Physics news update 616--December 4, 2002

December 04, 2002

ACOUSTIC MICROSCOPY

At this week's First Pan American/Iberian Meeting on Acoustics in Cancun, researchers presented results on acoustic microscopy, a burgeoning technique that could provide new kinds of medically useful information on biological tissue. Unlike many other microscopy techniques, acoustical microscopy can be performed on living tissue and even inside the body, with the use of small ultrasound probes. And unlike optical microscopy of biological specimens, acoustic microscopy does not require tissue staining.

In the technique, an ultrasound probe makes contact with a tissue sample, then yields an image based on how the tissue responds to the ultrasound. Although the resolution of acoustical microscopy is ultimately limited to about the cell level, rather than the molecular level (its maximum resolution is about 0.1 microns, about a hundredth of the width of a red blood cell), it can provide unique information on a biological tissue's mechanical properties. For many materials, the mechanical properties have a wider range of values than the optical properties, so the technique could come in handy for characterizing Alzheimer's plaques, to name one example. In principle, an acoustic microscope could also yield quick assessments on the pathology of skin lesions, without a biopsy and long before other techniques could provide information.

At the meeting, researchers described how acoustic microscopy is already advancing cardiology, specifically in the area of intravascular ultrasound (IVUS), in which a small ultrasound camera is threaded into the body to detect artery blockage. Using a scanning acoustic microscope to gather basic data on artery plaque, Yoshifumi Saijo of Tohoku University (saijo@idac.tohoku.ac.jp) and his colleagues are helping clinicians better interpret IVUS images. Employing knowledge from acoustical microscopy, Ton van der Steen (vandersteen@tch.fgg.eur.nl) of the Erasmus Medical Center in the Netherlands and colleagues have developed a clinical technique called IVUS elasticity imaging, which can detect vulnerable artery plaques, a hard-to-catch condition which kills up to 250,000 people every year in the US alone. (Session 1pBB at the meeting; Background information at http://www.acoustics.org/press/144th/Jones.htm and http://www.eur.nl/fgg/thorax/elasto/).

LONGEST ATOMIC STATE LIFETIME MEASURED FROM SPONTANEOUS DECAY IN UV

The internal state of an atom can change by absorbing or emitting bits of light. In a warm gas or plasma the electrons are frequently shuttling back and forth from one state to another. Some of these states are longer lived than others, though, because of extenuating circumstances. For instance, many transitions from an excited state to the ground state occur in nanoseconds, but some can last for tens of seconds or longer. Measuring the true lifetime of the longer-lived of these transitions is difficult for the simple reason that even when a sample of atoms is dilute, an atom is being bumped so often that de-excitations come about before the state decays radiatively.

When even the best laboratory vacuum on Earth is still too crowded for making such delicate measurements, persistent scientists turn to outer space. Tomas Brage of Lund University (Lund, Sweden), Philip Judge of the High Altitude Observatory at NCAR (Boulder, CO), and Charles Proffitt of the Computer Science Corporation (Baltimore, MD) resort to viewing excited atoms in the planetary nebula NGC3918 where, amid the wreckage of a dying star, there is enough energy to excite atoms but a density low enough (a few 1000 per cubic centimeter) that mutual pumping isn't a problem (see figure at http://www.aip.org/mgr/png/2002/171.htm). Using the Hubble Space Telescope, the three scientists looked at the emissions of excited triply ionized nitrogen atoms and observed a lifetime of 2500 seconds for one particular hyperfine transition. Why is this state so robust? Brage (tomas.brage@fysik.lu.se, 46-46-222-7724) says that angular momentum can be preserved in this transition only if, in addition to the electron emitting an ultraviolet photon, the nucleus itself flips over. Other than adding to basic knowledge about atomic physics, studies like these should provide spectroscopic information for studying the deaths of stars. (Brage et al., upcoming article in Physical Review Letters, probably 16 December.
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American Institute of Physics

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