1. A system for displaying three dimensional surface structures comprising: means for storing three dimensional signal patterns representing the value of at least one physical property associated with a three dimensional body at regularly spaced grid locations within said body; means for retrieving the eight three dimensional signal pattern values associated with each set of cubically adjacent grid locations within said body; means for comparing each set of said eight values with a predetermined threshold value to generate an eight bit binary vector each of whose elements is zero or one, based on the result of said comparison; means for generating a set of coordinate values for each distinct binary vector, said coordinate values representing the vertices of at least one predetermined polygonal surface which approximates the intersection of surfaces determined by said threshold value with the volume defined by said eight grid points, said coordinate values also being dependent on the location of said eight grid locations within said body; display processor means for receiving said coordinate values and for converting said coordinate values to a display format; and means for displaying surfaces determined by said threshold, said display means being driven by said display processor.