Different tomographic images of the same subject matter are optimally related to one another through a two-step procedure whereby, first, movements are plotted on a subregion level, producing a vector map relating subregions in a first set of scan frames to subregions in another set of scan frames; and then, based on the aggregate of subregion movements, the invention characterizes the "global" displacement relating the volume represented by the first set of scan frames to that represented by the second set. More specifically, the invention generates translation and rotation vectors that describe the movements underlying shifts in subject matter from one scan to the other, and which can be applied to one frame set to align it with the other.