1. A method of image segmentation and modelling, within a space spanned by a set of medical data, the portion of the space occupied by an object, the method comprising: (a) defining a level set function, a level set of the level set function being the boundary of a deformable model of the object; (b) iteratively performing at least once the set of steps: (i) calculating, at each of a plurality of locations, a corresponding force, said force being calculated using the medical data and the boundary of the model; and (ii) modifying the level set function at the plurality of locations according to the calculated corresponding forces; and (c) modelling the boundary of said object as the level set of the modified level set function.