The frame has two elongated parallel frame parts which are imaged by a marker substance on the tomographical picture, and between which is placed the positioner for the implement. The two elongated parallel frame parts are joined by a transverse strut near their adjacent ends. The positioner is fixed to the transverse strut or to one of the parallel frame parts. The positioner unit is rotatable or movably in the longitudinal direction of the transverse strut and or of the elongated parallel frame parts and or of a third axle.
