A system for determining a position of a probe (302) relative to an object such as a head (390) of a body of a patient. The head includes a surface such as a forehead (394) having a contour. The head is placed in a cradle (392) equipped with an arc (393). The cross sectional images of the head are determined relative to the arc. A hand held unit (380) optically scans the forehead and the arc. During scanning to generate the cross sectional images, the optical scanner (380) is used to determine the position of the forehead (394) relative to the cradle (392). During surgery, the optical scanner (380) also determines the position of the forehead (394) relative to a base ring (306). An array (300) for receiving radiation emitted from the probe (302) and from the base ring (306) generates signals indicating the position of the tip of the probe (302) relative to the base ring (306). A stereotactic imaging system selects and displays the image of the head closest to the measured position of the tip of the probe (302).