Eliminating the second-intersection contributions to spiral scan cone beam CT

A number of filtered-backprojection type quasi-exact algorithms have been recently developed to reconstruct images in cone beam spiral scan. A key common feature in these algorithms is a masking process employed in filtering the cone beam image. The masking process is an approximation to restrict the line integrals intersecting the cone beam image to the appropriate angular range required for data combination. Errors in the approximation are caused by portions of some integration line segments which do not conform to proper data combination. In this paper the line segments which constitute the major source of errors in the masking process are identified. It is found that these line segments are localized in the detector projection space. The errors can be corrected by applying the filtering process to this group of line segments. It is also shown in the paper that the correction procedure can be simplified by using 1D Hilbert transforms. Improvements in image quality brought about by the correction procedure are demonstrated in simulation reconstructions