Intravascular visualisation by subtraction CT angiography and nonrigid registration

Computed tomography angiography (CTA) is an established tool for vascular imaging. However, high-intense non-vascular structures in the contrast image can seriously hamper luminal visualisation. This is an issue for three-dimensional visualisation, where high-intense structures might cover the underlying vasculature. But also in two dimensions, the closeness of calcified plaques and contrastenhanced vessel lumen impedes correct determination of the vessel boundary. High-intense structures can be eliminated using subtraction CTA, where a native image is subtracted from the contrast image. However, patient and organ motion limits the widespread application of this technique. Within this paper, we use nonrigid image registration to solve this problem. The native image is registered to the contrast image using an automatic intensity-based nonrigid three-dimensional registration algorithm. Both images are merged in a fused image, allowing the user to switch between a view of the arteries, the bone or both. This procedure has been used to image the carotids of 95 patients. In all cases, the artifacts are strongly reduced by the nonrigid registration. The average calculation time of the whole procedure is about 30 minutes.