Evaluation of volumetric differences through 3-dimensional display of distance of closest approach

Treatment planning programs offer the ability to view quantitative information on serial CT sections. Many of these programs also offer the ability to view structures of anatomical interest from a beam's-eye view. Such displays may be of wire-frame objects or of shaded surface reconstructions derived from contours drawn in serial CT or MRI sections. Such surface reconstructions are not only shaded to give a 3-dimensional perspective, but can be colored in order to display information about some underlying property of a structure. Color has, for example, been used to demonstrate the distribution of dose on the surface of volumes of interest. We introduce a technique in which color is used to show the spatial differences between volumes of interest by displaying the surface of one volume and encoding in color the distance of closest approach of another volume. Regions in which the two surfaces are within a specified distance are shown in gray; regions in which the first structure lies outside the second are shown in reddish/yellow hues; and regions where the second is outside the first are shown in bluish/green hues with the hue being proportional to the distance apart. Such displays offer insight into anatomical relationships not otherwise easily obtained.