Measurements of automatically controlled exposure rate in radiodiagnostic screening units

Abstract

Two similar surveys have been conducted in hospitals in Glasgow and the Mersey Region in order to investigate the levels of exposure rate at which the automatic controls were set on radiodiagnostic screening units. The results from both surveys give rise to concern. The operation of such automatic systems depends upon a feedback device which monitors either the light output from the image intensifier or the video signal level and adjusts the tube factors so that the monitored quantity is maintained at a certain preset level. This level may be expressed in terms of the exposure rate in μR sec⁻¹, at the input phosphor of the intensifier. Normally the manufacturers offer a facility to switch between two or three different levels claimed to lie at specific values between 20 and 100 μR sec⁻¹ at the input phosphor. A further automatic control is also normally fitted in the television chain to maintain a constant monitor image brightness. This may well disguise mis-settings and malfunctions of the primary automatic control, so that high settings of the automatic exposure rate control may still give rise to an image, on the television monitor, of normal appearance. In order to investigate the operation of automatically controlled systems, measurements of the exposure rates at the intensifier have been made on a variety of units in the two areas.