1. An image registration method comprising: computing a similarity measure of first and second images offset at a plurality of relative axial offsets, the computing including aligning corresponding slices of the axially offset first and second images, the slices being oriented generally transverse to the axial direction, wherein the aligning of corresponding slices of the axially offset first and second images comprises aligning a body profile of an imaged subject in corresponding slices, the similarity measure being computed based on the aligned corresponding slices; identifying a starting relative axial offset between the first and second images based on the computed similarity measures; and performing an iterative nonrigid or deformational image registration process to relatively register the first and second images using the identified starting relative axial offset between the first and second images as an initial condition for the iterative nonrigid or deformational image registration process; wherein the computing, identifying, and performing are performed by a computer.