Abstract: Procedure for positioning a small internal portion of a body at a predetermined point relative to an external source of shock waves or the like, for treatment of the positioned body portion, wherein a volume of the body including the body portion is imaged with penetrating radiation by each of two detectors respectively aligned with viewing axes intersecting at the predetermined point; the coordinates of the body portion on each of the two images thus produced are measured and corrected for imaging distortions; from these corrected coordinates, there is derived a set of three-dimensional spatial coordinates for the body portion, defining a displacement vector giving the magnitude and direction of body motion required to position the body portion at the predetermined point; and the body is displaced with a resultant motion of that magnitude and direction.