Text 17, 93 rader
Skriven 2004-08-01 13:25:59 av Herman Trivilino (1:106/2000.7)
Ärende: PNU 694
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PHYSICS NEWS UPDATE
The American Institute of Physics Bulletin of Physics News
Number 694 July 29, 2004
by Phillip F. Schewe, Ben Stein
                                
3D NEUTRON IMAGING FOR MEDICINE.  To take pictures of the body, medical
professionals conventionally use x rays, magnetic fields (MRI), ultrasound, and
in some cases, radioactive isotopes (PET scans).  At this week's annual meeting
of the American Association of Physicists in Medicine in Pittsburgh, Duke
University researchers presented the first 3D pictures (of an inorganic test
object) from a
new technique that employs neutrons.   Why use neutrons for medical imaging?
Compared to other particles, neutrons are highly penetrating, and therefore can
image deeply buried body structures that cannot be reached by other probes. In
addition, neutrons can easily identify almost every naturally occurring
chemical element in the body. Called Neutron Stimulated Emission Computed
Tomography (NSECT), the technique involves illuminating the body with fast
neutrons (those with energies between 1 and 10 MeV).  The neutrons cause the
nuclei of atoms and molecules in the body to emit gamma-ray photons with
distinctive energies that depend on the specific chemical identities of the
atoms and molecules to which the nuclei belong.  The only two elements that
won't show up on a NSECT scan are the lightest elements: helium, which emits
gammas at 25 MeV, and hydrogen, which has no excited nuclear states and
therefore does not emit gammas. At the AAPM meeting, Carey Floyd
(cef@deckard.duhs.duke.edu) presented the first 3D images ever reconstructed
from the emission of characteristic gamma rays stimulated by fast neutrons. The
images, of an iron-copper sample, demonstrate the technique's ability to
completely distinguish between the iron and copper that made up the object. 
With further development, NSECT could potentially diagnose breast cancer early
by looking for differences in the concentration of trace elements that are
known to exist between benign and malignant tissue.  Neutrons could identify
cancer by the way it changes concentrations of chemical elements in tissue long
before the cancer has begun to cause the anatomical changes (such as the
formation of dense tumors or microcalcifications) that are detected by
conventional methods.  While an individual neutron is more damaging to the body
than a single x ray of equal energy, the researchers' preliminary calculations
indicate that an accurate test for breast cancer could be performed at a dose
similar to that of a current mammography examination.  As an intermediate step
towards this goal, the group next plans to develop a prototype system that can
image the distribution of iron in the liver in order to diagnose
hemochromatosis (iron overload in the liver) without the need for a biopsy.
(Meeting Paper WE-D-315-6; lay-language paper with pictures at
www.aapm.org/meetings/04AM/VirtualPressRoom/NeutronImaging.pdf)
                                                                               
        
SPEECH SCIENCE SINGS A NEW QUANTUM TUNE.  Physicists at King's College in
London (Barbara Forbes, forbes@phonologica.com) have devised the most precise
way yet of reproducing the natural resonance frequencies, or formants, of the
human vocal tract.  To achieve this, they apply the methods of wave mechanics,
more usually associated with quantum physics, to a classical acoustics problem
for the first time.  Their results may lead to better speech recognition
devices, which currently do not take vocal tract physics into account and can't
adapt to natural human speech styles, such as ordinary conversation.  In their
paper, the researchers analyze a simple organ pipe, which speech researchers
often study to gain basic insights into sound production in the vocal tract. 
The researchers show that adding curvatures--dents or bumps--at optimal
positions in a straight organ pipe allows its natural resonance frequencies to
be shifted up or down, largely independently of each other. The analysis
substantially advances a  long-held 1878 result of Lord Rayleigh.  Using the
tools of classical physics, Rayleigh concluded that constricting an organ pipe
at an antinode (region of maximum air pressure) raised its resonance frequency
while expanding the pipe lowered it.  To simplify his analysis, he assumed that
denting or expanding the pipe would not change two key quantities of the air
inside it: the kinetic energy density (related to the average velocity of air
particles) and the potential energy density (essentially the air's degree of
compression, proportional to the square of the air pressure).  Because of this
assumption, Rayleigh could not take into account wave dispersion, in which a
pulse of sound (typically made of many sinusoidal waves each of a different
frequency) changes its shape as it passes through a region of pipe where the
wall is dented. In the new quantum-mechanics-based analysis, the researchers
are able to model this wave dispersion.  To do so, they examine how changing
the pipe cross-section alters the potential energy density of the air in the
vicinity of the dented pipe.  Of course, the acoustical system is a macroscopic
physical system and the wave functions within a pipe are real, measurable
quantities.  Therefore, quantum phenomena involving uncertainty and probability
do not arise in the acoustical case.  In their analysis, their biggest surprise
was to find, contra Rayleigh, that constricting the pipe exactly at a node
(region of minimum air pressure) does not make any contribution to shifting a
resonance frequency.  Instead, wave phenomena in the vicinity of the node cause
the shifts.  Since vowels in human speech can be distinguished by the relative
positions of the vowel's 2-3 lowermost resonance frequencies (formants), this
finding may provide a more sophisticated understanding of the physical
phenomena that create the characteristic sets of frequencies for all phonetic
sounds.  The researchers' more precise knowledge of the adjustments that can
alter a pipe's resonance frequencies may also provide a very robust and
efficient way of programming a machine to recognize natural phonetic sounds, a
line of research they are currently pursuing. They also intend to apply their
method to a structure that more closely approximates the physiological
conditions in the vocal tract.  (Forbes and Pike, Physical Review Letters, 30
July 2004)

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 * Origin: Big Bang (1:106/2000.7)