1. A medical image data processing apparatus comprising:— a registration unit configured to perform a non-rigid registration of a first set of medical image data and a second set of medical image data thereby to determine a deformation field; and a region identification unit configured to identify at least one region for which the deformation field is approximately homogeneous, and, for at least part of said at least one identified region, to obtain a substantially homogeneous approximation of the deformation field.