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 commo n radiographic projections were investigated as a function of half-val ue-layer, HVL, and total equivalent filtration for the following cases : (1) with the patient exit dose held constant and (2) with the film d ensity held constant. As expected, the absorbed dose to organs proxima l to the x-ray beam entry point tracked with skin dose as a function o f HVL, whereas organ dose distal to the x-ray beam entry point was alm ost independent of HVL. Dose to organs near mid-line showed an interme diate HVL dependence. For the nine radiographic projections, increasin g the total filtration from 1.5 to 4.0 mm Al while holding the kVp fix ed resulted in mean decreases in the effective dose of 17% for the cas e of a constant exit dose, and 25% for a constant film density with a ''400 speed'' rare-earth screen-him system. The decreases in the mean skin entrance doses were 38% and 45%, respectively. With the screen-fi lm 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 betwe en 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 gener al diagnostic x-ray tubes. They also suggest, using the linear no-thre shold radiation risk model, that further significant reductions in sto chastic risk to the U.S. population can be achieved by raising the min imum beyond 2.5 mm. Experience over a 12 year period in our tertiary c are teaching hospital indicates that adding 1-1.5 mm Al filtration bey ond 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. (C) 1998 American Association of Physi cists in Medicine.