A system for determining a position of a probe relative to an object such as a head of a body of a patient. The head includes a surface such as a forehead having a contour. Cross sectional images of the head are scanned and stored as a function of the forehead contour. If the forehead contour does not appear in the scan images, then the position of the forehead contour relative to the scan images is determined with an optical scanner and a ring. During surgery, the optical scanner also determines the position of the forehead relative to the ring. An array for receiving radiation emitted from the probe and from the ring generates signals indicating the position of the tip of the probe relative to the ring. A stereotactic imaging system generates and displays an image of the head corresponding to the measured position of the tip of the probe. The system may also display scan images from different scanning technologies which scan images correspond to the same position in the head.