Comparison of Three Different Projectors for Cone Beam CT

This paper compares three different projectors for the iterative image reconstruction in cone beam computed tomography (CBCT), used to compute both the forward projection as well as the backprojection in practically all iterative reconstruction algorithms. The tested projectors are an improved version of the Siddon's algorithm, an interpolation-based projector, and the branchless distance-driven projector. For the forward projection of the interpolation-based projector, the effect of the sampling distance is also examined. The improved Siddon projector is a ray-driven projector while the interpolation and branchless projectors are ray-driven in forward projection and voxel-driven in backprojection. All projectors were implemented for GPUs using OpenCL and the interpolation-based and branchless distance-driven projector utilized the built-in hardware-based interpolation available in GPUs. The projectors were tested using experimental phantom data from Planmeca CBCT scanner. The results show that quality-wise the differences between the interpolation-based and branchless projectors are small, while the improved Siddon tends to cause aliasing artifacts and Moiré pattern unless sufficient sampling or point spread function blurring is used. Computationally the interpolation-based projector is fastest while the improved Siddon is the slowest. The branchless distance-driven projector can also exhibit noise pattern if the dynamic range of the integral image used with the projector is too wide. Thus the interpolation-based projector is both the computationally fastest method, while also providing very similar quality in CBCT as well as being more robust than the branchless distance-driven method. The sampling distance of the interpolation-based projector, on the other hand, has quite a small effect on the final image quality and thus distances of even four voxels can be used without noticeable effect on image quality.