By Davide Castelvecchi
From Science News, January 26th, 2008; Vol.173 #4 (p. 53)
Ordinary X-ray machines have their limits in screening luggage. For one thing, they can’t always tell the difference between plastic explosives and cheese. A new imaging technique, based on how materials scatter rather than absorb X rays, could improve airport security, and may even lead to better cancer detection.
In standard radiography, materials appear darker or lighter depending on how well they block X rays. Sometimes that’s not enough to tell materials apart. For example, cheese absorbs X rays about as readily as some plastic explosives do, says Franz Pfeiffer, a physicist at the École Polytechnique Fédérale in Lausanne, Switzerland.
In next month’s Nature Materials, Pfeiffer and his collaborators describe a new method for getting more information out of X rays. The team took inspiration from a standard trick used in microscopy to enhance the details of small, translucent objects such as certain insects. In what is called dark-field imaging, an arrangement of screens prevents light from passing straight through the sample and into the lens. Instead, only light scattered by the sample can enter the microscope.
Pfeiffer’s team placed a silicon wafer in front of its X-ray source. Plain silicon would let X rays through, but the wafer had a metal grating etched onto it, making it look like a venetian blind with micrometer-wide slits. The rays passed through the grating, and then the sample and two more gratings, before reaching a detector.
Depending on the sample’s composition, some of the rays went straight through, and some were deflected by a small angle, comparable to “a hair seen from 1 meter away,” says Pfeiffer.
The researchers took up to eight shots of each sample, each time shifting the third grating by a fraction of a micrometer. Computer analysis of the differences between these images enabled the researchers to separate the scattered X rays from those that went through. The effect was similar to viewing the sample from slightly different angles, says Pfeiffer.
While cheese is smooth, plastic explosive has a microscopic granular structure that increases the scattering of X rays, so its image appeared brighter in tests. The researchers also imaged chicken wings, with the bones revealed in greater contrast than with conventional X-ray imaging thanks to their porous structure. With improvements, the technique might detect tumors, which tend to scatter X rays more than healthy tissue does, Pfeiffer says.
While dark-field X-ray imaging has been done in the past, Pfeiffer says that the new technique is the first that could be practical to use with ordinary radiography equipment, as opposed to sophisticated and expensive research beams.
Steve Wilkins, a researcher at Australia’s Commonwealth Scientific and Industrial Research Organisation in Clayton South, calls the results “very promising.” However, Wilkins points out that right now the technique takes tens of seconds to produce an image. That’s too slow for applications such as medical imaging, he cautions, since a patient would need to stay perfectly still.