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Updated: July 8th, 2008 05:26 PM CDT

Imaging Technology: A Discerning View

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The Economist
via NewsEdge Corporation

A new way of processing X-rays gives much clearer images

X-RAYS are the mysterious phenomenon for which Wilhelm Röntgen was awarded the first Nobel prize in physics, in 1901. Since then, they have shed their mystery and found widespread use in medicine and industry, where they are used to reveal the inner properties of solid bodies.

Some properties, however, are more easily discerned than others. Conventional X-ray imaging relies on the fact that different materials absorb the radiation to different degrees. In a medical context, for example, bones absorb X-rays readily, and so show up white on an X-radiograph, which is a photographic negative. But X-rays are less good at discriminating between different forms of soft tissue, such as muscles, tendons, fat and blood vessels. That, however, could soon change. For Franz Pfeiffer of the Paul Scherrer Institute in Villigen, Switzerland, and his colleagues report, in the April edition of Nature Physics, that they have manipulated standard X-ray imaging techniques to show many more details of the inner body.

The trick needed to discern this fine detail, according to Dr Pfeiffer, is a simple one. The researchers took advantage not only of how tissues absorb X-rays but also of how much they slow their passage. This slowing can be seen as changes in the phase of the radiation that emerges--in other words of the relative positions of the peaks and troughs of the waves of which X-rays are composed.

Subtle changes in phase are easily picked up, so doctors can detect even small variations in the composition of the tissue under investigation, such as might be caused by the early stages of breast cancer. Indeed, this trick--known as phase-contrast imaging--is already used routinely in optical microscopy and transmission electron microscopy. Until now, however, no one had thought to use it for medical X-radiography.

To perform their trick, the researchers used a series of three devices called transmission gratings. They placed one between the source of the X-rays and the body under examination, and two between the body and the X-ray detector that forms the image. The first grating gathers information on the phases of the X-rays passing through it. The second and third work together to produce the detailed phase-contrasted image. The approach generates two separate images--the classic X-ray image and the phase-contrasted image--which can then be combined to produce a high-resolution picture.

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