A new 'phase' for biological imaging

Friday, 30 December, 2005

Researchers have demonstrated a practical x-ray device that provides 2- and 3-dimensional images of soft biological tissue with details that are ordinarily hard to discern with conventional x-ray imaging.

Performed by researchers at the Paul Scherrer Institut in Switzerland and the European Synchrotron Radiation Facility in France, this work may help facilitate advanced medical applications of x-rays, such as the ability to detect cancerous breast tissue directly, rather than the hard-tissue calcifications that are produced in later stages of the disease.

X-rays excel at imaging hard tissue - such as teeth - as well as the contrast between hard and soft tissue - such as bones and skin in the human hand. However, x-rays are ordinarily not good at distinguishing between different types of soft tissue, such as normal and cancerous breast cells.

Optics researchers have long shown that x-rays have the potential to image different kinds of soft tissue through a technique known as 'phase' imaging. When an x-ray encounters the boundary of two types of material, such as normal tissue and cancerous tissue, it will undergo a 'phase shift': the peak of the wave will move backward by a small amount relative to the position where it would be if there were no sample in the beam. By measuring the phase shifts as x-rays pass from one type of soft tissue to another, researchers can distinguish between the two, and can produce a practical image unattainable before.

While phase-based imaging devices have been previously constructed, none has yet been widely adopted for medical diagnosis. The new device has three attributes needed for widespread medical use - compact size (only a few centimetres in length), large field of view (up to 20 x 20 cm2), and the ability to use polychromatic x-rays rather than more difficult-to-obtain monochromatic sources.

The main innovation in the new design is that it uses a pair of gratings - each a thin slab of material with narrow, closely spaced parallel lines etched deeply into them, like little slits carved into the millimetre marks of a ruler. As they pass through the object to be imaged, the x-rays undergo a series of phase shifts. Passing next through the first grating, the x-ray's stream is diffracted into multiple waves that combine and interfere to produce a series of fringes (bright and dark stripes). The second grating extracts from this pattern precise information on the inner details of the object. Using this technique, the researchers imaged a small spider, revealing internal structures that would be difficult to image with any other method. The researchers believe that the modest requirements of this technique, in terms of the x-ray source, laboratory space and materials, may make phase-based imaging practical for a wide range of biological and medical applications.

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