Measurement of Semicircular Canal Geometry from High-Resolution MRI
Petra SCHMALBROCK1, Robert HOLTMAN2, Antonio ALGAZE3
1The Ohio State University, Department of Radiology, 1630 Upham Drive, Columbus, Ohio United States; 2The Ohio State University,
Biomedical Engineering, Columbus, Ohio United Stated; 3The Ohio State University, Biomedical Engineering, Columbus, Ohio United States;
or loss along the canal and excessive torsion. In all cases, the accuracy
of the final path was evaluated by visual inspection and repeated
analysis. Curvature radius and angles between the canals could be
determined in all cases. A variable amount of torsion was observed in
most canals.
Measurements of curvature radius and statistical errors for the radius
(5% or about 1 pixel) were comparable to previous work [2-4]. Using
multi-factor ANOVA analysis we found statistically significant
differences in canal size between the three canals (P<0.0005) and
between men and women (P=0.006). Unlike in previous work, we did
not see a statistically significant difference in size between the SSC
and PSC. However, both SSC and PSC were larger then the LSC. In
agreement with earlier findings, men typically had larger canals than
women (Fig2). There was no difference between right and left ears.
Dihedral angles measured between ipsilateral canals had statistical
errors of 1-4 degrees and were comparable to prior work [1]. T-test
analysis was used to assess the orthogonal arrangement of the canals.
The angle between the LSC and SSC (91.2±11.7deg) was not
significantly different from 90deg (P=0.68), whereas the angle
between the SSC and PSC (86.9±3.9deg) was statistically significantly
(P=0.003) smaller than 90deg, and the angle between the PSC and
LSC (93.1±4.9deg) was larger than 90 deg (P=0.012). No statistically
significant differences were found between the right and left ear or
between men and women.
Introduction
It is of interest to study the geometry of the semicircular canals
because their geometric arrangement, size and orientation in the head
determine their function. Thus substantial research has been done to
measure curvature radius, out-of-plane bending (torsion) and both ipsiand contralateral angles between canal planes [1-5]. Previous
measurements used surgical exposure, rubber casts or 3D histology
and all were destructive. We present here a method to obtain similar
measurements of semicircular canal geometry based on high resolution
MRI.
Methods
Axial T2 weighted images of 9 healthy subjects (4 female, 5 male)
were acquired using a segment-interleaved motion compensated
acquisition in the steady state (SIMCAST) [6] with a final cubic voxel
resolution of 0.18mm. Geometric measurements were based on a set of
centroid points through cross sections of each semicircular canal,
which were determined by a semiautomatic search algorithm. The
algorithm implemented in IDL makes use of the circular shape of the
canals. Using a viewer tool that depicts appropriate regions of the inner
ear in all three principal planes, the user has to select three points P1,
P2 and P3 at both ends and in the most distal portion of each canal.
The algorithm then fits a circle equation through these three points and
computes equidistant points Qi along the circle. These points are used
to guide the search algorithm. Guidance is needed because of the
complex geometry and interconnectedness of the labyrinth (Fig.1).
2D-region growing in all three planes through all Qi is used and the
plane with the smallest area is assumed to be the cross section.
Centroids of all cross sectional planes are computed and stored in a
data array. For geometric assessment, a circle is fit through all centroid
points to determine the canal curvature radius. Next, a plane through
all points is determined and the distance of each point from the plane is
computed. The largest distance from the plane is a measure for canal
torsion. Finally, dihedral angles between all canal planes were
computed.
Fig.2: Canal Curvature Radius
Discussion
Our study shows that semicircular canal geometry can be measured
with high-resolution MRI with accuracy comparable to prior work that
used destructive methods. This may make MRI a suitable tool for
study of balance disorders. For example, benign paroxysmal positional
vertigo (BPPV) is thought to be caused by particles floating in the
canals and is treated by head rotation particle repositioning maneuvers
[7]. Patient specific information regarding the canal orientation may
improve the outcome. Further, an abnormal location of the posterior
semicircular canal has been observed in Meniere's patients [8], but to
the best of our knowledge no systematic evaluation of the orientation
of all canals has been done, because of the invasive nature of prior
measurement methods.
References
1. Blanks RHI et al, Acta Otolaryngol 80: 1975; 185-196
2. Curthoys IS et al, J. Morphol. 151: 1977; 1
3. Muren C et al, Acta Radiol Diagnosis 27: 1986; 2
4. Sato H et al, Acta Otolaryngol 1993: 113; 171-175
5. Takagi A et al, Acta Otolaryngol 1989; 107: 362-365
6. Kurucay S et al, JMRI, 1997:7: 1060-1068
7. Buckingham RA, Laryngoscope 1999: 109:717-722
8. Schmalbrock P et al, AJNR 1996: 17:1707-1716
Fig 1: Volume Display from MRI
Results
Since semicircular canal geometry measurements are based on
centroid paths, their precision is based on the accuracy of the path and
the success of the algorithm finding it. In a majority of cases the
semiautomatic search algorithm completed successfully. Reasons for
occasional failure of the search algorithm included, inappropriate
automatic selection of the search algorithm threshold, signal variability
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