Distance-driven projection and backprojection

Projection and backprojection are important processes in computed tomography (CT). They are used in iterative reconstruction, simulation, and artifact correction, as well as routine (filtered-backprojection based) reconstruction. Existing methods either have poor performance or result in artifacts. A new method for projecting and backprojecting rays through pixels is presented that has good performance and eliminates artifacts, and could potentially enable routine iterative reconstruction in clinical CT systems. The new method, which we call distance-driven, reconciles the advantages of the common pixel-driven and ray-driven methods. It avoids an artifact-inducing approximation made by the previous methods and has very favorable computational properties. The method is applicable to parallel-beam, fanbeam, and cone-beam geometries. Its performance and artifact behavior are evaluated on a two-dimensional fan-beam geometry with flat detector and compared to the pixel-driven and ray-driven approaches. The distance-driven method prevents the artifacts that are generated in the pixel-driven projection and in the ray-driven backprojection. It outperforms pixel-driven and ray-driven methods from a computational standpoint and is amenable to hardware implementation.