Non-rigid registration for fusion of carotid vascular ultrasound and MRI volumetric datasets

In carotid plaque imaging, MRI provides exquisite soft-tissue characterization, but lacks the temporal resolu-tion for tissue strain imaging that real-time 3D ultrasound (3DUS) can provide. On the other hand, real-time3DUS currently lacks the spatial resolution of carotid MRI. Non-rigid alignment of ultrasound and MRI data isessential for integrating complementary morphology and biomechanical information for carotid vascular assess-ment. We assessed non-rigid registration for fusion of 3DUSand MRI carotid data based on deformable modelswhich are warped to maximize voxel similarity. We performed validation in vitro using isolated carotid arteryimaging. These samples were subjected to soft-tissue deformations during 3DUS and were imaged in a staticconÞguration with standard MR carotid pulse sequences. Registration of the source ultrasound sequences to thetarget MR volume was performed and the mean absolute distance between Þducials within the ultrasound andMR datasets was measured to determine inter-modality alignment quality. Our results indicate that registrationerrors on the order of 1mm are possible in vitro despite the low-resolution of current generation 3DUStransduc-ers. Registration performance should be further improved with the use of higher frequency 3DUS prototypesand efforts are underway to test those probes for in vivo 3DUS carotid imaging.Keywords: carotid imaging, vascular imaging, multimodal imaging, registration, fusion, 3-D ultrasound, mag-netic resonance imaging, atherosclerosis, vulnerable plaque