There is the most suitable photon energy in digital mammography depending on breast thickness and density. The most suitable photon energy is measured by CNR and SDNR. The purpose of this study is to construct a system for calculating CNR in digital mammography by simulation and to analyze the mechanism in which the most suitable photon energy is produced. Using the system to calculate CNR, we obtained the following results. Due to the effects of EDR, simultaneously, the contrast became constant and there was a negative peak of noise. When the negative peak of noise appears, photon energy accorded with the maximum value of CNR. In addition, we examined the effects of scattered radiation or a fixed noise on CNR.
