Dynamic range requirements in digital mammography

The dynamic range and the number of gray levels, gamma s, required for digital mammography has been evaluated using an energy transport model. The effects of molybdenum (Mo) and tungsten (W) target spectra and the energy-dependent attenuation by elemental filters, breast tissue, and a phosphor screen were included in the model. For detectors with ideal optical coupling and no inherent detector noise, 3,100 gray levels are discernable (requiring 12 bits per pixel), assuming a 40 kVp, W target spectrum (1.0 mm A1 filtration), a mean glandular dose to a 5 cm thick breast of 0.6 mGy, and an ideal observer with a 5 mm diam viewing aperture. The effects of inherent detector noise and realistic coupling efficiency on gamma s were also examined. For the 40 kVp, W spectrum, a detector with total coupling efficiency of 16 electrons (e-) per x-ray interaction and a dynamic range of 3000 (maximum carrier signal of 1.93 x 10(5) e-/pixel and inherent detector noise of 64 e- pixel) would decrease the number of gray levels that could be resolved by only 2% compared to a detector with ideal coupling and no inherent noise. A detector with a total coupling efficiency of 2.0 electrons per x-ray interaction and a dynamic range of 240 (maximum carrier signal 2.41 x 10(4) e-/pixel and inherent detector noise of 100 e-/pixel) would reduce the number of gray levels by 26% for the 40 kVp spectrum. On the basis of dynamic range, W spectra are preferable for digital mammography, since Mo spectra yielding the same signal-to-noise ratio require a detector with dynamic range twice as large, and with a 30% greater saturation signal. When no scatter rejection method is used, scattered radiation over a 254 cm2 imaging field reduces the number of discernable gray levels by 23% for a 5 cm thick breast and 34% for an 8 cm thick breast.