Reciprocity law failure with film/screen combinations for mammography

Abstract

In radiology the validity of the reciprocity law is usually assumed: the film density for a given X-ray spectrum depends only on the tube exposure, and not on the exposure time. This reciprocity law holds for non-screen films. For screen/film combinations, where most of the film density is due to exposure to light from the screen, reciprocity law failure may occur. For materials used nowadays in general radiology the effect is negligible for exposure times from 0.005 to 1 s. During the course of measurements on film/screen combinations used in [human] mammography, exposure times of up to 20 min had to be used. Therefore reciprocity law failure of these materials had to be investigated. The results for 4 common film/screen combinations are reproduced. Included for comparison are measurements on a double-coated film used for general radiology, and a 2nd curve for 1 of the mammography systems. Although the exposure times used in the experiments are much longer than exposure times used during routine mammography, the results indicate that reciprocity law failure might not be negligible at exposure times of a few seconds common in clinical routine.