GB1328843A - X-ray apparatus - Google Patents

Abstract

1328843 X-ray exposure control PHILIPS ELECTRONIC & ASSOCIATED INDUSTRIES Ltd 8 Sept 1970 42942/70 Heading G1A X-ray exposure control apparatus, for use mainly in sectional (tomographic) radiography in which the exposure time is predetermined by the length of the scan, is designed so that the intensity of the X-ray beam may be controlled to compensate for the different builds of patients, which affect differentially absorption of the beam. The apparatus includes an X-ray tube 1 supplied through transformer 2 and auto-transformer 4, and an X-ray detector 20, e.g. an ionization chamber, located behind the object 19, sensing the intensity passing therethrough and associated with comparison circuitry to determine the adjustments to the cathode heater current and, if required, the high voltage to the X-ray tube to attain a predetermined dose or film exposure density. Sectional Radiography. When the apparatus operates for use in sectional radiography, the switch 23 is closed and 15 and 16 are open. The exposure is timed by switch 13, circuit 8 acts as an amplifier, and circuit 9 is arranged not to affect the detector output. The input to 10 is thus a dose-rate dependent signal, which is compared in circuit 10 with a pre-set value for dose-rate. Any difference results in the cathode current of the tube being altered (through 18) to tend to reduce the difference. However, the cathode heater current control is relatively slow acting and if differences above predetermined amount either negative or positive occur initially, the appropriate switch 5 or 7 is closed via amplifier 11 and switch 12. This causes the tube voltage to be changed, which has a much greater effect than current control, on the dose rate. If the difference does not go over the predetermined: amount, cathode current control is effective alone. Instead of two switches 5, 7 a greater number may be provided, or continuous control by use of a thyristor may be provided. In order to modify the predetermined level set in circuit 10 to allow for the change in image contrast produced when switching by 12 to a different supply voltage, a link is provided from 12 to 10 via switch 23. A link is also provided (not shown) from 13 to 10 to modify the setting therein when the pre-set time is changed. Conventional radiography. In this, switch 23 is open and 15, 16 closed, and automatic exposure time control is provided. 'In this mode, the device 8 integrates the detector output to obtain a dose representative signal. When the dose has reached a predetermined value set into device 8, the exposure is terminated by switch 13 via 15. During this mode, circuit 9 acts a differentiator so that a dose-rate dependent signal is again obtained at 10. Control of the high voltage or switch-off time is effected as described in Specification 1328376, but cathode heater control is not effective.