Optically coupled digital radiography: sources of inefficiency

Imaging systems comprised of a small detector which "looks" at a large area of a light emitting phosphor with a lens coupling system are commercially available. These imaging systems can be quite inefficient depending on the characteristics of the optical coupling. We have implemented an optically coupled imaging system in our laboratory for use as a test bed. The system includes a Kodak MR-2190 screen and a digital detector, i.e., a CCD camera. The laboratory system has provided the ability to investigate when a system of this configuration starts to develop a "secondary quantum sink" as a result of poor optical coupling. Experimental measurements have been made of the large area gray-scale transfer, resolution and noise properties of the imaging system at two different values of optical demagnification factor and three levels of x-ray exposure. The values of the detective quantum efficiency calculated from these measurements demonstrate a significant increase in absolute value and bandwidth when the demagnification decreases by a factor of approximately two.