GB812077A - Improvements in or relating to radiographic apparatus - Google Patents

Abstract

812,077. X-ray apparatus. GENERAL ELECTRIC CO. Ltd. April 17, 1957 [April 18, 1956], No. 11825/56. Class 98(1) In radiographic apparatus for use with an X-ray tube energized by a pulsating unidirectional electric current, a Potter- Bucky diaphragm is oscillated at constant amplitude in synchronism with the pulsating current during an exposure in such a way that reversal of the movement of the diaphragm, and the relatively slow movement of the diaphragm immediately before and after the reversal, occur at times when there is little or no flow of current through the X-ray tube, and the diaphragm is oscillated by a vibratory element or system supported at a nodal point or points. As showr in Fig. 1, the Potter-Bucky diaphragm 1 is connected by flanged plates 3 and limbs 4 to a rectangular bar 5 of high tensile steel extending perpendicular in the direction of movement of the diaphragm 1. The bar 5 is connected to a further bar 9 (see also Fig. 2), and the bars are clamped together at their free ends by clamps 6, and are driven through a clamp 7 through their centre points, being supported by clamps 8 between their ends and the middle clamp 7. As shown in Fig. 4, the bar 5 and the bar 9 are held together by the clamp 7 which is of U-shape construction and may be tightened by bolts 13. The bar 9, except where it actually passes through the clamps, is slightly flattened on opposite sides and may be rotated within the clamp 7. A bracket 11 attached to clamp 7 is connected by a wire 19 to a moving coil driving means 10, by which the system is vibrated. The bracket 11 is also connected to a body 12 which acts as a counter-weight to the grid 1. As shown in Fig. 5, the clamps 8 at the nodal point of the system are mounted on a support plate 16 through resilient bushes 21. With one of the clamps 8 is associated a twoarmed link 23 (see also Fig. 1) which is connected by a lever system to a handle 36, which may be moved in order to rotate the bar 9 to alter its natural frequency of vibration. In order to measure the frequency of oscillation of the system, a magnet 37 is mounted on one of the brackets 3, so as to induce currents in a coil 39 wound on a laminated core 40. A series of mild steel plates 38 secured to the other flange plate 3 counterweight the magnet 37. The current induced in the coil 39 is proportional, at any instant, to the velocity of the grid 1. In operation, the moving coil driving means 10 maintains the system in oscillation by means of the 50 cycles per second alternating current supply which also provides the high voltage for the X-ray tube. It is ensured that the change of direction of the grid is synchronized with the periods during which the primary radiation from the X-ray tube is at a low value by altering the resonant frequency of the system by rotating the tube by means of the handle 36. This results in the phase relationship between the vibration of the rods 5, 9 and of the pulsating electric current being altered so that the desired synchronization results. In order that this adjustment may be made, the output from the coil 39 is applied to an auxiliary circuit whose function is to compare the phase and frequency of this current with the phase and frequency of the pulsating unidirectional electric current applied to the X-ray tube. The resonant frequency for the bars adopted when the X-ray voltage is supplied by full-wave rectification is also suitable when half-way rectification is used, or in the latter case the frequency of vibration of the rod system may be half the supply frequency. If desired, the two bars 5, 9 may be replaced by a single vibratory element.