A study of reconstruction artifacts in cone beam tomography using filtered backprojection and iterative EM algorithms

Reconstruction artifacts in cone beam tomography are studied for filtered backprojection (Feldkamp) and iterative EM algorithms. The filtered backprojection algorithm uses a voxel-driven, interpolated backprojection to reconstruct the cone beam data, whereas the iterative EM algorithm performs ray-driven projection and backprojection operations for each iteration. Two weighting schemes for the projection and backprojection operations in the EM algorithm are studied. One weights each voxel by the length of the ray through the voxel and the other equates the value of a voxel to the functional value of the midpoint of the line intersecting the voxel, which is obtained by interpolating between eight neighboring voxels. Cone beam reconstruction artifacts such as rings, bright vertical extremities, and slice-to-slice cross-talk are not found with parallel beam and fan beam geometries. When using filtered backprojection and iterative EM algorithms, the line-length weighting is susceptible to ring artifacts which are improved by using interpolated projector-backprojectors