A user interface for a computer-implemented Multi-Planar Reformatting (MPR) system comprises three 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; six selectable icons each representing one of six possible combinations of a plane parallel to one of the orthogonal MPR images and a direction substantially normal to a plane parallel to one of the other two MPR images, using which the user selects an approximate direction for the curve; and a fourth display area displaying a curve-related MPR image related to one or more points along the curve. The icons may be individually selectable to allow user input of adequate orientational information for a curved MPR image, or selectable in pairs for input of directional information for a cross-curve MPR image. Alternatively, three icons may be provided for a cross-curve MPR system. A tool to allow rotation of a curved MPR image about the curve can be provided, as can visual indicators of the depth of the curve within the volume, and of the orientation of the cross-curve image.