Automatic segmentation of intracochlear anatomy in conventional CT

Abstract

Cochlear implant surgery is a procedure performed to treat profound hearing loss. Clinical results suggest that implanting the electrode in the scala tympani, one of the two principal cavities inside the cochlea, may result in better hearing restoration. Segmentation of intracochlear cavities could thus aid the surgeon to choose the point of entry and angle of approach that maximize the likelihood of successful implant insertion, which may lead to more substantial hearing restoration. However, because the membrane that separates the intracochlear cavities is too thin to be seen in conventional in vivo imaging, traditional segmentation techniques are inadequate. In this paper, we circumvent this problem by creating an active shape model with micro CT (μCT) scans of the cochlea acquired ex vivo. We then use this model to segment conventional CT scans. The model is fitted to the partial information available in the conventional scans and used to estimate the position of structures not visible in these images. Quantitative evaluation of our method, made possible by the set of μCTs, results in Dice similarity coefficients averaging 0.75. Mean and maximum surface errors average 0.21 and 0.80 mm.

Figure 1

Superior-to-inferior view (left) and lateral-to-medial view (right) of ear anatomy. Shown are the cochlear labyrinth (yellow), facial nerve (purple), chorda tympani (green), auditory canal (blue-green), scala tympani (red), scala vestibuli (blue), and the traditional surgical approach (orange tube).

Figure 2

CT (top) and μCT (bottom) of a cochlea specimen. Delineated in the right panels are the scala tympani (red) and scala vestibuli (blue).

Figure 3

Constructing a point distribution model from a set of surfaces.

Figure 4

Performing segmentation with the active shape model.

Figure 5

First (top row) and second (bottom row) modes of variation of the scala tympani (red) and vestibuli (blue) in the point distribution model. On the left are (from left to right) −2, 0, and +2 standard deviations from the mean in Posterior-to-Anterior view. The same modes are shown on the right in Medial-to-Lateral view.

Figure 6

Quantitative segmentation results. Shown are the distributions of the DSC (left), mean surface distance in mm (middle), and max surface distance in mm (right) for the results of the active shape model (A.S.) and registration alone (Reg.). Arrows indicate the results for the experiment shown in Figure 7.

Figure 7

Contours of representative segmentation results. Automatic segmentation results for the scala tympani (red) and scala vestibuli (blue) are shown overlaid with the conventional CT (top row), and registered μCT (middle and bottom rows), and are compared to manually delineated contours of the scala tympani (light blue) and scala vestibuli (green).

Figure 8

Segmentations color encoded with error in mm for the experiments 1–5 (Up to Down). (Left to Right) Active shape model segmentation of the scala tympani, scala vestibuli, atlas-based segmentation of the scala tympani, scala vestibuli.