Low-cost x-ray imaging

Active-matrix flat-panel imagers (AMFPIs) represent the state of the art in medical x-ray imaging. They combine high sensitivity with the convenience and flexibility of digital imaging. For direct-conversion systems, a layer of amorphous selenium (a-Se) converts incoming x rays to charge carriers that drift, under the influence of an electric field, into an array of thin-film transistors. But because each pixel is addressed by its own tiny transistor, AMFPIs are expensive; a single unit typically costs between $100 000 and $200 000. University of Toronto’s John Rowlands and his team have now developed a low-cost alternative—the x-ray light valve. Like the AMFPI, the XLV relies on a-Se to convert x rays into charge. Unlike the AMFPI, it doesn’t measure the charge distribution directly. Rather, the XLV views the effect of charge on a liquid-crystal screen. The figure outlines the process. An x ray creates a cloud of electron–hole pairs in a-Se. With an electric field applied, some pairs drift toward oppositely charged surfaces of the photoconductor. The charge distribution induces a visible image in the liquid crystal, thanks to the crystal’s birefringence. The optical image is then digitized using an off-the-shelf scanner. Rowlands envisions the device as initially useful for static imaging and chest x rays, but research is directed toward fluoroscopy, real-time x-ray imaging. This month he presents the concepts behind his prototype at the Indo-US Workshop on Low-Cost Diagnostic and Therapeutic Technologies in Hyderabad, India. (R. D. MacDougall, I. Koprinarov, J. A. Rowlands, Med. Phys. 35, 4216, 2008) — R. Mark Wilson