1. A system for determining patient-specific anatomical dimensions for structures which have a branching configuration using scanned 2-D image data, said system comprising: a set of scanned 2-D images assembled into a 3-D database, said 3-D database comprising a volumetric data set; means for segmenting said volumetric data set into a set of 3-D locations corresponding to a specific anatomical structure to be measured; means for defining a plurality of 2-D image planes parallel to and spaced from one another and transversely intersecting said set of 3-D locations corresponding to said specific anatomical structure, each of said 2-D image planes including a set of co-ordinates located within said anatomical structure to be measured; means for specifying, for each branching structure contained within said set of 3-D locations corresponding to said specific anatomical structure, a branch line in said volumetric data set that uniquely indicates said branch structure, said branch line being specified by a start location and an end location disposed in selected first and second ones of said plurality of 2-D image planes, respectively; means for calculating, for each branching structure contained within said set of 3-D locations corresponding to said specific anatomical structure, a centroid path in said volumetric data set for that branching structure, said centroid path being determined by averaging said set of co-ordinates located within each of said 2-D image planes between said first and said second ones of said plurality of image planes so as to define a centroid for the portion of said branch structure contained in that particular 2-D image plane, and applying a curve-fitting algorithm to said centroids so as to supply data for any portions of said set of 3-D locations corresponding to said specific anatomical structure which may lie between said 2-D image planes, and for smoothing out noise that may occur in said system, whereby to create resulting space curves; and, means for calculating said desired anatomical dimensions from said resulting space curves.