Fast voxel-based 2D/3D registration algorithm using a volume rendering method based on the shear-warp factorization

2D/3D registration makes it possible to use pre-operative CT scans during X-ray fluoroscopy guided interventions for navigation purposes. For this registration task a fast voxel-based method is presented, which uses a recently introduced similarity measure (pattern intensity). This measure is especially suitable for 2D/3D registration, because it is robust with respect to structures as, for instance, a stent visible in the X-ray fluoroscopy image but not in the CT scan. The method uses only a part of the CT scan for the generation of digitally reconstructed radiographs (DRRs) to accelerate their computation. Nevertheless, computation time is crucial for intra-operative application, because numerous DRRs must be computed. The suitability of different volume rendering methods for 2D/3D registration has been investigated therefore. A method based on the shear-warp factorization turned out to be especially suitable and builds the basis of the registration algorithm. The algorithm has been applied to images of a spine phantom and to clinical images. For comparison, registration results have been calculated using ray-casting. The shear-warp factorization based rendering method accelerates registration by a factor up to seven compared to ray-casting without degrading registration accuracy. Using a vertebra as feature for registration, computation time is in the range of 3-4s (Sun UltraSparc, 300 MHz) which is acceptable for intra-operative application.