the calculus being shown on the monitor 8 at position a referenced to the reference mark 14 the x ray tube 2 is then pivoted by the angle u03b1 the treatment site calculus migrates in the video image on the monitor 8 by a distance s which is proportional to its path on the input luminescent screen of the x ray image intensifier 3 the treatment site is now located at position b on the monitor 8 the location of the treatment site at position b is marked on the monitor by any one of a number of data entry devices such as a light pen 15 a mouse 16 a tracking ball not shown or the like this results in the mark 17 being shown on the monitor 8 the difference between the position a corresponding to the reference mark 14 and the position b corresponding to the mark 17 i e the path s is entered in the computer 12 the height of the treatment site above the pivot point is calculated in the computer 12 from the path s the position of the pivot point for the x ray tube 2 the angle u03b1 and the distance of the focus image plane the current device positions corresponding to the distance s the position of the pivot point the angle u03b1 and the distance of the focus image plane are entered at line 18 the position of the treatment site in space can now be defined the computer 12 now.