Automatic pre- to intra-operative CT registration for image-guided cochlear implant surgery

Percutaneous cochlear implantation (PCI) is a minimally invasive image-guided cochlear implant approach, where access to the cochlea is achieved by drilling a linear channel from the outer skull to the cochlea. The PCI approach requires pre- and intra-operative planning. Segmentation of critical ear anatomy and computation of a safe drilling trajectory are performed in a pre-operative CT. The computed safe drilling trajectory must then be mapped to the intraoperative space. The mapping can be done using the transformation matrix that registers the pre- and intra-operative CTs. However, the difference in orientation between the pre- and intra-operative CTs is too extreme to be recovered by standard, gradient descent-based registration methods. Thus, we have so far relied on an expert to manually initialize the registration. In this work we present a method that aligns the scans automatically. We compared the performance of the automatic approach to the registration approach when an expert does the manual initialization on ten pairs of scans. There is a maximum difference of 0.19 mm between the entry and target points resulting from the automatic and manually initialized registration processes. This suggests that the automatic registration method is accurate enough to be used in a PCI surgery.