method to bring out a temporal difference between corresponding structures in a reference image r and a floating image f by convolving the reference image r and the floating image f with a window function h w to generate r w and f w , applying a non - rigid transformation resulting in a transformation field g mapping every location r r to a corresponding location r f in the floating image f and generating a subtraction image by performing subtraction r w − f w ) wherein r represents a voxel in reference image r .