A radiation detector system is provided that includes plural detector units and at least one processor. The detector units are configured to acquire imaging information at plural corresponding projection angles. The at least one processor is configured to acquire projections at the projection angles； organize the projections into groups based on the projection angles； and, for each group of projections, rotate a corresponding image from an original orientation so that the group of projections are parallel to a first axis of the rotated image, convolute and sum slices from the group of projections using kernels to provide a corresponding coordinate set forward projection； perform a back projection to provide back projections； and rotate the back projections to the original orientation and sum the rotated back projections to provide a back projected transaxial image.
