Fast projection algorithm for voxel arrays with object dependent boundaries

One-pass list-mode EM (OPL-EM), previously named ROPLE, is a new list-mode iterative reconstruction algorithm for high resolution, high count-rate and large field of view (FOV) PET scanners. The list-mode EM algorithm requires projecting every list-mode event from and into a large FOV with a large number of sub-millimeter voxels, and so substantial computing time is spent on back-projection and forward-projection operations. This paper presents an improved ray-tracing method, which extends the previously used projection algorithm, known as the Siddon algorithm, for a FOV with an irregularly bounded set of voxels instead of the traditional cubic array. This method can remove redundant projection operations in regions where no activity is present. This leads to typically a factor of 5 speed up compared to the conventional Siddon algorithm, and up to a factor of 2 speed up compared to the incremental approach.