An image transformation method, performed by a computer, for determining a transformation between two medical images of an anatomical structure, respectively taken at a first and second point in time, said method comprising the steps of: acquiring first and second image datasets which represent the first and second image, respectively; determining an image-based transformation between the first image and the second image; acquiring an elastic model of the anatomical structure at the first point in time, wherein the elastic model comprises a plurality of nodes and the relationships between the nodes; calculating the displacements of a subset of the nodes in accordance with the image-based transformation; calculating the displacements of the other nodes in accordance with the relationships between the nodes as represented by the elastic model, while keeping the displacements of the nodes in the subset of nodes constant; and calculating the transformation from the displacement vectors which represent the displac
