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MRI On the Cheap and On the Go
Berkeley, CA, September 5: When we hear the
term “MRI,” most of us probably think of a special treatment room in a hospital
with a huge doughnut-shaped machine that costs a lot of money and makes a lot of
noise. Researchers with the U.S. Department of Energy’s Lawrence Berkeley
National Laboratory (Berkeley Lab)
are looking to change that perception with the successful testing of a
laser-based MRI technique that would make the technology compact and portable,
relatively cheap, and quiet.
“We have developed a novel approach for the
detection of MRI based on optical atomic magnetometry,” said chemist Alexander
Pines, one of the world’s leading authorities on NMR/MRI technology. Pines holds
a joint appointment as a chemist with Berkeley Lab’s Materials Sciences Division
and with UC Berkeley, where he is the Glenn T. Seaborg Professor of Chemistry.
“Our technique provides a viable alternative for MRI detection with
substantially enhanced sensitivity and time resolution for various situations
where traditional MRI is not optimal.”
Pines led the development of this new MRI
technique along with Dmitry Budker, who holds a joint appointment with Berkeley
Lab’s Nuclear Science Division and UC Berkeley’s Physics Department. Shoujun Xu,
a member of Pines’ research group, conducted the MRI measurements. The three
were co-authors of a paper about this technique which appeared in the Aug 22
edition of the Proceedings of the National Academy of Science (PNAS). Other
authors of the PNAS paper were Valeriy Yashchuk, Marcus Donaldson and Simon
Rochester.
MRI, which stands for magnetic resonance imaging,
and its sister technology, nuclear magnetic resonance (NMR) spectroscopy, are
based on a property of atomic nuclei with an unpaired proton or neutron called
“spin.” Such nuclei spin on an axis like miniature tops, giving rise to a
magnetic moment, which means the nuclei act as if they were bar magnets with a
north and south pole. When exposed to an external magnetic field, these
spinning "bar magnets" attempt to align their axes along the lines of magnetic
force. Since the alignment is not exact, the result is a wobbling rotation, or
“precession,” that’s unique to each type of atom.
If, while exposed to the magnetic field, the
precessing nuclei are also hit with a radiofrequency (rf) pulse, they will
absorb and re-emit energy at specific frequencies according to their rate of
precession. When the rf pulse is combined with magnetic field gradients, a
spatially encoded signal is produced that can be detected and translated into
images.
Obtaining a spatially encoded MRI signal from a
sample depends upon polarizing the spins of its precessing nuclei so that an
excess points in one direction, either “up” or “down.” Conventional MRI
technology uses an exceptionally strong external magnetic field to produce a
detectable signal. The stronger the magnetic field, the stronger the signal,
which means a large and expensive cryogenic high-field magnet.
A smaller magnet results in less polarization and
a weaker MRI signal, which therefore requires a more sensitive means of signal
detection. One alternative being explored is the use of SQUIDs (superconducting
quantum interference devices), which can detect the faintest of magnetic signals
but must be cooled to a temperature of near absolute zero. This requirement
makes SQUIDs expensive and somewhat tricky devices to use. It also limits the
situations in which they can be effectively deployed.
The alternative MRI technology being developed by
Pines, Xu, Budker and their colleagues is also highly sensitive to low-field
magnetic signals but offers the enormous advantage of being operable at room
temperatures.
Said Xu, “Our technique has comparable
sensitivity with SQUIDs, but the fact that it does not require superconducting
magnets or cryogenics significantly reduces the cost and maintenance of the
apparatus, and opens the technology up to a broader range of applications.
Furthermore, our technique has simple electronics that can be easily integrated
into detector arrays.”
This new laser-based approach to MRI is derived
from two technological advances. One, developed by the Pines’ research group,
physically separates the two basic steps of MRI, signal encoding and detection.
Physically separating these two steps enables each to be optimized for
sensitivity. The other advance, developed by the Budker research group, is a
highly sensitive atomic magnetometer that’s based on a phenomenon called
“nonlinear magneto-optical rotation.” With this magnetometer, a sample of
alkali atoms featuring a single unpaired electron is vaporized in a glass cell.
The unpaired electron makes the atoms themselves act like spinning bar magnets,
with a magnetic moment three orders of magnitude stronger than that of
precessing nuclei.
A beam of laser light “pumps” the atoms so that
their spins are polarized, then “probes” the polarized atoms for an MRI signal.
According to Budker, instead of the multimillion
dollar costs of a conventional MRI system, this alternative MRI technology would
cost only a few thousand dollars to implement.
“Our system is fundamentally simple and does not
involve any single expensive component,” Budker said. “We anticipate that the
whole apparatus will become quite compact and deployable as a battery-powered
portable device.”
In the MRI system that the Berkeley researchers
tested, the fluid to be imaged, water, was passed through two small cells for
signal encoding, then transported to a U-shaped detection area for interrogation
by a pair of Budker’s magnetometers. The magnetometers were oriented so that
they detected the MRI signal with opposite signs. This configuration
dramatically improved the signal-to-noise ratio, enabling the researchers to
detect an MRI signal from microliters of water in 0.1 second without the
presence of a strong magnet.
“We are continuing to optimize our system, in
both sensitivity and detection efficiency, to make this technique suitable for
microfluidics and biological objects with sizes in the micrometer regime,” said
Xu. “In addition, further consolidation of the apparatus is underway so that the
whole setup becomes portable and therefore can be conveniently utilized as an
in-line analytical instrument for monitoring chemical reactions and biological
processes.”
Berkeley Lab is a U.S. Department of Energy
national laboratory located in Berkeley, California. It conducts unclassified
scientific research and is managed by the University of California. Visit our
Website at www.lbl.gov.
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