Measurement of HVL on fluoroscopy units

The total filtration of a fluoroscopy x-ray tube assembly may be deduced from the half value layer (HVL) of the x-ray beam at a specified tube voltage using published data tables. The HVL is usually measured by placing increasing thicknesses of Al plates between the focus and a dosimeter, and measuring the output while keeping the kV constant. However, on many of the newer fluoroscopy units, it is not possible to set the tube potential manually. This can create difficulties when measuring the HVL since the increasing thicknesses of aluminum trigger the automatic brightness control to adjust the kV and mA automatically, thus altering the tube output and HVL. This communication describes a simple but effective method to measure the amount of filtration on such units, which has the advantage of not requiring any extra equipment.