a method for identifying non - body structures in digitized medical images including the steps of providing a digitized image comprising a plurality of intensities corresponding to a domain of points on an n - dimensional grid , wherein said image includes a representation of a body and of non - body structures separate from said body , initializing a surface in said image on a side of said non - body structures opposite from said body , defining a plurality of forces acting on said surface , and displacing said surface through said non - body structures using said forces until said body is encountered .