate within any rigid bone. It is therefore a generalized tool, and is rapidly adaptable to
new applications.
System Accuracy Study
Fig. 8. Graphical user interface allowing precise C-arm
alignment. Holes 1 and 2 are represented by small circles
about the nail axis. The nearly horizontal line represents the
nail?s long axis. The larger circle represents the C-arm?s
field of view. In addition, analog and numerical displays
(bottom) indicate the angular position of the C-arm relative
to a hole axis for fine-tuning.
the patient/implant reference is preoperatively fixed
to the nail (nail reference). Prior to nail insertion,
the positions and orientations of the locking holes
are defined with respect to the nail reference using
a special optoelectronic pointing tool. When the
nail has been implanted and the C-arm is ready to
be aligned, these landmarks are visualized in realtime before obtaining an image. Each locking hole
is represented by a pair of small circles, a long
straight line specifies the position of the nail axis
(Fig. 8), and the large circle predicts the field of
view of the C-arm image. A perfect C-arm alignment is achieved when two circles belonging to a
hole are matched and located in the center of the
field of view. Fine-tuning of the C-arm rotations
can be performed through feedback from the two
sliders at the bottom of the interface. Once an
image aligned directly through the hole is obtained
(Fig. 9, right), the system can be switched to guide
the C-arm view perpendicular to the hole axis (Fig.
9, left).
To verify the total system accuracy, i.e., the accuracy with which a surgical tool is represented on
acquired C-arm images, a dedicated optoelectronically tracked phantom was constructed. This accuracy phantom contains eight spherical steel markers
(2 mm in diameter), aligned in two planes (Fig. 10).
The coordinates of the sphere centers relative to the
optoelectronic markers were measured with a maximum error of 0.2 mm. Imaging this phantom allows verification of the system accuracy by comparing the actual sphere representations to those
calculated by the system. For visual estimation of
the errors, small crosses are superimposed on the
image in the calculated sphere center position: If
the cross is located outside the associated sphere,
the error is .1 mm. An exact error quantification,
as required for an accuracy study, is performed by
metric measurements on image printouts.
The C-arm used for the accuracy study (Comet Telam C125, Liebefeld, Switzerland) was calibrated using a calibration plate with 137 markers
and a grid width of 10 mm. We then acquired a
total of 40 images with the C-arm in various positions and at various distances from the described
User Interface
A simple graphical user interface was implemented
to allow easy use of the system in the operating
theater. The image acquisition mode enables the
user to load and display up to nine C-arm images.
Four of these are displayed in the navigation mode
Fig. 9. Real-time image interactive guidance of surgical
drill used to distally lock femoral intramedullary nails. The
left view shows the countersinks and the right view is
aligned straight down the axis of the hole.
Hofstetter et al.: Fluoroscopy for Surgical Navigation
Fig. 10. Optoelectronically tracked accuracy verification
phantom. The representations of contained steel spheres in
an X-ray image are compared to those determined by the
system.
phantom covering the range of its intraoperative
use. This led to a total of 232 visible spheres
providing an equivalent number of error values. We
evaluated all of these visible spheres, including
those on the border outside the range of the distortion correction. The resulting mean error was 0.55
mm, with a standard deviation of 0.47 mm and a
maximum value of 2.34 mm. The error distribution
is given in Figure 11.
Laboratory Pilot Study
Distal locking of femoral nails was chosen to demonstrate the feasibility of this technology. A wide
73
variety of tools and surgical techniques, such as
locked mechanical devices, guidance systems, fluoroscopic control, and radiolucent drills, have previously been proposed for the accurate and safe
placement of the locking screws. The key problem
lies in the significant deformation of the distal end
of the nail upon insertion into the femur.22 As a
result, the extensive near-real-time use of the Carm is commonly accepted. However, besides causing significant radiation exposure to the patient and
staff, the surgical outcome is still not optimal, since
a direct link between the intraoperative images and
the surgical action cannot be provided with current
techniques.
Using our proposed system, we suggest the
following modified surgical procedure: Before insertion of the nail, a dynamic reference base with
LEDs is attached to the proximal end of the nail.
The position of every distal hole is defined with a
pointing tool, and computer-aided C-arm alignment
ensures images are aligned with the axes of the
locking holes (Fig. 9, right). The computer guides
the C-arm through a 90� rotation and a second
image is taken on which the counter-sinks of the
holes can be easily identified (Fig. 9, left). Realtime image-interactive navigation of a surgical tool
is now available to the surgeon: The position of the
drill can be controlled in two images simultaneously, giving a pseudo?3-D guidance, and the
locking holes can be safely prepared. No additional
Fig. 11. Distribution of the total system error determined in the accuracy study with a total of 232 samples in 40 C-arm shots.
74
Hofstetter et al.: Fluoroscopy for Surgical Navigation
fluoroscopy updates are necessary, resulting in significantly reduced radiation exposure.
Note that precise computer-guided alignment
of the C-arm with respect to the locking holes is
done under the assumption that deformations applied to the nail during its insertion do not cause
significant rotation to the axes of the locking holes
or torsion to the nail, but rather lead to translational
displacements of the locking holes, which do not
significantly affect the C-arm orientation alignment. This assumption has been shown to be true
for the tibial nail in a preliminary study performed
in connection with the development of a guidance
device.23,24 Furthermore, we have found that small
view misalignments due to nail bending are indicated in the C-arm image by a slightly oval appearance of the locking hole, and this can be compensated for during surgery. It is important to note that
the image is taken with the nail already bent, so
navigation accuracy? unlike C-arm view alignment?is unaffected by this phenomenon.
Three test series were carried out: (a) a laboratory study on 25 plastic femurs involving a total
of 50 locking holes; (b) an in vitro study using 10
human cadaver femurs with 20 locking holes; and
(c) an application on two full cadavers with a total
of four locking holes. In all three series, the new
system allowed easy and successful insertion of the
locking screws. In some cases (20% in series a,
10% in series b, and 0% in series c), the drill
touched the nail slightly before entering the hole,
with no significant damage or consequences to the
locking procedure. The mean X-ray exposure time
per pair of prepared holes was 1.78 s for series a,
1.63 s for series b, and 1.65 s for series c. These
studies are described in detail in a forthcoming
paper (Slomczykowski M, Hofstetter R, Sati M,
Krettek C, Nolte L-P. A novel computer assisted
fluoroscopy system for intra-operative guidance:
feasibility study for distal locking of unreamed
femoral nails. Submitted to Journal of Orthopaedic
Trauma).
DISCUSSION
A novel kind of computer-assisted freehand navigation system has been proposed that is based
solely on intraoperatively acquired fluoroscopic
images. In addition, rapid acquisition of these images is supported by computer-guided alignment of
the C-arm. This technology was used to perform
computerized distal locking of femoral nails in a
laboratory setup on plastic and cadaver bones. The
results indicate potential surgical benefits for in
vivo use of the system, and the concept of an
entirely precalibrated system keeps its intraoperative handling as simple as possible.
In the in vitro studies, the prototype was used
with caution and a redundant functional check was
performed before preparation of each surgical action. Before the concept can be used in clinics on a
routine basis, the mechanical and software components must be made more sturdy and reliable to
fulfil the practical requirements of an intraoperative
tool. Hardware and software must also be adjusted
for different C-arm models to prove that the
method yields consistent accuracy.
A key issue during development of the navigator was the total system accuracy. Because the
C-arm was primarily used as an imaging device
rather than as a means of measurement, geometric
linearity and independence of external influence
were not an issue for most C-arm manufacturers.
Thus, the C-arm itself has been found to be the
main source of inaccuracy in the concept. Deformation of the C-arm frame, as well as image distortions, are reasons for such inaccuracies, and
have to be compensated for to achieve reasonable
system accuracy. Our compensation methods significantly decrease? but do not eliminate?the errors. With the C-arm model used in this study,
remaining errors arising, for example, from variable image intensifier distortions caused by external magnetic fields limited the mean system accuracy to 0.55 mm and led in rare cases to errors of up
to 2.34 mm. We are aware that magnetic distortions
can be a larger source of error for certain C-arm
models; this is the subject of our ongoing work.
However, distal locking of the femoral nail (locking hole � 5 mm, hole sink � 6 mm, drill � 4 mm)
could successfully be performed with the technique
presented here. This is because the specially concave hole sinks and the conical point of the drill
mechanically guide the drill through the hole even
when the drill is slightly off target.
A potentially more accurate solution is to
update the intrinsic and extrinsic calibration in every C-arm image using marker phantoms within the
operation as proposed in Pietka and Huang,17 and
Weese et al.7 However, this has drawbacks in practice, since the optoelectronically tracked, radiolucent phantoms have to be placed within the field of
view in the operation. This is cumbersome and, for
many orthopedic procedures, impossible: a typical
tradeoff between system accuracy and practical usability. Recently, a method for C-arm precalibration was proposed which allows correction for distortions caused by the earth?s magnetic field.25 The
C-arm has to undergo a preoperative ?learning?
Hofstetter et al.: Fluoroscopy for Surgical Navigation
procedure that determines and stores the distortions
for all possible C-arm positions relative to the
earth?s magnetic field. We recently developed a
more practical solution that allows the removal of
distortions caused by all kinds of magnetic fields;
this work will be presented in a future publication.
A second source of error is the position sensor that?although the most accurate available?
has limited measurement accuracy for rigid body
transformations. Five consecutive coordinate transformations based on the position sensor directly or
indirectly affect the accuracy of a tool?s representation in a C-arm image. By increasing the number
of LEDs used and prudently positioning them on
the associated rigid bodies, we were able to increase the system accuracy.
The system presented here differs from that
of Viant et al.10 in that the surgeon navigates directly and freehand on multiple computer-aligned
C-arm images; the system of Viant et al. uses stereo
reconstruction of the locking hole to define a trajectory that is carried out with an electromechanical
locking arm.
A fluoroscopy-based system can be seen as a
complement to CT-based systems. Its advantage of
instant availability without preoperative preparations (CT acquisition, segmentation, and registration) allows it to be used for several applications in
the field of traumatology. It is also open for use in
other fields where acquisition of CTs cannot be
justified owing to cost or radiation exposure issues.
Since no manual registration procedure is required,
fluoroscopy-based navigation may be advantageous
in fields where bone structures do not allow recognition of the landmarks needed for CT-image registration. Of particular interest is the potential for
minimally invasive procedures. Since the C-arm is
precalibrated, a minimally invasive DRB attachment is sufficient for keyhole surgical approaches
under multiplanar X-ray control. CT-based methods are more suited in applications where the image
quality of the C-arm is not sufficient or where
attainable perspective views do not provide enough
3-D information.
CONCLUSION
A new computer-assisted surgery system has been
developed integrating a standard C-arm into a freehand navigation system. To our knowledge, it is the
first system allowing surgical navigation based
solely on multiplanar static C-arm images. A
graphical user interface was developed that allows
real-time image-interactive guidance of surgical instruments without further fluoroscopy updates.
75
Based on the results of the technical accuracy study
and laboratory pilot study presented in this article,
two potential advantages of the new computerbased technique, as compared to current manual
techniques, can be identified:
?
?
significantly reduced radiation exposure
improvement of surgical accuracy and safety.
The system holds the promise for a generalized surgical tool that can be adapted to a wide
variety of applications in the field of orthopedic
surgery and offers the possibility of performing
minimally invasive approaches.
ACKNOWLEDGMENTS
The authors thank Yvan Bourquin and Heinz
Waelti for their help in software programming,
Professor C. Krettek for performing the cadaver
study, and Medivision AG, Oberdorf, Switzerland,
for supporting this work by providing electrical and
mechanical hardware.
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