1. A method of image segmentation and modeling, within a space spanned by a set of medical data, a portion of the space occupied by an object, the method comprising carrying out on a computer: (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 first locations on the boundary, a corresponding force, said force being calculated using the medical data; (ii) calculating, at each of a plurality of second locations which are defined using the boundary and which are not on the boundary, a corresponding force, said force being calculated using said calculated forces at the first locations, wherein the force at said second locations is calculated by defining, for each said second location, a corresponding neighbourhood, and calculating the force at the second location using the average of said calculated forces at each said first location within the corresponding neighbourhood; and (iii) modifying the level set function at the first and second locations according to the calculated corresponding forces; the iteration terminating when a minimum of an energy function is reached, the energy function being related to the calculated forces at the first locations on the boundary; and (c) modeling the boundary of said object as the level set of the modified level set function.