Scatter-to-primary ratio and absorption efficiency in screen-film and computed radiography systems

Purpose: To determine whether the scatter-to-primary (s/p) ratio, measured on a screen-film or computed radiographic (CR) system, varies with chemical composition or primary attenuation properties. Materials and methods: The s/p ratios of four Gd₂O₂S screens, three CaWO₄ screens and a CR screen, were determined under uniform irradiation conditions. One simulated low scatter conditions, such as in lung imaging, and the other simulated high scatter conditions as in abdominal imaging. The absorption efficiency of each screen was measured and calculated. Results: The measured s/p ratio varied within each set of screens. The s/p ratio was related to the absorption efficiency of primary radiation for the screen-film and CR system. No difference in the s/p ratio was seen between screen-film and CR systems when normalised for the primary absorption efficiency. Conclusions: Absorption efficiency of the imaging system is a major indicator of s/p ratio, for both screen-film and CR systems.