Hybrid rigid and non-rigid registration algorithm for alignment of intra-subject thoracic and abdominal images

A hybrid rigid and non-rigid registration algorithm has been presented to register thoracic and abdominal CT images of the same subject scanned at different times. The bony structures are first segmented from two different time CT images, respectively. Then, the segmented bony structures in the two respective images are registered based on their boundary points using a soft correspondence matching algorithm, with a rigid transformation constraint on each bony structure. With estimated correspondences in bony structures, the dense deformations in the entire images are interpolated by a thin plate spline (TPS) interpolation technique. To improve the alignment of soft tissues in the images as well, a normalised mutual information based B-spline registration algorithm is used to iteratively refine the registration of soft tissues, and at the same time keep the rigid transformation for each bony structure. This registration refinement procedure is repeated until the algorithm converges. The proposed hybrid registration algorithm has been applied to the clinical data with very encouraging results as measured by two clinical radiologists.