Patent GB-1503938-A (GB)
Title: Method and apparatus for selecting x-ray measurement fields suitable for controlling an x-ray exposure
IPC: H05G1/26
Filed: 1975-03-07
Abstract: 1503938 X-ray exposure control PHILIPS ELECTRONIC & ASSOCIATED INDUSTRIES Ltd 7 March 1975 [12 March 1974] 9578/75 Headings GIA and G1U Apparatus for selecting x-ray measurement fields suitable for controlling a radiographic exposure of an object in a radiographic apparatus utilizes an x-ray dose or dose-rate measuring assembly having a plurality of measuring fields e.g. an ionization chamber and includes a circuit for determining the maximum and minimum dose incident on the measuring fields, for switching off measuring fields not exposed or totally exposed to the x-radiation and for determining those measuring fields in which the dose is within a predetermined range of the mean dose in the fields not switched off. The device includes a television camera providing a video signal of a fluoroscopic image which signal is fed to a peak rectifier 1 which derives maximum value of the video signal during a first frame which value is used to determine the reference voltage αU applied to comparator 2. Subsequent video signals in excess of this reference cause, via the comparator, closure of switch 3 whereby the capacitors of a capacitor matrix 5 are connected to a current source 4. The capacitor connected to the source is determined by the video horizontal and vertical sync. pulses whereby a specific column and row Fig. 2 are selected by appropriate switches in conductors 8, 9 so that the capacitor, associated with a measuring field corresponding to the area of the television image scanned by the video signal, is selected. The voltages to which the capacitors are charged are compared with a reference voltage by comparator 7 which acts with store 6 so as to cause those measuring fields for which the capacitor voltage exceeds the reference value to be deactivated for the subsequent exposure. The value is chosen so that those measuring fields which are exposed to direct x-ray radiation are excluded from those determining the x-ray exposure. Similarly those measuring fields which are not exposed at all to x-ray radiation at the particular diaphragm setting obtaining are also to be excluded. To this end the minimum value of the video signal is determined by inverting the video signal so high amplitude corresponds to low dose and then passing the signal to the rectifier 1 and so on as before. Next, the mean brightness (and hence mean-rate dose) is determined by an integrating circuit constituted by amplifier 10, resistor 11 and a selected capacitor connected between the output and inverting input of the amplifier. This circuit forms a mean value of the video signal which is applied via input 21 to the resistor 11 which mean value is stored in the selected capacitor, whereby the mean value of the video signal is 'written' into the various capacitors during an image frame. Subsequently the highest and lowest capacitor voltage, corresponding to the highest and lowest dose in the measuring fields which have not been excluded, are determined by a circuit as in Fig. 3. Thus capacitor 15 is initially charged to a high value and capacitor 15 is discharged. Capacitor charging voltages for the matrix 5 are fed via conductor 12, and the charging voltage for the first capacitor (top left, Fig. 2) since it is lower than the initial charge on 15 causes the appearance of a logic L at the output of comparator 16 and is fed to one input of AND gate 17. Provided that the associated measuring field is not one of those to be excluded from the determination of exposure, the other input of the AND gate 17 also receives a logic L so switch 3 is closed to charge the capacitor 15 to the voltage on conductor 12. When the next matrix capacitor is selected, depending on whether its voltage is higher or lower than that of the previously selected capacitor the switch 13 or 13' will close to charge capacitor 15 or 15' to charge to a higher or lower voltage respectively. Thus at the end of the image frame the maximum capacitor voltage of the matrix 5 is stored in the capacitor 15 and the minimum capacitor voltage is stored in the capacitor 15'.