Nearly 15 years ago, University of Toronto’s John Rowlands helped pioneer what has become the state of the art in digital x-ray imaging—the active-matrix flat-panel imager. In the device, a layer of amorphous selenium (a-Se) converts incoming x rays directly to charge carriers that migrate, under the influence of an electric field, into an embedded array of thin-film transistors, amplifiers, and subsequent analog-to-digital converters. The digitized signal can then be displayed, processed, and stored.
The same kind of flat-panel system can also be based on indirect conversion, using both a phosphor layer that emits light when hit by an x ray and an array of photodiodes that convert the light into an electrical signal. But light scattering in the phosphor makes that a lower-resolution approach. (See the article by Rowlands and Safa Kasap in Physics Today November 1997, page 24 .)
In both cases, image quality is excellent, electronic...
