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Skriven 2005-08-13 14:38:26 av Herman Trivilino (1:106/2000.7)
Ärende: PNU 741
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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@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@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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