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Nylon may take X rays into digital age.

A century ago, the novelty of X rays captured the public's fancy. X-ray machines were everywhere: Anyone craving a "bone portrait" needed to go no farther than the local department store. So widespread were X rays that entrepreneurs even sold lead-lined underwear designed to protect the modest from the prying gaze of the X-ray camera.

Those early entrepreneurs would have loved a new material described in the Aug. 2 Science: X-ray-absorbing nylon. Researchers developed this material for a more high-tech purpose, however. The specially prepared nylon was created to be a key part of a system designed to make radiography-the most widely used diagnostic technique-digital. By eliminating the need for film, scientists could manipulate X-ray images by computer, send them by modem, and store them on disk-advantages shared by newer methods like magnetic resonance imaging (MRI).

The material, developed by Ying Wang and Norman Herron of the DuPont Experimental Station in Wilmington, Del., consists of tiny particles of X- ray-absorbing bismuth triiodide mixed into flexible nylon. Like many composites, this one combines the best of both compounds. Pure bismuth triiodide absorbs X rays well but is very difficult to spread into a thin film, Wang says. Nor can it handle the large electric field that would be necessary in a digital X-ray system. Nylon's properties make up for those deficiencies.

The new composite is actually a nanocomposite, which means that most of the particles are less than a billionth of a meter long. If the particles are too big-even millionths of a meter-the material loses its desirable properties and behaves more like pure bismuth triiodide.

The optimum concentration is about 50 percent bismuth triiodide by weight.

"There's a natural limit, since we're basically using nylon as a solvent," Wang says. "To go beyond that limit, you have to do some more work to chemically attach the particles to the nylon."

Increasing the concentration to about 65 percent would make the X-ray absorption close to that of selenium, the best candidate material so far.

Selenium, however, is toxic and can be difficult to prepare into thin films.

A digital X-ray machine that uses a nanocomposite would work something like a photocopier, Wang says. In a photocopier, a static charge is placed across a plate. The plate discharges when it's exposed to light, leaving a fine pattern of static wherever lettering or other images block the light. Powdered ink, or toner, then clings to the charges and is fused onto a piece of paper.

In a digital X-ray machine, the nanocomposite would replace the X-ray film and act like the charged plate in the photocopier. X rays passing through soft body tissue would cause the material to discharge accordingly. Some kind of electrical detection system-perhaps an array of transistors-would then transmit the remaining charge pattern to a computer.

An advantage of this system, says Clinton M. Logan of Lawrence Livermore (Calif.) National Laboratory, who is developing a different digital X-ray technique, is that the new imaging device would plug directly into existing hospital X-ray cameras.
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Title Annotation:nylon composite developed for use in digital X-ray machine
Author:Wu, Corinna
Publication:Science News
Article Type:Brief Article
Date:Aug 3, 1996
Words:503
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