GB1489377A - Controlling x-ray sources - Google Patents

Abstract

1489377 X-ray exposure control PHILIPS ELECTRONIC & ASSOCIATED INDUSTRIES Ltd 9 Oct 1974 [12 Oct 1973] 43725/74 Heading G1A The voltage and current of an x-ray tube are controlled to give predetermined dose or exposure density as a result of an exposure of predetermined length by (a) Measuring the dose with the tube operating at V test , I test and using the exposure meter to calculate the current (I w1 ) needed at voltage V test to give the required dose. (b) mechanically or electrically forming a function (2) which passes through V test , I w1 and which represents voltage and current values for the required dose, (c) mechanically or electrically forming a function (1) which defines the operating region of the tube at the said exposure length. (d) determining the intercept of functions 1 and 2 with the lower voltage value (V min1 , I 1 ) and (e) setting these values on the apparatus ready for the next exposure. Preferably, as shown, the current is plotted as its logarithm. If the curve 2 is not completely accurate, the process may be repeated using initial values V m in 1 , I, and generating a function (3) (similar to (2) but displaced in the log I direction) from which operating parameters V min2 , I 2 can be determined. The mechanical determinations can be carried out using a jig by which the curve (2) or (3) can be moved parallel to the log I axis; curve 1 being fixed relative to the axes. In an electrical embodiment, x-ray tube 5 has its voltage controlled from 7 and its current from 9. The radiation intensity at the film is detected at 41 and fed to exposure meter 35 which calculates the tube voltage necessary to give the desired dose and supplies this at output 51. Circuits 69 and 32 give, for an input voltage V, the corresponding values of log I from curves 2 and 1 respectively of Fig. 1. Exposure timing is controlled by a duration signal from 47. For the test reading V test is supplied from input 21 to capacitor 85, an output log I test being produced at output 31 of circuit 32. During the exposure, the exposure duration signal at 63 causes circuit 61 to set itself to its input value of, log I test . After the exposure, switch 83 is closed, closing loop 17. Adder 55 receives the output from exposure meter 35, circuit 32 and circuit 61 (which supplies the difference between the value stored during exposure and the output of circuit 69) and the value of V in the circuit alters until it reaches V min 1 , which is stored on capacitor 85. This value may be used for subsequent exposures or may be refined by a similar procedure with initial values V min1 and log I test .