An approximation with a simpler numerical technique is adapted for calculating Vin/V. In one embodiment, the voxel that crosses the edge of the fiducial marker is divided into m???n???p subvoxels, where m, n, p are an integer, then designating a subvoxel as being inside the fiducial marker if its centroid is inside the fiducial marker and as being outside the fiducial marker if its centroid is out