Signal-to-noise properties of mammographic film-screen systems

The signal-to-noise ratio (SNR) and the detective quantum efficiency (DQE) have been experimentally determined as a function of spatial frequency for several mammographic film-screen systems. These two parameters were determined from our measurements of noise power spectra and sensitometric properties of each system along with modulation transfer function (MTF) data for the screens which were obtained from others. From the noise power spectra, it was found that film noise contributes significantly to the total noise of mammographic film-screen systems, comprising 30%-50% of the total noise at 1 cycle/mm and as much as 75% at 5 cycles/mm. All systems had approximately the same SNR below 1.5 cycles/mm, but differed at higher frequencies due to differences in screen MTF and in the gradient of the film's sensitometric curve. The DQE curves varied between systems at all frequencies, however, due to differences in system speed, MTF, and gradient. Generally, the DQE of mammographic film-screen systems is between 10%-30% at frequencies below 1 cycle/mm and decreases to about 1% between 8 and 12 cycles/mm. Compared to film-screen systems used in general radiography, mammographic systems have similar DQE values at low frequencies, but are superior at higher frequencies.