1. A system for creating a synthetic computed-tomography (CT) image volume, comprising: a rigid registration algorithm configured to register a reference CT image volume to a first clinical image volume and thereby produce a first registration; a smoothing function configured to further refine said first registration and thereby produce a smoothed registration; and a voxelwise intensity algorithm configured to further refine said smoothed registration and thereby produce a synthetic CT image volume having characteristics correlated to those of said first clinical image volume.