Comparison of low contrast detectability between a digital amorphous silicon and a screen-film based imaging system for thoracic radiography

Low contrast threshold detectability is investigated theoretically and expe rimentally for an amorphous silicon (a-Si) x-ray detector designed for digi tal radiography and for a standard thoracic screen-film combination. A theo retical signal-to-noise ratio is described with a human observer signal det ection model. It is calculated using the detective quantum efficiency (DQE) and the modulation transfer function of the imaging system, as well as a s patial response function for the human visual system. Using a four-alternat ive forced choice observer perception study, the threshold contrasts of dis k shaped objects ranging in size from 0.3 to 4.0 mm are determined. Signifi cantly better contrast detectability is obtained from the digital detector, which is attributed to its higher DQE. On average, the disk shaped objects can be detected at 45% less contrast than required for screen-him. With no free parameters the experimental data agree well with those predicted by t he observer model. Based upon the data, the model predicts that x-ray expos ure for the a-Si detector only needs to be 30% of the exposure required to perform equally well on the contrast-detail detectability task using screen -film. (C) 1999 American Association of Physicists in Medicine. [S0094-2405 (99)01507-2].