Space carving and filtered back-projection as preconditioners for proton computed tomography reconstruction

This paper considers how to determine the boundary of an object by comparing two methods: space carving (SC) and filtered back-projection (FBP). Determination of the boundary is an important first step in proton CT. The boundary is used to set up the large sparse linear system of equations, which are then solved to determine the relative stopping power of each element (voxel) in the object. For instance, to find the path of the proton through the object, the entry and exit points on the boundary must first be found. The boundary also becomes important in the iterative reconstruction, as only voxels inside the object are updated, to reduce computational complexity and prevent external artifacts from forming. SC and FBP are compared on speed and boundary results for four cases: (1) noiseless simulated data, (2) noisy simulated data, (3) a real pediatric head phantom, and (4) a real rat. The usefulness and potential of both methods are discussed and future directions are outlined.