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Title of Journal: JARO

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Abbravation: Journal of the Association for Research in Otolaryngology

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Orientation of Human Semicircular Canals Measured by Three-Dimensional Multiplanar CT Reconstruction

Authors: Charles C. Della Santina, Valeria Potyagaylo, Americo A. Migliaccio, Lloyd B. Minor, John P. Carey,

Publish Date: 2005/08/09
Volume: 6, Issue:3, Pages: 191-206
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Analysis of vestibulo-ocular reflex experiments requires knowledge of the absolute orientations (with respect to skull landmarks) of semicircular canals (SCC). Data relating SCC orientations to accessible skull landmarks in humans are sparse, apart from a classic study of 10 skulls, which concluded that the horizontal and anterior SCC are not mutually orthogonal (111 ± 7.6°). Multiple studies of isolated labyrinths have shown the inter-SCC angles are close to 90°. We hypothesized that a larger sample would yield mean absolute SCC orientations closer to the mutual orthogonality demonstrated for isolated labyrinths. We measured canal orientations with respect to accessible skull landmarks using 3-D multiplanar reconstructions of computerized tomography scans of the temporal bones of 22 human subjects. Images were acquired with 0.5-mm thickness and reconstructed with in-plane resolution of 234 μm. There was no significant difference between the left and a mirror image of the right (p > 0.57 on multiway ANOVA of orientation vector coefficients), so data were pooled for the 44 labyrinths. The angle between the anterior and posterior SCC was 94.0 ± 4.0° (mean ± SD). The angle between the anterior and horizontal SCC was 90.6 ± 6.2°. The angle between the horizontal and posterior SCC was 90.4 ± 4.9°. The direction angles between a vector normal to the left horizontal SCC and the positive Reid's stereotaxic X (+nasal), Y (+left), and Z (+superior) axes were 108.7 ± 7.5°, 92.2 ± 5.7°, and 19.9 ± 7.0°, respectively. The angles between a vector normal to the left anterior SCC and the positive Reid's stereotaxic X, Y, and Z axes were 125.9 ± 5.2°, 38.4 ± 5.1°, and 100.1 ± 6.2°, respectively. The angles between a vector normal to the left posterior SCC and the positive Reid's stereotaxic X, Y, and Z axes were 133.6 ± 5.3°, 131.5 ± 5.1°, and 105.6 ± 6.6°, respectively. The mean anterior SCC–contralateral posterior SCC angle was 15.3 ± 7.2°. The absolute orientations of human SCC are more nearly orthogonal than previously reported.The semicircular canals (SCC) of the vestibular labyrinth encode head rotational velocity and provide input to the vestibulo-ocular reflex (VOR), vestibulo-collic reflex, vestibulo-spinal system, vestibulo-reticular system, cerebellum, and cortex (Ewald 1892; Cohen et al. 1964; Suzuki and Cohen 1964; Wilson et al. 1995; Maeda et al. 1975; Shinoda et al. 1994, 1997; Peterson and Abzug 1975; Wilson et al. 1976; Ebata et al. 2004). Accurate knowledge of the absolute orientation (with respect to skull landmarks) and position of SCC is essential for design of experiments studying the vestibular system and for understanding exam findings during clinical evaluation of patients with vestibular disorders. Apart from a widely cited classic study of 10 human skulls by Blanks et al. (Blanks et al. 1975; Curthoys et al. 1977), few of the many studies of human labyrinth morphology have related SCC orientations to accessible skull landmarks that can guide an investigator in orienting a subject's head for canal-plane-specific vestibular stimulation. Blanks et al. reported that the angles between ipsilateral SCC of one labyrinth depart significantly from mutual orthogonality, with the angle between the anterior SCC and horizontal SCC equal to 111.8 ± 7.6° (mean ± SD). Multiple studies of the inter-SCC angles of isolated human labyrinths using high-resolution histologic or radiographic reconstructions suggest that SCC orientations are much more nearly orthogonal to one another (Archer et al. 1988; Harada et al. 1990; Hashimoto 2003; Ifediba et al. 2004; Takagi et al. 1989; Spoor and Zonneveld 1995, 1998). This disparity suggests that one or more of the absolute SCC orientations reported by Blanks et al. may have been up to ∼20° off of the true population mean, a deviation that would be significant for many experimental designs.We hypothesized that the departure from canal orthogonality observed by Blanks et al. does not represent the true population mean, and that a larger sample would yield mean canal orientations that are closer to mutual orthogonality. To test our hypothesis, we measured SCC orientations with respect to accessible skull landmarks using three-dimensional multiplanar reconstructions of high-resolution computed tomography (CT) scans of 44 labyrinths in 22 human subjects.Computed tomography scans were acquired for both ears of 7 male and 15 female subjects aged 48 ± 15 years. CT scans were performed for accepted clinical indications in all patients. The indication for CT imaging was headache with disequilibrium in nine subjects; motion sickness, fatigue, lightheadedness, or anxiety associated with dizziness in six subjects; evaluation of middle ear or mastoid inflammatory disease in three subjects; conductive hearing loss in two subjects; benign paroxysmal positional vertigo (BPPV) in one subject; and sensorineural hearing loss in one subject. Diagnosis after 1.5 years of follow-up was migraine in 10 subjects; middle ear or mastoid inflammatory disease in three subjects; otosclerosis in two subjects; depression or anxiety in two subjects; and obstructive sleep apnea, enlarged vestibular aqueduct, orthostatic hypotension, BPPV, and Eustachian tube dysfunction in one subject each. No subject had a history of temporal bone fracture. Except for the one subject whose CT scan revealed mildly enlarged vestibular aqueducts without abnormal SCC configuration or significant deviation in SCC orientation from the rest of the dataset, no patient had cochlear or labyrinthine dysmorphism (abnormal location, patency, caliber, shape, size, or orientation) as judged by the neuroradiologists reading the CT for purposes of routine clinical care and by two of the authors (C.C.D.S., J.P.C.). This study was a review of existing clinical data with patient identifiers removed. The Joint Committee on Clinical Investigation of the Johns Hopkins University School of Medicine determined that the study was exempt from a protocol requirement and that it qualified for exemption from an institutional review board protocol based on Department of Health and Human Services criteria 45 CFR 46.101(b)(4).The CT scans were acquired on a 16-detector Toshiba Aquilion CT scanner, using a bone algorithm. To avoid loss of in-plane resolution that can occur in helical scanning modes, scans were performed in step-scan mode. Each axial slice was scanned with the subject stationary, and the table was axially stepped 0.5 mm before each subsequent acquisition. The x-ray beam was generated with a 300 mA/100 kVP source and collimated to 0.5-mm slice thickness at full-width half-maximum. Each temporal bone was separately encompassed by 120-mm diameter region of interest using a 512 × 512 voxel matrix, creating voxels of 0.234 × 0.234 × 0.5 mm. Axial scans of the whole head were acquired using a 240-mm region of interest, yielding 0.468 × 0.468 × 0.5 mm voxels. The 3-D datasets were exported to a workstation for multiplanar analysis using Vitrea® software (Vital Images, Inc., Plymouth, MN) and Matlab (Mathworks, Cambridge, MA). The higher-resolution temporal bone reconstructions were used to measure distances and angles reported for the semicircular canals, and whole-head reconstructions were used to define Reid's planes with respect to skull landmarks (as defined below).The Vitrea® software system allows one to view a CT dataset simultaneously using a 3-D rendered image and a set of three 2-D images that represent mutually orthogonal “slices” through the dataset. We measured a SCC's orientation by optimally aligning one slice plane with the SCC, then extracting the coordinate transformation matrix Vitrea® used to reach that alignment from a space-fixed coordinate frame defined by the CT dataset volume's boundaries (Kuipers 1999). Measurements were made with 1-voxel resolution (∼0.22° angular, 0.234 mm in-plane translational).Measuring semicircular canal (SCC) orientations and centroid positions using 3D multiplanar reconstructions of left temporal bone CT of a 56-year-old woman. Within each column, images show three mutually orthogonal slices through the volume data in the orientations shown by the color-coded planes in the 3D rendering at bottom. All three planes are shown in each image, but two planes appear as lines in each slice image (ABC, EFG, IJK), because they are viewed on edge. In all images, red arrows = horizontal SCC (HSCC); green arrows = posterior SCC (PSCC); blue arrows = anterior SCC (ASCC); iac = internal auditory canal; sig = sigmoid sinus; va = vestibular aqueduct; mc = mandibular condyle; A/P/R/L/S/I = anterior/posterior/right/left/superior/inferior. In the first set (A–D), the red plane is aligned with the HSCC; the blue and green planes are arbitrary. In the second set (E–H), the green plane is aligned with the PSCC; the red and blue planes are arbitrary. In the third set (I–L), the blue plane is aligned with the ASCC; the red and green planes are arbitrary.Confirming optimal alignment of image plane with a semicircular canal plane. (A) Image plane tentatively aligned with canal lumen of the anterior SCC of Figure 1K. Stepping one (B) then two (C) voxel lengths (∼234 μm each) along the image plane normal causes uniform disappearance of canal lumen.



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