 The resulting projection matrix can be sparsely stored and output if needed, and the adjoint of X-ray transform can be also computed by the algorithm .
<meaning-changed>  The resulting projection matrix can be sparsely stored and output if needed, and the adjoint of X-ray transform can be also computed by the algorithm .
 The comparison study demonstrates the proposed algorithm is fast, more complete, and is more flexible with respect to different scanning geometries and different basis functions .