Fast distance-driven projection and truncation management for iterative cone-beam CT reconstruction

The purpose of this study was to improve the speed of the Simultaneous Algebraic Reconstruction Technique (SART) for truncated cone-beam (CB) projections using a distance-driven forward projector. First, we optimized the innermost loop of the projector for a cone-beam scanner with a circular source trajectory and a flat-panel imager. We showed that the innermost loop of the distance-driven projection reduces to the mapping of two segments of samples with constant spacings. Second, we implemented the projector to handle a multi-resolution grid in the planes orthogonal to the rotation axis in order to reduce the overhead required to manage the truncation of the conebeam projections. The distance-driven projector was 1.4 times faster than an optimized ray-driven projection with Joseph interpolation. Moreover, the multi-resolution reduced 4 times the overhead required to handle an half-truncated Shepp-Logan phantom.