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Computer Aided Surgery
4:45? 49 (1999)
Clinical Paper
Clinical Evaluation of Multimodality Registration
in Frameless Stereotaxy
Hunaldo Villalobos, m.d., and Isabelle M. Germano, m.d.
Department of Neurosurgery, Mount Sinai Medical Center,
1 Gustave Levy Place, New York, New York
Computer-assisted frameless neurosurgery bases its accuracy and reliability on
registration. The aim of this prospective study was to compare the clinical accuracy of different
registration techniques used for computer-assisted frameless neurosurgery.
Ninety-eight registrations in 44 patients were used to compare the clinical accuracy of
self-adhesive marker (MR) and facial landmark (FR) registrations used alone or in conjunction with
surface-fit registration (MR/SR and FR/SR, respectively) for cranial neurosurgery. The computer
estimated error (CEE) of each registration was compared to the real error (RE). This was obtained
by holding the frameless pointer at the center of three different markers and measuring the distance
from the real-time representation on the computer three-planar images to the center of the marker
on the screen.
The most accurate registration was obtained using MR; the RE of MR was 1.6 6 0.1 mm
compared to 3.4 6 0.4 mm for FR. Although the smallest CEE error was obtained using MR/SR, this
was not sustained by the RE. Furthermore, the RE of FR/SR was significantly larger than the CEE
(Student t test, p < .001).
This study corroborates previous results showing that, in the clinical setting, self-adhesive
marker registration is more accurate than facial landmark registration. Furthermore, although surface-fit
registration can be used in conjunction with self-adhesive marker registration, this does not improve the
degree of real accuracy for cranial registration. Comp Aid Surg 4:45?49 (1999). �99 Wiley-Liss, Inc.
Key words: frameless stereotaxy, computer-assisted, cranial, neuronavigation, accuracy, surface-fit,
point registration, facial landmarks registration
During the past decade, several centers have developed computer-assisted frameless stereotactic systems to be applied in neurological surgery.9,18,22?24
These systems are integrated by a three-dimensional (3D) digitizer connected to a computer
workstation that displays the position of the instruments in a set of reformatted images. Image-guided
computer-assisted surgery has become a tool of
inestimable value for neurosurgeons.4 ? 8,23
Registration is the cornerstone of frameless
stereotaxy. The majority of systems use different
types of external reference points or fiducials to
register the patient?s head in the surgical field to the
data set of images in the computer. This process
involves preoperative acquisition of images with
fiducial markers in place. Intraoperatively, these
are overlapped to the patient?s own anatomy by
entering into the computer the same point on the
Received September 11, 1998; accepted March 11, 1999.
Address correspondence/reprint requests to: I. M. Germano. E-mail:
�99 Wiley-Liss, Inc.
Villalobos and Germano: Registration in Frameless Stereotaxy
images and on the patient anatomy. The fiducials
can consist of self-adhesive markers, implantable
soft-tissue/bone screws, or facial landmarks.15,16
Previous studies have suggested that self-adhesive
markers provide a more accurate registration than
facial landmarks. The former, however, require
trained personnel to place them appropriately and a
new set of preoperative images that is often redundant.
The purpose of this prospective study was to
compare the clinical accuracy of frameless registration using self-adhesive markers (MR) with facial landmarks (FR) used alone or in conjunction
with surface-fit registration (MR/SR and FR/SR,
respectively). This study was performed with the
StealthStation frameless stereotaxy system (Sofamor-Danek, Broomfield, CO).4
During the period September 1997 through February 1998, 44 consecutive patients undergoing a
cranial frameless neurosurgical procedure performed by the same surgeon (IMG) were entered in
this prospective study. Preoperatively, 8 ?10 selfadhesive fiducial markers (Medical Products, Baltimore, MD) were applied to the patient?s scalp in
the standard noncollinear fashion minutes before a
magnetic resonance image (MRI) of the brain was
obtained. Prior to applying the markers, the hair
was minimally shaved and Benzoin tincture was
applied to the skin to improve adhesion. Each fiducial marker was then marked in the center with
an indelible marker pen and a circumferential head
wrap was placed around the patient?s head. The
MRI parameters were as follows: TR 5 600; TE 5
8; thickness 5 2 mm; interspace 5 0 mm; NEX 5
2; FOV 5 34; matrix 5 256 3 256. In all cases, the
patient was brought to the operating room immediately after the MRI. After adequate anesthesia
had been achieved, the patient?s head was fixed in
the Mayfield head holder. Care was taken to place
the Mayfield pins at least 2 cm away from the skin
markers. If one of the pins was closer than 2 cm to
a skin marker, this was not used for registration. All
registrations were performed by the same person
(HV). Clinical correlation accuracy was determined
by comparing four registration techniques as described below.
Clinical Experience: Registration
Marker Registration (MR)
The self-adhesive markers were numbered on the
computer screen in counterclockwise fashion be-
ginning with the right frontal one. The same marker
chosen on the computer screen was then identified
on the patient?s head using the blunt-tip probe.
When all markers had been entered, the registration
transformation and error were calculated by the
computer. For point registration, this error is
known as computer estimated error (CEE), which is
a mean fiducial error. A CEE , 4 mm was accepted. To measure the true accuracy of the registration between the image and physical space, the
frameless pointer was then placed in the center of
three different markers. The real-time projections
of the pointer appeared on the computer screen as
crosshairs on the three-planar images. The distance
from the real-time projection of the pointer on the
images to the center of the marker was then digitized in all three planes. The longest distance for
each marker in all three planes was recorded in
each patient. The average of these measurements
was called real error (RE).
Surface Registration after Marker
Registration (MR/SR)
In all cases where MR was completed with a CEE
of ,4 mm and a predicted accuracy (PA)3 at 10 cm
of ,6.0 mm, surface registration (SR) was performed. For surface registration, approximately 40
points along the surface of a patient?s head were
digitized and stored in the computer until a measure
of registration uniqueness known as geometric constraint was satisfied. The location of these points
was randomly chosen and had to be within the
volume that was scanned. The points were entered
randomly, touching the skin very lightly. They
were primarily selected from the fronto-facial area,
as well as the calvarium and auriculo-temporal
area. The computer reconstructed a geometric
model of the surface of the patient?s head based on
the acquired images. The geometric model was
then aligned to the $40 points by the computer, by
finding a transformation which minimized the distance between them. In surface registration, an error measure based on the distances between each of
the digitized points and the corresponding geometric surface model can be used to estimate the quality of the registration. After registration was completed, RE was calculated as described above.
Facial Landmark Registration (FR)
For this registration method, anatomical landmarks
were used instead of self-adhesive markers. Any
distinct anatomical feature can be used for this
registration process. Typically, we used the following points in a counterclockwise fashion: the right
Villalobos and Germano: Registration in Frameless Stereotaxy
pinna, the right external cantus, the nasion, a midline frontal fiducial marker, the left external cantus,
and the left tragus (Fig. 1). Other facial landmarks
were selected when the above could not be used or
were outside the scanned field. The registration was
performed on the patient using the same sequence.
A CEE of ,4 mm was accepted, and the RE
between the image and physical space was calculated as described above.
Table 1. Comparison of Mean Computer
Estimated Error (CEE) and Real Error (RE)
in Clinical Setting Using Different
Registration Techniques*
Surface Registration for Facial Landmarks
Registration (MR/SR)
* Point registration with self-adhesive markers (MR), point registration using
self-adhesive markers followed by surface-fit algorithm (MR/SR), point
registration using facial landmarks (FR), and point registration using facial
landmarks followed by surface-fit algorithm (FR/SR).
In all cases where FR was completed with a CEE of
,5 mm and a PA at 10 cm of #6.0 mm, SR was
performed as described above.
Laboratory Experience: Registration
CEE (mm)
RE (mm)
The object was then scanned using the identical
MRI protocol used for the clinical study.
Cranial Phantom
Statistical Analysis
Five phantoms were used for evaluation of the
registration techniques under laboratory conditions.
To mimic facial landmarks, we used three hardskin fruits to simulate facial registration, and these
were sculpted on one side for that purpose. In
addition, 10 self-adhesive markers were used to
simulate point registration (MR). We used similar
MRI imaging and registration techniques and protocols as described above in Clinical Experience.
Computer estimated error and RE within registration modality were compared using Student?s unpaired t test. Analysis of variance (ANOVA) with
post hoc correction was used to compare CEE and
REE among registrations. A probability (p) , .05
was considered significant.
Fig. 1. StealthStation computer screen showing the localization of the anatomical landmarks used for facial registration.
Frameless stereotactic cranial procedures were performed in 44 patients aged 51 6 2.7 years (range
5? 86). Three children aged 8 6 3 years were
included in the study. The procedures were performed on the following locations: temporal (12),
parietal (10), frontal (9), basal ganglia (7), and
posterior fossa (6). Forty-four registrations and relative errors using MR were compared with 26
registrations using point registration followed by
MR/SR, 28 using FR, and 16 using FR/SR.
Table 1 summarizes the CEE and RE for each
registration type. The most accurate registration,
i.e., the smallest RE, was obtained using MR (RE:
1.6 6 0.1 mm; ANOVA, p , .0001). Although the
absolute smallest error was the CEE found using
MR/SR (1.1 6 0.1 mm), the RE showed a higher
value (2.9 6 0.3 mm). The mean number of registrations to reach a CEE , 4 mm (MR) or , 5 mm
(FR) and a PA at 10 cm , 6.0 mm was 2.8 6 0.3
using self-adhesive markers and 5.7 6 0.1 mm
using facial landmarks. The approximate time for
registration was 5 min with self-adhesive markers,
10 min with facial landmarks, and 30 min when
surface registration was used.
Data pertinent to the children included in this
study are reported in Table 2. In this subgroup, no
differences were found comparing the four modal-
Villalobos and Germano: Registration in Frameless Stereotaxy
Table 2. Comparison of Mean Computer
Estimated Error (CEE) and Real Error (RE)
in Clinical Setting in the Pediatric Population
CEE (mm)
RE (mm)
1.9 6 0.1
0.9 6 0.1
2.2 6 0.1
1.4 6 0.2
Abbreviations as in Table 1.
ities. Table 3 summarizes our laboratory experience
using the surface-fit registration on a rigid phantom. Although no statistical differences in accuracy
were found, the smallest RE was found with MR.
The accuracy of the StealthStation in the laboratory
with optimal parameters has been reported to have
a maximum error of 0.55 6 0.29 mm.13 In the
clinical setting, however, the registration accuracy
may depend on several factors including the registration techniques used. The two main registration
techniques used in neuronavigation are point and
surface-fit registration.14
For point registration, self-adhesive markers,
facial landmarks, or bone-implanted fiducials have
been used. In our practice, we do not use implanted
fiducials, as these are more invasive than the two
former types. Previous studies using a different
system showed that, compared with anatomical
landmarks followed by surface registration, fiducial
registration had greater accuracy.7,21 The higher
error found using facial landmarks can be partially
attributed to the fact that the points used for the
registration are fairly coplanar.25 In our study, we
found that the smallest RE was obtained using
point registration. This corroborated the previous
studies mentioned above.
Self-adhesive markers are less appealing than
facial landmarks because they have to be applied
prior to surgery by trained personnel and in most
cases require repetition of recent radiographic images.19 Originally developed for registration of
multiple 3D image data sets,17 surface-fit registration is based on an algorithm of the least-squares fit
between head and hat. Surface registration techniques have been compared with the landmark surface method and found to have a similar accuracy
within 2 mm.17 Surface-fit registration was developed
for different settings of imaging studies to improve
registration error.12,18,19 This method consists of digitizing the surface of the skin, which is then correlated
with a distance map created by the computer. This
technique is used for multimodality imaging coregistration with acceptable accuracy.10,12,16,20
The use of surface-fit registration to optimize
a registration done using facial landmarks is appealing. If proven to be accurate, this modality
would obviate the need for applying adhesive fiducials and duplicating preoperative imaging studies.
Our clinical experience showed, however, that facial registration followed by surface-fit registration
is not as accurate as marker registration alone:
RE 5 1.6 6 0.1 and 4.7 6 0.7 mm, respectively
(p , .001). Furthermore, our study showed that,
although the CEE using facial landmark registration followed by surface-fit is small (1.2 6 0.1
mm), this does not correspond to the RE (4.7 6 0.7
mm). Thus, this software must be used with great
care for cranial registration.
It should be noted, however, that using a rigid
model in the laboratory, the CEE and RE for surface-fit registration were small and not significantly
different. We believe that this discrepancy is due to
the skin flexibility and elasticity that may interfere
with the surface algorithm. Furthermore, pressure
deformity occurring during the scan and lack of
turgor with presence of wrinkles in the adult all
may contribute to the decreased accuracy using the
surface-fit registration for cranial procedures. In
our study, in the children in whom there was good
turgor and lack of wrinkles, the accuracy of the
surface-fit program was significantly better than the
results from the adult population. Furthermore, no
difference was found between CEE and RE using
the surface-fit algorithm. Similar findings were reported using a potentiometer arm system.1 To overcome the error secondary to skin depression
(known as ?shrinking sphere?2) during acquisition
of points with the manual digitizer, Henderson and
Bucholz proposed an ergonomic depth-sensing laser instrument that digitized the forehead contour in
1 min.11 This and other modalities should be further
investigated to ensure greater clinical accuracy.
In conclusion, this study showed that, in the
clinical setting with an adult population, the most
accurate registration is achieved using self-adhe-
Table 3. Comparison of Computer
Estimated Error (CEE) and Real Error (RE)
in Laboratory Using Rigid Phantoms
CEE (mm)
RE (mm)
1.9 6 0.1
1.1 6 0.2
0.9 6 0.2
1.8 6 0.4
Abbreviations as in Table 1.
Villalobos and Germano: Registration in Frameless Stereotaxy
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