Nonrigid registration for subtraction CT angiography applied to the carotids and cranial arteries

RATIONALE AND OBJECTIVES: Computed tomography angiography (CTA) is an established tool for vascular imaging. However, high-intensity nonvascular structures in the contrast image can seriously hamper luminal visualization. This is an issue for three-dimensional visualization, where high-intensity structures might cover the underlying vasculature. But also in two dimensions, calcified plaques adjacent to the contrast-enhanced vessel lumen impede correct determination of the vessel boundary. High-intensity 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. We propose to use nonrigid image registration to solve this problem. MATERIALS AND METHODS: For each patient, a native image and a contrast image are recorded, respectively, before and after contrast administration. 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. The procedure has been applied to 95 patients. RESULTS: In all cases, subtraction CTA using nonrigid registration allows for a significantly better artifacts removal than subtraction CTA without registration. Image quality of all images was judged adequate for clinical use. The average total processing time for each dataset is about 30 minutes. CONCLUSION: Nonrigid registration can allow for a great reduction in subtraction artifacts for subtraction CTA, resulting in a clear view of the vasculature.