1. A medical image data processing apparatus, comprising: processing circuitry configured to perform a non-rigid registration of a first set of medical image data and a second set of medical image data, wherein each of the first set of medical image data and the second set of medical image data comprises a respective set of voxels, each voxel having an intensity value, and wherein the non-rigid registration determines a deformation field that comprises vector values representing displacements of at least one of voxels of the first set of medical image data and the corresponding voxels of the second set of medical image data according to the registration, and process the vector values of the deformation field to identify at least one region having a size greater than a threshold size and for which the vector values of the deformation field are approximately homogeneous, and, for at least part of said at least one identified region, to obtain a substantially homogeneous approximation of the deformation field, wherein in identifying the at least one region for which the vector values of the deformation field are approximately homogeneous, the processing circuitry is further configured to identify at least one region for which at least one component of a variation of the deformation field is less than a threshold value.