1. A user interface for a computer-implemented multi-planar reformatting (MPR) system for allowing a user to define and view a curve through a three-dimensional volume data set, comprising: first, second and third display areas in which are displayed three orthogonal MPR images of the volume data set; a curve drawing tool to allow a user to define a curve through the volume data set, at least part of the curve being displayed in projected format on each of the MPR images; a plurality of selectable icons each representing a direction substantially normal to a plane parallel to one of the MPR images, using which the user can select an approximate direction to be taken by the curve through the volume data set; and a fourth display area, in which is displayed, after the user has defined at least part of the curve, a curve-related MPR image through the volume data set related to one or more positions along the curve.