1. Method for measuring geometric quantities intrinsic to an anatomical system, based on two geometrically calibrated, stereoscopic, two-dimensional images of the anatomical system of a patient, comprising (a) receiving registration data comprising at least one set of two-dimensional coordinates, each set of two-dimensional coordinates comprising a first pair of two-dimensional coordinates corresponding to a first point on one of the said two stereoscopic images and a second pair of two-dimensional coordinates corresponding to a second point on the other of the said two stereoscopic images, (b) by using geometric calibration information, determining at least one three-dimensional geometric primitive defined by at least one portion of the registration data received in step (a), (c) computing, based on at least one three-dimensional geometric primitive determined in step (b), a value of geometric quantity intrinsic to the anatomical system.