A New "Phase" for Biological Imaging

Number 741 #1, August 12, 2005 by Phil Schewe and Ben Stein
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@xxxxxx), 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)

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