1. A method of non-rigidly registering an atlas with an image, said method comprising the steps of: dividing a predefined domain into regular blocks, said regular blocks being hexahedrons, said atlas and said image being volumetric images defined in said predefined domain, said atlas comprising a plurality of labels L, each non-zero label L representing a labeled structure and the label L=0 representing an unlabeled structure; calculating a boundary displacement of at least one structure in said atlas according to labels L of that structure to register that structure with an equivalent structure in said image, said calculating step comprising the sub-steps of: calculating a displacement of centre points of each block, said displacement maximising a block similarity measure between corresponding blocks in said atlas and said image; and calculating said boundary displacement by calculating a displacement for points on said boundary of said structure by interpolating said displacements of neighbouring centre points; and warping the boundary of said structure in said atlas according to said boundary displacement using topology preserving front propagation of boundary points of said structure in said atlas according to labels L.