Use of computed tomography scans for cochlear implants

Abstract

While 3-dimensional (3D) imaging by computed tomography has long been desirable for research and treatment of cochlear-implant patients, technical challenges have limited its wide application. Recent developments in scanner hardware and image processing techniques now allow image quality improvements that make clinical applications feasible. Validation experiments were performed to characterize a new methodology and its imaging performance.

Fig. 1

Images from a clinical scanner of an electrode array inserted into a cochlear canal within an excised temporal bone specimen. Left panel is with a low-contrast window, demonstrating resolution of 11 individual electrodes along the array that lie in the plane of this section. Right panel has a higher-contrast window appropriate for bone/soft-tissue viewing, showing distortions in the canal wall in the vicinity of the electrode array.

Fig. 2

Left: micro-CT scan of specimen from Figure 1. Note higher spatial resolution, including the resolution of lead wires between individual electrodes. Right: Hounsfield scale profile of an electrode (approximate coordinates 240,200) in Figure 1, from clinical scanner and micro-CT device. The spatial resolution is related to the “rise time” of the edges, here being about 100 μm for the micro-CT and about 500 μm for the clinical CT.

Fig. 3

Images from clinical CT (left), micro-CT (middle), and OPFOS (right) for the same three orthogonal sections cut through a common donor cochlea. Soft tissue features are clearly rendered in OPFOS and well correlated with micro-CT landmarks. While not visible in the clinical CT (100-μm voxels), the location of these landmarks can be inferred in the clinical CT (36-μm voxels) with reference to the OPFOS (6-μm voxels) atlas.

Fig. 4

Rendered view of coregistered micro-CT and clinical CT scans of an implanted electrode array. Upper right, the rendered electrodes from the micro-CT scans (blue) are centered within the rendered electrodes from the clinical CT scanner (red). As shown in the three orthogonal sectional views, the micro-CT (blue) voxels lie within the thresholded array objects (red outline), which is symmetrically centered in the bloomed array volume from clinical CT.

Fig. 5

Analysis of registered cochlear wall shells. Left, plot of two registered wall outlines. A chamfer length, the distance to the closest neighboring wall point in the registered volume for each point on the wall, is computed. For this slice (corresponding to number 180 in right plot), the mean chamfer distance was 0.035 mm (SD = 0.06 mm), with a maximum individual distance of 0.2 mm. Right, plot of the average chamfer distance for each slice in the registered volume. The mean chamfer distance for the total volume was 0.042 mm. (In the first 50 slices, portions of the shell lay outside one of the segmented volumes, resulting in inflated estimates of closet distance.)