VISUALIZATION OF ANTERIOR SKULL BASE DEFECTS WITH
INTRAOPERATIVE CONE-BEAM CT
Gideon Bachar, MD,1* Emma Barker, MD,1* Harley Chan, PhD,2Michael J. Daly, MSc,2
Sajendra Nithiananthan, BSc,2,3Al Vescan, MD,4Jonathan C. Irish, MD,1
Jeffrey H. Siewerdsen, PhD2,3,5
1Department of Otolaryngology–Head and Neck Surgery, Department of Surgical Oncology,
Princess Margaret Hospital, Toronto, Ontario, Canada. E-mail: email@example.com
2Ontario Cancer Institute, Princess Margaret Hospital, University Health Network,
The Institute of Medical Science at the University of Toronto, Toronto, Ontario, Canada
3Department of Medical Biophysics, University of Toronto, Toronto, Ontario, Canada
4Department of Otolaryngology–Head and Neck Surgery, Mount Sinai Hospital, Toronto, Canada
5Department of Biomedical Engineering, Johns Hopkins University, Baltimore, MD
Accepted 17 June 2009
Published online 19 August 2009 in Wiley InterScience (www.interscience.wiley.com). DOI: 10.1002/hed.21219
in demonstrating anterior skull base defects (ASBDs), differing
in size and location, was investigated. The study was designed
to describe the potential advantage of CBCT in the setting of
an intraoperative cerebrospinal fluid (CSF) leak.
Methods. In all, 120 ASBD were evaluated in 5 cadaver
heads. Orthogonal and oblique slices were reconstructed. Ob-
server studies assessed the visibility of ASBD in each location
as a function of defect size.
Results. For 1-, 2-, and 4-mm defects, the percentage that
were undetectable ranged from 20% to 33%, 0% to 14%, and
0% to 5%, respectively. Confident breach detection increased
with defect size and was most challenging in the lateral
lamella and cribriform. CBCT permitted confident detection of
ASBD as small as about 2 mm in the fovea ethmoidalis and
planum. Oblique views were found to be superior to orthogo-
Background. The role of cone-beam CT (CBCT)
Conclusions. The ability to identify ASBD depended on the
size and location of defect. Oblique viewing planes were opti-
mal for ASBD visualization.
Head Neck 32: 504–512, 2010
C 2009 Wiley Periodicals, Inc.
intraoperative; imaging; cerebrospinal fluid (CSF)
cone-beam CT; sinus; skullbase;surgery;
Cerebrospinal fluid (CSF) rhinorrhea results
from a breach in the anterior skull base, leading
to a direct communication between the subarach-
noid space and either the sinus or nasal cavity.
mately 90% of cases are a result of trauma,
either an accident or a postsurgical (iatrogenic)
CSF rhinorrhea resulting from an iatrogenic
surgical complication is an increasing concern
as a result of the increasing popularity of endo-
scopic sinus surgery and closed neurosurgical
procedures. The size and anatomic location of
Correspondence to: G. Bachar
*The first 2 authors contributed equally to this work.
C 2009 Wiley Periodicals, Inc.
504 Intraoperative Cone-Beam CTHEAD & NECK—DOI 10.1002/hed April 2010
an anterior skull base defect are multifactoral,
depending on the type of procedure, extent and
location of disease, surgical instruments used,
and surgical expertise.
Skull base anatomy presents a complicated
architecture predisposed to iatrogenic CSF leaks
that can be difficult to detect intraoperatively.3
CSF rhinorrhea may be associated with both sig-
nificant morbidity and mortality. Meningitis is
the most common complication, with a reported
incidence of 19% in patients with persistent CSF
rhinorrhea.4Other complications include ortho-
static (upright) headache, intracranial infections,
pneumocephalus, and bleeding.3,4
CSF leaks that are diagnosed intraoperatively
are treated immediately. In cases in which the
leak is diagnosed a few days after surgery, a con-
servative approach is generally adopted.5Patients
with CSF leaks present a diagnostic challenge to
the surgeon. Various preoperative imaging modal-
ities are available for the diagnosis and identifica-
tion of a CSF leak. High-resolution CT scans, T2-
weighted MRI, MR cisternography, and intrathe-
cal fluorescein have all been described.5–8How-
ever, both radionuclide
contrast-enhanced CT cisternography have been
the mainstay of evaluation of a skull base defect.
The clear disadvantage of the cystenography
studies is the need for intrathecal injection. These
are invasive, time consuming, and include poten-
tial side effects of headache, nausea, vomiting,
seizures, and intracerebral hemorrhage.9
ability to identify a CSF leak during the surgical
procedure has a significant advantage. It enables
the surgeon to immediately seal the skull base
defect and to minimize the risk of further trauma
and postoperative complications.
To overcome the limitations associated with
guidance based on preoperative image data, we
described a technology for 3-dimensional (3D)
cone-beam CT (CBCT) imaging using a mobile C-
arm modified to include a high-performance flat-
panel x-ray detector.10–12The method operates
similar to CT, except CBCT captures an entire
volume image in a single half-rotation (?180?)
about the patient at lower radiation dose. Its
application in head and neck surgery has been
reported in a variety of studies,10–12and a system
for CBCT-guided head and neck surgery has
entered research trials at our institution. In the
present study, we investigated the role of CBCT
in demonstrating anterior skull base defects
(ASBDs) varying in size and anatomic location.
The study was designed to evaluate the perform-
ance of CBCT for intraoperative detection of an-
terior skull base CSF leak.
MATERIALS AND METHODS
C-Arm for Cone-Beam CT Imaging.
mobile isocentric C-arm (Siemens PowerMobil)
was developed in collaboration with Siemens
Healthcare (Special Products Division, Erlangen,
Germany) for intraoperative CBCT. As illustrated
in Figure 1A, the C-arm was modified to include
FIGURE 1. (A) Experimental setup showing the C-arm and cadaveric head. (B) Endoscopic photograph illustrating skull base defects
after total ethmoidectomy, medial maxillectomy, sphenoidotomy, and excision of the pituitary gland. Eight locations were systematically
breached: (1 and 2) left and right cribriform plate; (3 and 4) left and right lateral lamella; (5 and 6) left and right fovea ethmoidalis; and
(7 and 8) left and right planum, respectively. The figure shows a specimen with 2-mm-diameter defects introduced at each location.
[Color figure can be viewed in the online issue, which is available at www.interscience.wiley.com.]
Intraoperative Cone-Beam CTHEAD & NECK—DOI 10.1002/hedApril 2010505
a flat-panel detector (FPD; PaxScan 4030CB,
Varian, Palo Alto, CA), a motorized orbital drive,
geometric calibration, and a computer control sys-
tem for 3D reconstruction. CBCT acquisition
involved collection of projections across a rota-
tional arc of about 178?, nominally 200 projections
acquired in about 60 s (?3.3 fps). The 3D field of
view (FOV, ?20 cm ? 20 cm ? 15 cm) is sufficient
to encompass the skull base.13–15The dose associ-
ated with CBCT imaging on the C-arm has been
previously described10: approximately 2.9 mGy
(0.10 mSv) and 9.6 mGy (0.35 mSv) were shown
sufficient for visualization of bone and soft tis-
sues, respectively. Such dose levels represent a
fraction (?1/10–1/3) that of diagnostic CT.
Specimen Preparation and CBCT Imaging.
cadaver heads were acquired from the Division of
Anatomy, University of Toronto, in accord with
the Anatomy Act of Ontario. All heads were ini-
tially scanned on a 16-slice diagnostic CT scanner
(Discovery PET-CT; General Electric, Milwaukee,
WI) to review any anatomic variations and
excluded any anatomic abnormality or disease.
During the head preparation, each head under-
went a complete surgical clearance of the eth-
moid, maxillary, and sphenoidal sinuses. Air
space along the skull base (attributed to postfixa-
tion brain shrinkage) was filled with water-equiv-
alent gel via craniotomy. Subsequently, the
cranial bone flap was reopposed and secured.
After preparation, a baseline CBCT scan was
taken. Each head was immobilized in a carbon-
fiber frame using 6 to 8 cranial pins, and was
supported at the C-arm isocenter for each scan.
Eight locations on the anterior skull base were
selected for introduction of skull base breach as
illustrated in Figure 1B: the (left and right) cri-
briform plate, fovea ethmoidalis, lateral lamella,
and planum. After the baseline scan, an experi-
enced skull base surgeon used a 1-mm-diameter
drill to introduce skull base defects into each
site, and a second CBCT scan was acquired.
Then each of the 1-mm defects was widened
using a 2-mm drill (2-mm defects are shown in
Figure 2), followed by a third CBCT scan.
Finally, all sites were drilled to a 4-mm diame-
ter, and a final CBCT scan was acquired.
Visualization of Skull Base Defects: Observer Study
and Data Analysis.
The images were viewed in 3D
visualization software (3D Slicer v3.2; Brigham &
Women’s Hospital, Boston, MA, and Massachu-
setts Institute of Technology, Cambridge, MA).
The 3D coordinates of each defect were iden-
tified by an independent observer. For smaller
defects that were difficult to identify (eg, 1 mm),
the coordinates of the large defects (4 mm) were
translated to the earlier scans, using anatomic
landmarks to account for possible displacement
of the specimen between scans. In all, 120
defects (5 heads ? 8 locations ? 3 diameters)
were thus localized.
For the observer studies described in the fol-
lowing text, image slices through each defect
were extracted from CBCT volume reconstruc-
tions using the ‘‘3D Slicer’’ software. These
included ‘‘orthogonal’’ slices that correspond to
conventional triplanar views (coronal, sagittal,
and axial) as well as ‘‘oblique’’ slices. The
oblique slices included both a coronal and a lon-
gitudinal slice (ie, a quasi-sagittal slice in the
plane of the drill) and a tangential slice (ie, a
quasi-axial slice in the plane of the defect).
For each defect, an area of interest was
cropped from the image such that the defect
was presented in the center of the image. Slice
triples (either orthogonal or oblique) were dis-
played in comparison with the corresponding
‘‘baseline’’ area of interest (ie, the identical area
of skull base before introduction of a defect).
Example images are shown in Figures 2 and 3
for each anatomic location.
Observer studies were conducted to assess
visibility of skull base defects in each location as
a function of defect size. Five expert observers
participated, including 1 radiologist, 2 skull
base surgeons, and 2 head and neck surgeons.
The study was conducted under controlled view-
ing conditions: a darkened reading room on a
diagnostic workstation (Dell Precision 380, 3-
GHz Pentium 4 with dual-head Barco displays,
1536 ? 2048 resolution, 8-bit grayscale; Dell
Inc., Round Rock, TX). A fixed viewing distance
of 50 cm was recommended but not enforced.
Observers received identical instructions and
training. Reading order across 120 cases was in-
dependently randomized for each observer. For
each case, observers were shown (1) the full
FOV (nominal resolution) coronal and sagittal
images for purposes of orientation; (2) the base-
line images (orthogonal or oblique slices with no
defect); and (3) the defect images (orthogonal or
oblique slices with a defect). Observers were
then asked to rate their satisfaction in detect-
ing, localizing, and characterizing the defect on
a scale of 1 to 5 (1 ¼ Unable to identify any
breach, 2 ¼ The breach could be overlooked, 3 ¼
506 Intraoperative Cone-Beam CTHEAD & NECK—DOI 10.1002/hed April 2010
The breach is visible, but detection and charac-
terization of subtle features are a bit challeng-
ing, 4 ¼ The Breach is visible, 5 ¼ The breach
is perfectly obvious).
The results were analyzed using both a
mixed-regression model and trend testing. Mean
values (and SD) in observer scores were ana-
lyzed as a function of anatomic location (4 sites)
and defect size (3 diameters). Results were also
analyzed for orthogonal views compared with
oblique views to see whether the latter more
clearly demonstrated defects. A paired t test
(Wilcoxon) was used to test statistical signifi-
cance in observer scores between orthogonal and
Of the 120 defects (5 heads ? 8 locations ? 3
diameters), 102 were deemed successful ([5 ? 2
? 3] ¼ 30 in the planum, 30 in the fovea eth-
moidalis, 29 in the lateral lamella, and 13 in the
cribriform). The shortfall of cases in the cribri-
form was attributed to difficulty in drilling large
(4-mm) defects, which was technically challeng-
ing because of the thickness of the skull base at
this point, resulting in a much bigger defect
being created than desired and thus leading to
rejection of nearly 17 cases from the analysis.
The percentage of defects that were judged
undetectable (a score of 1) depended on the loca-
tion and size of the defect. For the 1-mm defects,
from 20% to 33% (specifically, 30% [cribriform],
33% [lateral lamella], 20% [fovea ethmoidalis],
and 21% [planum]). For the 2-mm defects, the
percentage judged undetectable was consider-
ably lower, 0% to 14% (12% [cribriform], 14% [lat-
eral lamella], 0% [fovea ethmoidalis], and 1%
FIGURE 2. Illustration of variable defect size (baseline, 1-, 2-, and 4-mm-diameter breach). Examples correspond to defects in the pla-
num demonstrated in orthogonal (coronal, sagittal, and axial) views. Cone-beam computed tomographic (CBCT) images. Throughout
the following figures, the full field-of-view (FOV) images (left) at ‘‘nominal’’ resolution (0.8-mm voxels) illustrate the surrounding ana-
tomic context. The zoomed images (right) at ‘‘high’’ resolution (0.4-mm voxels) illustrate the region of the breach. In each case, the
breach is near the center of the image, and its location is demarked by gray triangles at the image boundaries.
Intraoperative Cone-Beam CT HEAD & NECK—DOI 10.1002/hedApril 2010507
[planum]). The 4-mm defects were almost always
detectable, with 0% to 5% rated undetectable (5%
[cribriform], 2% [lateral lamella], 1% [fovea eth-
moidalis], and 0% [planum]). Besides demon-
strating more confident detection for larger
defects, theseresults indicatethat breach
FIGURE 3. Illustration of skull base breach in regions of the cribriform plate (A), fovea ethmoidalis (B), lateral lamella (C), and planum
(D). Each example shows baseline images in comparison to images of a 2-mm defect in oblique views.
508 Intraoperative Cone-Beam CT HEAD & NECK—DOI 10.1002/hed April 2010
detection was most challenging in the lateral
lamella and cribriform and most conspicuous in
the fovea ethmoidalis and planum.
Similarly, the results indicate the degree to
which CBCT enables confident detection of skull
base breach (score of ?4 in observer rating). For
the 1-mmdefects, confident
achieved in 3% to 14% of cases (specifically, 3%
[cribriform], 7% [lateral lamella], 7% [fovea eth-
moidalis], and 14% [planum]). For the 2-mm
defects, confident detection was achieved in 44%
to 69% of cases (specifically, 45% [cribriform],
44% [lateral lamella], 69% [fovea ethmoidalis],
and 67% [planum]). Finally, for the 4-mm
defects, confident detection was achieved in 47%
to 91% of cases (specifically, 47% [cribriform],
71% [lateral lamella], 87% [fovea ethmoidalis],
and 91% [planum]).
A mixed-regression model allowed analysis of
statistical significance in the differences arising
from defect size, in a manner that adjusts for
the dependence on anatomic site, showing the
observer scores for the 3 defect sizes (1, 2, and
4 mm) to be significantly different (p < .0001).
As shown in Figure
increased with defect size: average score ¼ 2.51
for 1 mm, 3.82 for 2 mm, and 4.45 for 4 mm.
The regression model also tested the linear de-
4, the averagescore
pendence in scores from 1 mm to 4 mm, suggest-
ing a trend that is significantly greater than 0.
Specifically, observer score increased by 0.60 for
every 1-mm increase in defect size (p < .0001).
After considering various anatomic sites, the
slopes of regression were found to be 0.68 (pla-
num), 0.64 (lateral lamella), 0.65 (fovea ethmoi-
dalis),and 0.30 (cribriform).
consistent with the previously noted point that
breaches were more difficult to detect in the cri-
briform. Interestingly, the intercept points for
the 4 sites plotted in Figure 4 were not signifi-
cantly different (p ¼ .14).
There was a significant difference in the
scores associated with orthogonal versus oblique
views, as shown in Figure 5. The intercept
implied by the mixed regression analysis was
significantly different (p ¼ .02) for the 2 types of
views, with a lower intercept for orthogonal
views (1.75) than that for oblique views (2.06).
Interestingly, the slopes in the observer scores
(ie, the change in score per unit size of the
breach) of the 2 types of views were not signifi-
cantly different (p ¼ .26). A paired t test
between scores in orthogonal and oblique views
suggests a significant difference in the planum,
combining defect sizes (p < .001) and also
when all 4 sites were combined (p < .001) (see
FIGURE 4. Plots summarizing the visibility of skull base breach (observer score 1–5) as a function of the defect size (1–4 mm). Mixed
linear regression is consistent with the finding that visibility of breach in the cribriform was significantly more challenging than that in
the fovea ethmoidalis, lateral lamella, and planum.
Intraoperative Cone-Beam CTHEAD & NECK—DOI 10.1002/hedApril 2010509
Figure 5). However, the cribriform plate, lateral
lamella, and fovea ethmoidalis, individually, did
not show a significant difference between or-
thogonal and oblique views. In comparing differ-
ent anatomic sites, there was a significant
difference for 1-mm defects (p < .001) and 2-mm
defects (p < .002), but not 4-mm defects (p ¼ .4).
This study is the first to investigate the role of
CBCT imaging for visualization of ASBD. The
cadaver model provided a somewhat idealized,
but reasonably robust, tool to evaluate CBCT in
the detection of known breaches of variable size
and within distinct anatomic locations of the
anterior skull base. The advantage of the cur-
rent study is that cadaveric specimens allowed a
strict protocol, enabling the creation of bony
defects of specific size in predetermined loca-
tions. We were therefore able to evaluate the
identification rate/sensitivity of each site sepa-
rately and assess how easily individual defects
could be detected.
To date, there is no universally accepted mo-
dality or algorithm to evaluate anterior skull
base defects. Various authors suggest different
imaging modalities to evaluate iatrogenic ante-
rior skull base defects,6,8,16but the reported
sensitivity and specificity of such tests are
inconsistent, probably the result of different
imaging modalities and small numbers of cases.
In addition, the visibility of a defect depends on
its location and size, an uncontrolled variable in
previous studies. The results in this report con-
firm this point: a 2-mm breach in the cribriform
plate was significantly more challenging to iden-
tify than was a breach of identical size in the
planum. Shetty et al7found that high-resolution
CT was accurate in 93% of patients, whereas
MR cisternography was accurate in 89% of
patients with clinically suspected CSF rhinor-
rhea. The combination of high-resolution CT
and MR cisternography was accurate in 96% of
patients. In contrast, Eljamel et al17demon-
strated the use of high-resolution CT with an
identification rate of only 47%.
For small (1-mm) skull base defects, 20% to
33% of cases did not demonstrate visualization
of the defect in CBCT, despite observers know-
ing of its existence in the image. This is in con-
trast to just 2% of cases for which a 4-mm
defect could not be identified. Of the 4 anatomic
FIGURE 5. Plots summarizing the visibility of skull base breach (observer score 1–5) for orthogonal views (light gray bars) and oblique
views (black bars). For individual anatomic sites, only the planum (D) illustrated a statistically significant difference between orthogonal
and oblique views, whereas pooling over all sites and defect sizes demonstrated an overall improvement (p < .001) for oblique views.
510 Intraoperative Cone-Beam CTHEAD & NECK—DOI 10.1002/hed April 2010
sites, the planum and fovea ethmoidalis demon-
strated clearer visibility of defects of any size.
These and the lateral lamella exhibited similar
‘‘slopes’’ in detectability versus size of breach
(from 1 mm to 4 mm), suggesting an improve-
ment at larger defect identification (2 mm then
4 mm). The detection rate within the cribriform
plate was significantly lower, suggesting that
even larger defects (eg, 2 and 4 mm) in the cri-
briform were not as easy to identify as in other
anatomic sites. Interestingly, the ‘‘y-intercept’’
on plots of detectability versus size of breach
were not significantly different among the 4
anatomic sites, implying that a small breach
(<1 mm) would become unidentifiable irrespec-
tive of anatomic site.
Overall, CBCT provided confident detection
(score of ?4) in 44% to 69% of 2-mm defects and
47% to 91% of 4-mm defects, across all anatomic
sites tested. We acknowledge that the identifica-
tion rate may be falsely elevated by the prede-
fect skull preparation (including ethmoidectomy
and mucosa stripping). However, to create a
comparable model, between individual skulls,
this preparation was necessary.
In a comparison of conventional orthogonal
triplanar views (coronal, axial, and sagittal
planes) with oblique views (coronal, longitudi-
nal, and tangential planes through the defect),
the results confirm that breaches were better
demonstrated in the latter. Therefore, if 3D vis-
ualization software permits convenient selection
of oblique planes to more accurately contain the
true plane of the breach, the identification rate
CSF leaks are typically intermittent, and
some imaging (cisternography) requires active
leakage during the examination to improve sen-
sitivity. Moreover, cisternography also requires
an intrathecal injection and a transfer to the CT
scanner, before the operating room. The use of
CBCT overcomes these obstacles and would be
ideal in sinus/skull base surgery if there were a
risk of skull base breach. It has the potential to
demonstrate skull base defects intraoperatively,
and does not depend on the presence of CSF
flow during the examination. It can be com-
pleted in the operating theater without the need
of injection or patient transfer. Moreover, the
CBCT provides a noninvasive, near real-time
images at 1/10 of the time and radiation dose
delivered by conventional CT. Therefore, in
cases in which suspicion of a skull base breach
exists, the defect can potentially be localized
and treated during the primary procedure using
CBCT. This would minimize the need for a sec-
ond procedure, reduce patient stress and dis-
comfort, and decrease both operating room costs
and time. In addition, practice that aims to
repair a CSF leak as a second procedure con-
tends with a previously operated field with asso-
ciated scar tissue and fibrosis. The ability to
identify and repair a CSF leak during the pri-
mary surgery can eliminate such problems. In
any case, we are not suggesting that CBCT will
replace the need for good preoperative imaging.
This study has a variety of limitations. First,
the study was performed on cadaver heads and
not on living patients with an actual CSF leak.
Second, the observers were asked to identify the
skull base breach while reviewing a narrow part
of the image, unable to scroll through adjacent
slices, and the observers were informed that the
breach did, in fact, exist in the image. This is
in contrast to the clinical setting in which the
surgeon or radiologist is required to search an
entire volume to locate the bony defect from
multiple images collected during the CT scan. In
addition, the test (satisfaction rating scale) does
not allow quantitation of sensitivity or specific-
ity of breach detection.
In summary, a cadaver model was used to
test intraoperative CBCT detection of ASBD,
investigating detectability thresholds as a func-
tion of defect size and location, each found to
affect detectability. In addition, optimal viewing
planes (oblique) were identified. The utility of
this experimental model was demonstrated, and
implementation of the system in a clinical intra-
operative environment is under way.
thanks to the Guided Therapeutics Program
(GTx) at the University Health Network (Toronto,
ON) and to Dr. Eugene Yu (University Health
Network, Toronto, ON) for expert assistance with
the observer studies. The work was supported by
the Princess Margaret Hospital Foundation and a
grant from the National Institutes of Health
The authors extend their
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