PHYSICS NEWS UPDATE
The American Institute of Physics Bulletin of Physics News
Number 741   August 12, 2005
by Phillip F. Schewe, 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{at}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 cm^2), 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, text available at
http://www.opticsexpress.org/abstract.cfm?URI=OPEX-13-16-6296; For
background information, see "Phase Sensitive X-ray Imaging" in
Physics Today, July 2000; graphics and more details at
osa.org/news/release/08.2005/contrast_imager_newphasemed.asp)

PHOTONIC CRYSTAL ACCELERATOR.  At many universities and national labs,
electrons are accelerated to high speeds by electric fields imparted by
gusts of microwaves.  The cavities in which these microwaves are delivered
(by devices called klystrons) will support a main radiation mode and other,
wakefield, modes, or overtones, as well.  For example, at SLAC, which uses
microwaves at a frequency of 3 GHz, the presence of the overtones is not a
big problem, but for future machines, such as the prospective Next Linear
Collider (30 miles long), problems could arise.  If this machine were to
operate with superconducting equipment, overtones might eject electrons
from the main beam, causing them to smash into the sides of the
accelerating channel, causing a loss of superconductivity and the shutdown
of the accelerator.  Now, however, physicists at MIT have used photonic
crystals, material structures which allow the passage of light at some
frequencies but not others, to greatly limit overtones in an accelerator
cavity.   This represents the first time a photonic crystal (also referred
to as a photonic bandgap, or PBG) structure has acted as an accelerator. 
Furthermore, in this case the acceleration gradient, an important measure
of an accelerator's efficacy, was 35 MeV/m.  This is twice the value one
normally obtains at the MIT linac being used for this test, where an
electron beam with an energy of 17 MeV was boosted by an additional 1.4 MeV
in the photonic-crystal structure, which consists of arrays of tiny rods
and operates at a frequency of 17 GHz.  The next step, says MIT scientist
Evgenya Smirnova (now at Los Alamos, smirnova{at}lanl.gov, 505-667-5634), is
to build a longer accelerator structure and use much more klystron power. 
With this, a much higher acceleration gradient should be possible. 
(Smirnova et al., Physical Review Letters, 12 August; lab website:
www.psfc.mit.edu/wab/novel-ele.html)

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