An investigation of X-ray equipment factors influencing patient dose in radiography.

Abstract

Dose audit is a quality control tool for assessing whether patient doses are kept as low as reasonably practicable. Limitations of audit for radiographic patients are the inherent imprecision in assessment and the lack of information provided for the identification of high levels of dose. In this report, four equipment parameters are identified as influencing entrance surface dose (ESD). They are film speed, radiation quality represented by the X-ray transmission through a 20 mm aluminium filter, attenuation of the table top and grid, and a geometric factor to account for inverse square law attenuation. They have been combined into a system speed index (SSI). This was measured for 17 X-ray installations. To test the influence of SSI on dose, ESD was measured using a phantom to simulate the attenuation for an anteroposterior view of the lumbar spine. A radiographic dose index (RDI) was defined as the ESD required to produce a reference optical density of 1.2 at 80 kV. For the measurements it was found that RDI was equal to 16.1/SSI (r = 0.960). The report recommends the use of SSI as an index of dose efficiency which can be used to interpret the results of patient dose audits and determine priorities for dose saving strategies.