Patent GB-1029859-A (GB)
Title: X-ray beam defining diaphragm system
IPC: A61N5/10
Filed: 1963-11-22
Abstract: 1,029,859. X-ray or electron therapy apparatus. VARIAN ASSOCIATES. Nov. 22, 1963 [Nov. 23, 1962], No. 46311/63. Heading H5R. In X-ray apparatus comprising means for producing and directing a beam of X-rays along a central axis and a beam defining system adapted to confine the beam to a predetermined field about the central axis, the beam defining system is rotatable about the central axis. Fig. 1 shows the apparatus in which a linear accelerator 15 produces a charged particle beam which is deflected through approximately 90 degrees to pass through the beam defining means 4 along a central axis 12. The beam defining system is rotatable about the axis 4 and in addition the accelerator and head 4 are rotatable about an axis 6. The patient is mounted on a treatment couch 8 having detachable wheels which is supported on a float table 10 arranged for dual speed vertical movement on a post 11. The table being rotatable about the axis 12. Fig. 2 shows in more detail the beam defining head in which the electron beam 31 is bent through 90 degrees to impinge on a target assembly 16 to produce X- rays, or the beam may be bent to by pass the target if the equipment is required for electron therapy &c. The X-ray beam is passed through a heavy metal primary collimator 17 to an ionization chamber and flattening filter 19 which may be retracted from the beam path by the rack-and-pinion mechanism 21 if the electron beam is required. The remainder of the beam defining mechanism is shown in section in Fig. 3, the X-ray beam passing through a wedge filter 71, and an optically reflecting plate 73, to the variable diaphragm section of the head. The diaphragm jaws, e.g. 53 and 53<SP>1</SP> comprise lead blocks having stepped tungsten face plates 76, tungsten being employed as a better absorber of X-rays in the 4-8 MEV range of operation. The jaws are mounted by means of rollers 44 which run in arcuate grooves 49 in a mounting plate 46 so that, as the jaws are moved to adjust the beam to the required size, the faces of the jaws remain parallel with the rays from the X-ray source. The jaws are moved by rotation of pinion 40<SP>1</SP>, e.g. by motor 33, Fig. 2, which moves racks 42<SP>1</SP> and 43<SP>1</SP> in opposite directions in a groove in the mounting plate, the ends of the racks being provided with U-shaped members 78 and 79 which engage pegs 98 and 99 mounted in the jaws, the pegs being eccentrically mounted so that the relative positions of the jaws may be accurately aligned. Two pairs of similarly mounted adjustable jaws are provided, the second pair being rotated 90 degrees to the first pair so as to define the other axis of the beam. The whole jaw assembly is mounted on a plate 24 which is rotatably mounted by ball-bearings 27 on plate 23. Electrical connection to the rotating portion, the rotation of which is limited to just over 360 degrees, is by means of a cable 30 coiled in an annular groove in the plate 24. An optical simulator system is provided comprising a prefocused lamp assembly 84 with a graticule and a zoom lens 92, light from which passes via reflecting plates 85<SP>1</SP> and 73 to the X-ray beam defining channel through which the light beam passes to form an optical image on the patient of crossed lines and possibly a scale to enable accurate setting of the X-ray beam. In addition, a range finder is provided by the lens 90 which projects an image of the filament of the lamp 89 which, when the apparatus is set to the required Focal skin distance, will be centred on the crossed line image projected by the simulator system. A flatness monitoring device 82 is mounted in the top edge of each of the lower jaws 76 and 76<SP>1</SP>, the monitoring device comprising a cadmium sulphide photo-cell embedded in epoxy resin in a light opaque conductive housing.