Cone-beam reprojection using projection-matrices

This paper addresses reprojection of three-dimensional (3-D) reconstructions obtained from cone-beam scans using a C-arm imaging equipment assisted by a pose-determining system. The emphasis is on reprojecting without decomposing the estimated projection matrix (P-matrix) associated with a pose. Both voxel- and ray-driven methods are considered. The voxel-driven reprojector follows the algorithm for backprojection using a P-matrix. The ray-driven reprojector is derived by extracting from the P-matrix the equation of the line joining a detector-pixel and the X-ray source position. This reprojector can be modified to a ray-driven backprojector. When the geometry is specified explicitly in terms of the physical parameters of the imaging system, the projection matrices can be constructed. The resulting "projection-matrix method" is advantageous, especially when the scanning trajectory is irregular. The algorithms presented are useful in iterative methods of image reconstruction and enhancement procedures, apart from their well-known role in visualization and volume rendering. Reprojections of 3-D patient data compare favorably with the original X-ray projections obtained from a prototype C-arm system. The algorithms for reprojection can be modified to compute perspective maximum intensity projection.