A system utilizing computed tomography (CT) as for surgery. A CT scanner provides data representative of or slices taken through a body member. Display apparatus reveals the slices. A reference apparatus (frame) affixed to the body, operates in conjunction with the scanner and the display apparatus. The frame includes: structure for affixation to the body, holder means for guiding the therapeutic instrument, and structure for providing reference indications. The system further includes a data processing structure and control means for transforming data between frame coordinates and slice coordinates.