Nonrigid Registration of 3-Dimensional Images of the Carotid Arteries

Atherosclerosis at the carotid bifurcation can result in cerebral emboli, which in turn can block the blood supply to the brain causing ischemic strokes. Non-invasive imaging tools that characterize arterial wall, and atherosclerotic plaque structure and composition may help to determine the factors, which lead to the development of unstable lesions, and identify patients at risk of plaque disruption. Registration of 3D ultrasound (US) images of carotid plaque obtained at different time points, and with Magnetic Resonance (MR) images are required for monitoring of plaque changes in volume and surface morphology, and combining the complementary information of the two modalities for better understanding of factors that define plaque vulnerability. These registration techniques should be nonrigid, to remove deformations caused by bending and torsion in the neck during image acquisition sessions. The high degrees of freedom and large number of parameters associated with nonrigid image registration methods causes several problems including unnatural plaque morphology alteration, high computational complexity, and low reliability. Thus, we used a “twisting and bending” model with only six parameters to model the natural movement of the neck for nonrigid registration. We calculated the Mean Registration Error (MRE) between the segmented vessel surfaces in the target and the registered images using the distance between “matched points” to evaluate registration results. We registered 3D US carotid images acquired at different head positions to simulate images acquired at different times, and obtained an average MRE of 0.8±0.3mm for nonrigid registration. We registered 3D US and MR carotid images at field strengths, 1.5T and 3.0T, of the same subject acquired on the same day, and obtained an average MRE of 1.4±0.3mm for 1.5T and 1.5±0.4mm for 3.0T, using nonrigid registration. Furthermore, we showed that the error metric used here was not significantly different from the widely accepted Target Registration Error (TRE)