Signal to noise ratio and radiation dose as function of photon energy in mammography

This study investigated the signal to noise ratio (SNR) and radiation dose as a function of photon energy in screen-film mammography. An analytical expression was derived for the SNR for monoenergetic photons incident on simulated masses and microcalcifications embedded in uniform slabs of Lucite. The mean dose, D, was determined by dividing the total energy deposited in the Lucite phantom by the corresponding phantom mass. SNR and dose data for different photon energies were normalized to a constant value of energy absorbed by a 34 mg/cm ² Gd ₂O ₂S screen. A figure of merit (FOM), defined as SNR ²/D, permitted the optimum photon energy to be determined for each imaging task. For microcalcifications, the optimum energy was dependent on the size of the microcalcification, and increased from 19 keV for 100 μm to 22 keV for 500 μm imaged in a 4 cm Lucite phantom. The optimal photon energy for microcalcifications was 16-19 keV for a 2 cm phantom, increasing to 24-26 keV for an 8 cm phantom. For simulated masses of all diameters (2 mm to 10 mm) and thickness (0.15 to 0.6 mm), the optimal photon energy was ∼ 17 keV for a 2 cm phantom, and increased to ∼32 keV for an 8 cm phantom.