It's much, much simpler than that. You just project a ray from the X-ray point source through the center of a voxel onto the detector, interpolate the X-ray energy detected, and add that energy to the sum in that voxel. Repeat for each voxel and each x-Ray taken (CT scans take a bunch of x-Rays going around in a circle), remove some noise, and boom you've voxelled the data. Fundamentally you're just summing the energy that passes through each point in space. It's trivially parallelizeable.