A tool for registration verification based on gradient correspondence
Primož Markelj, Franjo Pernuš, Boštjan Likar · Proceedings of SPIE, the International Society for Optical Engineering/Proceedings of SPIE · 2009
Verification of registration accuracy is paramount for assessing the validity and clinical feasibility of a registration method. When a ground truth registration is not available or when local misalignments need to be examined, a qualitative assessment of registration results must be performed. The verification of registration by analyzing correspondences of gradients derived from the rigidly registered CT and MR images was performed. Strongest local CT gradients were extracted and transformed into the MR gradient image. A local gradient correspondence search in the MR image was performed using discrete systematic displacements in the direction of the strongest local CT gradients. As the measure of gradient correspondence between the CT and MR gradients the absolute values and the directions of the gradients were considered. The directional information was integrated by means of a weighting function which was calculated as a product of absolute values of the strongest local CT gradient and the MR gradient weighted by the angle between these two gradients. Two correspondence visualization techniques and gradient displacement analysis were developed to highlight the misaligned gradients and provide qualitative assessment of local misregistration. The feasibility of the proposed approach was demonstrated on the CT and MR images of the RIRE database registered using the normalized mutual information similarity measure. Global and local misregistrations were detected. Furthermore, the acquisition artifacts of non-rectified MR images could be visualized and were shown to degrade the registration performance.