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High-Contrast X-Rays

Dark-field x-ray imaging could make for more-accurate mammograms and better security screens.

Swiss researchers have demonstrated the practicality of a new high-resolution x-ray imaging technique that reveals fine structures that are invisible using conventional techniques. Dark-field x-ray imaging can be used to generate highly detailed images of bones and to distinguish between substances that look identical in conventional x-ray images, such as explosives and cheese. The researchers are now investigating whether their approach might also increase the resolution of medical imaging techniques such as mammograms and computed-tomography (CT) scans.

Hot wings: A new x-ray imaging technique (bottom) relies on information about how a sample scatters the radiation, providing a higher contrast image of the bones in a chicken wing than conventional x-ray imaging (top) does. Conventional x-ray imaging relies on information about how the wing absorbs radiation.

Franz Pfeiffer, assistant professor of physics at Ecole Polytechnique Fédérale de Lausanne, in Switzerland, who developed the new technique, compares conventional x-ray images with shadows. Such images rely on information about how much radiation is absorbed as it passes through a sample, such as a patient’s limb. But more-complex interactions are happening, says Pfeiffer, and the more information that can be gleaned about these interactions, the better the contrast of the images. Dark-field imaging measures how a sample scatters light.

“These guys are showing that you can do things with x-rays that were only thought practical optically [with visible light],” says Richard Lanza, a senior research scientist at MIT’s department of nuclear science and engineering.

Previously, researchers including Pfeiffer had demonstrated dark-field imaging using a large, expensive particle accelerator called a synchrotron as an x-ray source. Synchrotrons provide very bright, finely focused beams of x-rays. Such a powerful source was necessary because the inefficient crystal optics used to focus the x-rays onto the sample could only cope with a narrow spectrum of wavelengths.

To replace the inefficient crystal optics, Pfeiffer developed silicon filters that work with the full spectrum of rays generated by low-power, conventional x-ray tubes. These filters are flat discs of silicon etched with 20-micrometer-long slits, some of which are filled with gold. To generate scattering images, these grates are placed between the x-ray source and the sample, and between the sample and the detector.

“Small structures like micro-cracks show up nicely in these images because they scatter radiation quite a bit,” says Pfeiffer. This suggests that the images could be useful for detecting osteoporosis or for finding flaws in mechanical structures such as turbines.

“Edges and boundaries are more clear in the dark-field images,” says Elizabeth Brainerd, an evolutionary biologist at Brown University, who uses x-rays to study the biomechanics of bones. (See “Catching Evolution on the Run.”) It can be difficult to distinguish small bones and joints in conventional x-rays. Brainerd agrees that dark-field images could be useful for detecting small fractures and bone spurs in patients, and she’s excited about the possibility of extending Pfeiffer’s technique to three-dimensional CT scans.

Pfeiffer’s approach could be used to improve security systems too. Conventional x-ray imagers like those at airport-security checkpoints can’t differentiate between many different kinds of materials–for example, chocolate and cheese appear identical to some explosives. But cheese and explosives scatter x-rays differently, so in Pfeiffer’s dark-field images, the differences between the two materials are apparent.

Pfeiffer has already begun making CT scans with conventional x-ray tubes using another contrast-enhancing technique he developed two years ago, called phase contrast. He says that he’s currently working to incorporate gratings for dark-field imaging into conventional CT devices. He’s also collaborating with researchers at the Center for Biomedical Imaging, an institute run by the University of Lausanne and the University of Geneva, to determine whether dark-field x-ray imaging can be used to tell healthy tissue from cancerous tissue. Cancers don’t absorb x-rays very differently than healthy tissue does, so x-ray systems that rely on other properties, such as scattering, might make for better mammograms, for example. Lanza’s group at MIT is also working to develop better cancer-detecting CT scanners that use a combination of absorption and refraction for contrast and also rely on nanofabricated gratings. (See “Changing the Physics behind X-Ray Imaging.”)

Dark-field imaging has been used for more than 20 years to enhance contrast and resolution in conventional optical microscopes. But applying the contrast-enhancing techniques that work well with visible light to x-rays has taken a long time, says Pfeiffer. Such a system is only now possible thanks to advances in photolithography and many years of basic science research using synchrotrons, he says.

Pfeiffer envisions that future x-ray imaging systems will be like what light microscopes are today: they will incorporate many complementary systems for enhancing contrast–absorption, refraction, scattering–and doctors and researchers will be able to use whichever combination works best for a given sample.

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