Planning a safe drilling path for cochlear implantation surgery using image registration techniques

The procedure currently used for cochlear implementation requires wide surgical exposure to identify anatomic landmarks. At our institution a minimally invasive technique is being developed that will permit to perform the procedure with a small burr hole. This technique does, however, require identifying pre-operatively a surgical path that reaches the cochlea without touching sensitive structures. This path can be found interactively by localizing a point in the facial recess and another point in the basal turn of the cochlea in the pre-operative CT images. Unfortunately, this is a difficult task because these structures are small and difficult to visualize. As an alternative, outlines of those two structures can be drawn first by a qualified surgeon in one image volume selected as an atlas. This atlas can then be registered to other image volumes to permit automatic localization. In this work a 12 parameter affine registration is performed first using mutual information as a similarity measure. After that a non rigid registration algorithm is applied to register the ear in the atlas to the patient's ear. The structures outlined in the atlas are deformed using the computed transformations and the resulting intensity centroids are used to draw the required safe path. The developed algorithm has been tested on eleven ears. In every instance, the path was deemed acceptable.