Errors in measurement of kVp and exposure time from the presence of a pre-magnetization pulse.

Abstract

The correct execution of quality assurance measurements on diagnostic X-ray apparatus demands an understanding of the limits of accuracy of the instruments being used. This note describes source of error in the measurement of exposure time and of peak accelerating potential (kVp) when measurements are made with two items of test equipment in common use. The errors are particularly important for measurements at high values of kVp or at short exposure times. Modern 3-phase X-ray generators commonly utilize a pre-magnetization pulse before the start of the true X-ray exposure, in order to lessen switch-on transients. The generation and character of this pulse varies between generators. An example which is not untypical, from a Siemens Triplex Optimatic 723 generator, is a pulse of 10 ms duration and accelerating potential 30 to 40 kV lower than that during the main exposure. It is possible to observe this pulse directly using a potential divider or, more conveniently, to monitor the exposure waveform using a solid-state semiconductor detector. The latter method is used in the Raycheck exposure timer (R T Labs Ltd., Wells, BA5 2AW) which was used in this study to measure times and evaluate exposure waveforms. This instrument has been described by Bullen and Bye (1979) and uses a set of semiconductor diodes as the radiation detector.