Piecewise affine initialized spline-based patient-specific registration of a high-resolution ear model for surgical guidance

Image guidance of ear surgery would enable an ENT surgeon to navigate about the components of the middle and inner ear, but the elaboration of anatomical models for this application is limited by the resolution of CT and its inability to distinguish among soft tissues. As a result, it is impossible to identify manually some tissues in clinical data, while visible tissues can only be identified with significant overhead. We propose a method for producing patient-specific description of the middle and inner ear on the basis of the minimally supervised registration of a high resolution model elaborated from micro-MR to patient CT, where the transformation among the model and the patient data is determined in a component-wise coarse-to-fine strategy. The first two stages feature a rough alignment on the basis of a few homologous point pairs, followed by a refinement based on a global affine transformation determined by mutual information. The middle stage involves a piecewise affine registration where each local affine transformation is given the global transformation as a starting point and is determined by mutual information over an appropriate anatomical mask. The final registration of each component is produced by mutual information-based thin-plate splines, whose anchor points overlap the affine-transformed mask.