818,870. Automatic control system for X-ray tubes. WESTINGHOUSE ELECTRIC INTERNATIONAL CO. Aug. 1 ,1957 [Aug. 10, 1956], No. 24378/57. Class 38 (4). [Also in Group XX] X-ray apparatus, having an X-ray tube and a screen whereon a luminous image of an irradiated object is formed, includes a partiallyreflecting mirror which reflects part of the light from the screen for visual observation of the image and transmits the other part to a photographic recording means, a fully-reflecting mirror which reflects the whole of the light from the screen for visual observation, and mechanism for bringing the mirrors into operation alternately. This mechanism may be associated with an electric switch so that the energy level of operation of the X-ray tube is increased when the partially-reflecting mirror is brought into operation. As shown in Fig. 1, a tower 5 is movably mounted on an X-ray table 9 supports a housing 16 for an electronic image intensifier in such a way that it moves lengthwise and crosswise of the table together with an X-ray tube (not shown) beneath the table. As shown in Fig. 3, the image intensifier 62 has an emissive screen 63 and a screen 65 whereon the intensified image is formed. In a modification, the image intensifier may be replaced by a fluorescent screen. Optical system.-Light from the screen 65 passes through a collecting lens 91 to a partially reflecting mirror 92. A large portion of the light passes through mirror 92 to the film 110 of a kinematograph camera mounted in a top housing 23 above the housing 21 for the optical system, which in turn is mounted above housing 16. The camera film-driving mechanism is actuated by a synchronous electric motor 112. Light reflected from mirror 92 is further reflected by a mirror 94 to a lens 99, and by further mirrors 95, 19 to the observer's eye. By this means, the image can be viewed throughout the operation of the camera. For fluoroscopic viewing, a fully reflecting mirror 93 is moved by a linkage 98 connected to a handle 97 from the inoperative position shown in Fig. 3 to the position shown in dotted lines in front of mirror 92. X-ray tube supply circuit.-A transformer 28 and rectifying circuit 35 supply the anode voltage for the X-ray tube 10 (Fig. 3). A rectifier bridge 33 and a variable resistor 34 are connected in series with the secondary windings 31, so that a D.C. voltage proportional to the tube current appears across the resistor 34. The tube filament current is supplied by a transformer 36, which is connected to the mains X, Y through a variable impedance circuit 32, termed the MA stabilizer, of the kind which controls the current in the A.C. circuit in response to variations in a D.C. voltage applied by a conductor 38. By means of a relay 81, the conductor 38 can be connected either to the resistor 34, so that the tube current is kept constant, or to a brightness amplifier 48 supplied by a photo-multiplier tube 54 energized by light from the screen 65, so that the image brightness is kept constant. Operation for fluoroscopy.-When the handle 97 is operated to bring mirror 93 into position for fluoroscopic viewing, a switch 64 is opened and contacts 66a, 66b of a switch 66 are closed. When a foot switch 85 is pressed, a time delay relay 73 is energized which, after an interval, closes contacts 74 and energizes a relay 83. This closes contacts 83a, which connect the A.C. power supply to transformer 28 through a contactor circuit 25, and the X-ray tube 10 is energized as long as switch 85 is held closed. The MA stabilizer 32 is connected through contacts 81a to the variable resistor 34, so that the tube current is kept constant as explained above. Operation for cinefluorography.-The mode of operation is changed from fluoroscopy to cinefluorography by operating handle 97, to remove mirror 93, to close switch 64 and to close contacts 66a, 66c of switch 66, thus energizing relays 75, 77. Motor 112, which is energized through contact 66c and relay contacts 75a, 77b drives a commutator 113 as well as the film driving mechanism. The commutator has conductive segments 115 which periodically complete a circuit through brushes 118, 120 to actuate contactor 25, which causes tube 10 to be energized periodically in synchronism with the film movement. Contact 77b is also connected to a time delay 88 which, after an interval, completes a circuit energizing relays 79, 81, 83. Contact 83a closes and applies power to contactor 25. A signal from the photomultiplier tube 54, which varies with the brightness of the image on the screen 65 is applied to a brightness pre-amplifier 48. The amplified D.C. voltage from the output of the pre-amplifier 48 is applied to a circuit termed the KV stabilizer 41, and also, through contact 81b, to the MA stabilizer 32. The MA stabilizer thus tends to keep the image brightness constant, but its effect is insufficient in itself, and is therefore reinforced by the effect of the KV stabilizer 41. This includes a valve 42, the grid of which is connected by contacts 79a to the brightness pre-amplifier 48. The anode 43 is connected to a D.C. voltage source through the D.C. control winding 49 of a saturable reactor 47. Variation in the output signal of the brightness pre-amplifier produces corresponding variation in the anode current of valve 42, and therefore a variation in the impedance of the A.C. windings 50 of the saturable reactor 47. The windings 50 are connected in series with a resistor 52 across the centre-tapped secondary winding 51 of a transformer supplied with A.C. The primary winding of a transformer 53 is connected across half the secondary winding 51 and a resistor 52. Thus, the transformer 53 will be supplied with A.C. of constant amplitude but of variable phase, depending on the impedance of the A.C. windings 50. The output from transformer 53 is fed, through secondary windings 57, to the contactor 25, which includes a pair of thyratrons connected inversely in parallel. The point in each cycle of A.C. at which the thyratrons conduct is thus dependent on the image brightness, and controls in turn the peak kilovoltage applied to the X-ray tube; and this action, together with the action of the MA stabilizer, keeps the image brightness constant throughout cinefluorography.
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