Spatially varying Riemannian elasticity regularization: Application to thoracic CT registration in image-guided radiotherapy

For deformable registration of computed tomography (CT) scans in image guided radiation therapy (IGRT) we apply Riemannian elasticity regularization. We explore the use of spatially varying elasticity parameters to encourage bone rigidity and local tissue volume change only in the gross tumor volume (GTV) and the lungs. We evaluate the method on the point-validated 4DCT breathing thorax POPI-model and demonstrate its use and properties in registration of pre- and post-chemo CT scans for contour propagation in a Hodgkin lymphoma (HL) case showing significant tumor shrinkage. For the POPI-model we achieved a total mean target registration error (TRE) of 0.92 ± 0.49 mm. Using spatially varying regularization for the HL case, deformation was limited to the GTV and lungs.