A New "Phase" for Biological Imaging

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 (Timm Weitkamp, timm.weitkamp@psi.ch), 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 centimeters in length), large field of view (up to 20x20 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 inch 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 rays 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. (Weitkamp et al., Optics Express, August 8, 2005; For background information, see "Phase Sensitive X-ray Imaging" in Physics Today, July 2000; graphics and more details here)