---------- Forwarded message ---------- Date: Fri, 12 Aug 2005 11:25:03 -0400 From: [EMAIL PROTECTED] To: [EMAIL PROTECTED] Subject: Physics News Update 741
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, [EMAIL PROTECTED]), 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, [EMAIL PROTECTED], 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) *********** PHYSICS NEWS UPDATE is a digest of physics news items arising from physics meetings, physics journals, newspapers and magazines, and other news sources. 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