Effective dose in diagnostic radiology as a function of x-ray beam filtration for a constant exit dose and constant film density.

Individual organ absorbed dose and total effective dose for nine common radiographic projections were investigated as a function of half-value-layer, HVL, and total equivalent filtration for the following cases: (1) with the patient exit dose held constant and (2) with the film density held constant. As expected, the absorbed dose to organs proximal to the x-ray beam entry point tracked with skin dose as a function of HVL, whereas organ dose distal to the x-ray beam entry point was almost independent of HVL. Dose to organs near mid-line showed an intermediate HVL dependence. For the nine radiographic projections, increasing the total filtration from 1.5 to 4.0 mm Al while holding the kVp fixed resulted in mean decreases in the effective dose of 17% for the case of a constant exit dose, and 25% for a constant film density with a "400 speed" rare-earth screen-film system. The decreases in the mean skin entrance doses were 38% and 45%, respectively. With the screen-film system, the average effective dose decreased at 16% per mm of added Al between 1.5 and 2.5 mm Al total filtration, and at 7% per mm between 2.5 and 4.0 mm. These results partially support the NCRP Report No. 102 recommendation that the minimum filtration be 2.5 mm Al for general diagnostic x-ray tubes. They also suggest, using the linear no-threshold radiation risk model, that further significant reductions in stochastic risk to the U.S. population can be achieved by raising the minimum beyond 2.5 mm. Experience over a 12 year period in our tertiary care teaching hospital indicates that adding 1-1.5 mm Al filtration beyond the 2.5 mm minimum does not pose a problem in terms of additional tube loading or reduction in image quality. However, these issues need to be more formally addressed.