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October 3, 2001
Scientists Image Material That Could Improve
MRI Technologies
EVANSTON, Ill. — Using a technique similar to
the magnetic resonance imaging (MRI) widely applied by hospitals
for medical diagnosis but at a resolution 10,000 times greater,
physicists at Northwestern University have gained insights into
a high-temperature superconductor that might one day benefit hospital
MRI technologies and the patients who rely on them.
The research, led by William Halperin, professor of
physics and astronomy, in collaboration with scientists from Northwestern,
Argonne National Laboratory and the National High Magnetic Field
Laboratory (NHMFL), was the highest magnetic field imaging experiment
ever conducted.
The findings, including the direct evidence of an
electronic Doppler effect, are reported in the Oct. 4 issue of the
journal Nature.
"Currently, hospitals use low-temperature superconductors in
MRI, but high-temperature superconductors — a relatively new
discovery — may be a better material, with the bonus of requiring
less cooling, thus reducing costs," said Halperin.
To advance its potential application in future technologies,
the researchers first need to understand the physical properties
of these materials and especially how they behave in the presence
of very large magnetic fields.
"The goal in the medical world is to get better
resolution in MRI technology in order to improve diagnoses —
the better the resolution the more detail for analysis," said
Halperin. "Our imaging method is a major technical advance
in the study of superconductors and one that has basic implications
for magnetic imaging."
For the first time, researchers were able to peer
into the cores of vortices — tiny electrical tornadoes swirling
around in the copper oxide compound, YBa2Cu3O7, the classic high-temperature
superconducting material. (The vortices result from magnetic fields
trapped inside the material.) The core of the vortex in the superconductor
is very much like the eye of a hurricane except that it is so small
it is hard to investigate. Each core measures only three nanometers
across (the width of about 10 atoms strung together). Extremely
high resolution — made possible by a large magnetic field —
was required to look inside the vortex core.
Halperin’s team took advantage of the recently
commissioned hybrid magnet in the Tallahassee Laboratory of the
NHMFL, which provided them with the highest steady magnetic field
in the world — a million times stronger than the Earth’s.
This allowed the scientists to distinguish the core from what is
around it.
All superconductors have two essential properties.
First, a superconductor, when cooled to its appropriate temperature,
conducts electricity without any resistance. In other words, an
electrical charge can flow without generating any heat, which means
no loss of energy. Second, a superconductor expels some of the magnetic
field inside it. In order to keep the desirable zero resistance
state, the magnetic fields remaining in the material (in the form
of vortices) need to be understood and controlled.
The most important component of the MRI technology
used in hospitals is the main magnet, which supplies the steady
magnetic field critical for high-quality imaging. Superconducting
magnets are the most widely used because they require less electricity,
thus reducing the cost of operation. Also important are the gradient
magnets that generate magnetic fields that vary in space. These
non-uniform fields make it possible to distinguish one region of
the body from another, generating three-dimensional images from
any angle and direction, in a non-invasive fashion.
In order to study superconducting vortices in tiny
crystals of YBa2Cu3O7, Halperin and his team relied on a similar
MRI technique. In this case, the hybrid magnet at NHMFL — weighing
34 tons and standing 22 feet tall — provided the steady magnetic
field, and the whirling electrical tornadoes — the vortices
in the material — provided the non-uniform magnetic fields.
The extremely high spatial resolution of their method
revealed new electrical and physical properties that the researchers
did not expect to find. They learned that the electrons behave very
differently inside the vortex’s core than outside. The core
is no longer superconducting due to the intense magnetic fields
generated by the vortex’s swirling "electronic winds."
Of particular interest was the observation that the
circulating electrical currents surrounding the vortex’s core
create an electronic Doppler effect. The acoustic version of this
effect is well known. For example, sound waves from the whistle
of an approaching train have a higher pitch than when the train
is receding. Similarly, an electron moving in the current circulating
around the vortex has its energy shifted upwards if it moves with
the current and downwards if it is moving against the current. The
electronic Doppler effect is key to understanding many properties
of high-temperature superconductors.
In addition to Halperin, other authors on the paper
are Vesna Mitrovic (principal author), Eric Sigmund and Nathan Bachman,
from Northwestern; Mathias Eschrig, from Argonne National Laboratory;
and William Moulton, Philip Kuhns and Arneil Reyes, from the National
High Magnetic Field Laboratory.
The research was supported by the National Science
Foundation.
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