Reduction of patient dose in medical radiography by utilizing scattered X-rays: Relation between permissible limit of scatter fraction,viewer brightness,and perceptibility of vision

This paper proposes a new technique for reducing the patient dose when employing medical radiographs prepared by using screen-film systems. In this technique the patient dose can be reduced by employing scattered X-rays in order to obtain the same film density as that realized without the use of scattered X-rays. The minimum perceptible thickness difference ΔX_min, which can be recognized by liminal vision, was psychophysically calculated by considering the energy spectrum of incident X-ray, sensitivity spectrum of the screen layer, and the perception capability of human vision. From the calculated ΔX_mins in various conditions, the permissible upper limit of scatter fraction for obtaining the same ΔX_min for three kinds of luminances, and the fraction of reduction in the primary X-rays were determined.As an example of the results, when the object size required for perception is 1.3 mm, a scatter fraction up to 42% can be permitted at a density D of 1.0 for a luminance of 2548 cd m^?2. When we increase the luminance of the viewer from 478 cd m^?2 to 2548 cd m^?2, the upper limit of the permitted scatter fraction varies from 30% to 42% at a D of 1.0, i.e., the patient dose can be reduced by 17% under the same perceptibility of ΔX_min by utilizing scattered X-rays. This reduction can be successfully achieved by changing the lead content of the grid from 0.45 to 0.38 g cm^?2.