Orientation of human semicircular canals measured by three-dimensional multiplanar CT reconstruction

Abstract

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 degrees). Multiple studies of isolated labyrinths have shown the inter-SCC angles are close to 90 degrees. 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 microm. 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 degrees (mean +/- SD). The angle between the anterior and horizontal SCC was 90.6 +/- 6.2 degrees. The angle between the horizontal and posterior SCC was 90.4 +/- 4.9 degrees. 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 degrees, 92.2 +/- 5.7 degrees, and 19.9 +/- 7.0 degrees, 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 degrees, 38.4 +/- 5.1 degrees, and 100.1 +/- 6.2 degrees, 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 degrees, 131.5 +/- 5.1 degrees, and 105.6 +/- 6.6 degrees, respectively. The mean anterior SCC-contralateral posterior SCC angle was 15.3 +/- 7.2 degrees. The absolute orientations of human SCC are more nearly orthogonal than previously reported.

Fig. 1

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.

Fig. 2

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.

Fig. 3

Measuring orientations of Reid's planes. (A–C) Mutually orthogonal slices through the head CT of one subject, with relative orientations, as illustrated in the color-coded three-dimensional rendering in panels D–F. In all images, red = Reid's horizontal plane Z_REID; green = Reid's coronal plane X_REID; dotted blue = Reid's midsagittal plane Y_REID. Solid blue = a plane parallel to Y_REID and passing through the left external auditory canal entrance center (LEAC, arrowheads in ABC) used as the starting point for measuring Reid's planes. All three planes are shown in each image, but two planes appear as lines in each slice image (ABC), because they are viewed on edge. A/P/R/L/S/I = anterior/posterior/right/left/superior/inferior. (A) Reid's horizontal plane (Z_REID), with X_REID plane viewed “on edge” overlying the interaural axis (which is also the Y_REID axis). (B) X_REID coronal plane as viewed from anterior; here, the Z_REID plane viewed “on edge” overlies the interaural axis. (C) A parasagittal slice through the LEAC center point parallel to Reid's midsagittal plane, viewed from the left. (Y_REID itself is later defined to lie midway between the center points of the left and right EAC entrances, perpendicular to the interaural axis connecting these points.) (D–F) Three-dimensional renderings showing relative orientation of planes within skull.

Fig. 4

(A) Unit length vectors normal to semicircular canals of left (blue, L) and right (red, R) horizontal (H), anterior (A), and posterior (P) semicircular canals of 22 human subjects, shown as viewed from each cardinal axis (A, B, C views from −X_REID, +Y_REID, +Z_REID axes, respectively) and left-high-posterior view (D), after vectors were rotated so as to align Reid's planes for all subjects. +X = anterior along +X_REID, perpendicular to the Reid coronal plane. +Y = left along +Y_REID, the interaural axis perpendicular to the Reid sagittal plane. +Z = superior along +Z_REID, perpendicular to Reid horizontal plane. (B) Mean normal vectors, same views as in A. Radius of each circle = geometric mean of standard deviations of direction angles between the mean vector and each of the three positive axes (+X_REID, +Y_REID, +Z_REID).

Fig. 5

Orientations of mean planes for the horizontal (red), posterior (blue), and anterior (green) SCC of the left labyrinth, with respect to Reid's cardinal axes (+X_REID, +Y_REID, +Z_REID). (A–D) Views from the +X_REID (anterior), +Y_REID (left), +Z_REID (superior), and −X_REID (posterior) axes, respectively. (E, F) High/left/anterior and high/left/posterior views, respectively. To simplify visual interpretation, the intersection of the SCC planes shown was translated to the intersection of the interaural axis and midsagittal plane [the center of the Reid coordinate system (X_REID, Y_REID, Z_REID) = (0, 0, 0)].

Fig. 6

Inter-SCC angles for left (red), right (blue), and pooled (black) canal pairs. Box plots show median (waist), estimated range of median (notch), first and third quartiles (box ends), expected data range (whiskers) and outliers (+). All are close to 90°, and there is no significant difference between sides.