A quantitative assessment of portal film contrast as a function of beam energy

Portal film contrast on a specially designed test phantom has been studied as a function of photon beam energy and object-to-film distance. The results provide important insights into the physical processes responsible for image contrast. In particular, theoretical calculations of Compton scatter reactions in the phantom can be used to predict visual film contrast. Good agreement between theory and experiment can be achieved by evaluating the double differential Compton cross sections [d sigma (E,theta)/dE d theta] in the test object without resorting to variable parameters or artificial normalization. These calculations demonstrate the importance of low-energy photons, object-to-film distance, and object size on portal film contrast.