Checking diagnostic X-ray beam quality

Abstract

A radiographic method is described for comparing the effective kilovoltage and inherent filtration of diagnostic X-ray machines. The apparatus consists of a modified X-ray cassette covered by a sheet of Pb perforated with 2 rows of holes; 1 row is associated with a strip of slow intensifying screen and provides a reference exposure, the other row has a strip of fast intensifying screen and a Cu step wedge. The thickness of Cu needed to give a film density which matches that of the reference exposure is called the Cu reference number and provides an index of the beam quality. The device is calibrated against a constant potential generator for which the kV values are accurately known. For determining the effective kV of a machine the beam is attenuated and hardened at source by means of a filter of 1.5 mm of Cu, corresponding approximately to the filtration (of the primary beam) provided by an abdomen plus Bucky grid, plus table top. The method is relatively independent of processing, exposure and film density. Further advantages are small size and the use of normal radiographic exposures on intact X-ray equipment. An accuracy of 1 kV is attainable.