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 ΔXmin, 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 ΔXmins in various conditions, the permissible upper limit of scatter fraction for obtaining the same ΔXmin 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 ΔXmin 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.