An evaluation of three simple phantoms used in quality assurance of automatic exposure control

Full text: Different attenuators are used to assess the performance of x-ray systems with automatic exposure control (AEC). Three attenuators, copper, water and hardboard, were compared in this assessment. The Health Physics Service (HPS), which is part of the Department of Medical Physics and Bioengineering at Christchurch Hospital, New Zealand, has chosen to use hardboard as its patient equivalent phantom when testing x-ray equipment simply because it is generally easier to handle compared to water phantoms. Tests were conducted on the hardboard phantom in order to evaluate its effect on radiation in terms of attenuation equivalence, production of scatter and sensitivity to different kilovoltages compared to water or copper. Entrance and exit dose comparison tests were conducted using the AEC on a Philips Optimus 50 x-ray machine. A Keithley Triad Field Service Kit Model 10100A was used to record doses. The first set of tests were designed to ascertain the thickness of water and copper required to produce a similar mAs to the particular thickness of hardboard being examined. The tests were repeated with a film in a cassette which was placed in the bucky tray. Film optical densities were then measured. The hardboard phantom shows a very similar response to radiation as water and hence the human body and is therefore a suitable material for use in the construction of phantoms. Copper produces a very different radiation spectrum and its scattering effects are quite different to water or hardboard. For these reasons we consider copper to be unsuitable for use as a phantom when programming AECs. Copyright (2000) Australasian College of Physical Scientists and Engineers in Medicine