  The key aspect of parallel-beam X-ray CT is forward and back projection, but
its computational burden continues to be an obstacle for applications. We
propose a method to improve the performance of related algorithms by
calculating the Gram filter exactly and interpolating the sinogram signal
optimally. In addition, the detector blur effect can be included in our model
efficiently. The improvements in speed and quality for back projection and
iterative reconstruction are shown in our experiments on both analytical
phantoms and real CT images.
