It’s really hard to hear what the brain is saying. Neural impulses --
currents of ions moving through channels between the brain’s 100 billion
neurons at a potential of about 0.1 volt -- produce magnetic fields in
the range of a trillionth of a tesla, about a 100 million times weaker
than Earth’s magnetic field.
That electrical activity can be
detected by electrodes placed on the scalp, or directly into the brain,
that measure the difference in voltage between different points. That
method, however, if used non-invasively, can’t identify the location of
the signal with sufficient accuracy for many uses.
“For
conditions such as epilepsy, doctors need to know where the epileptic
center is,” says Svenja Knappe of NIST’s Physical Measurement
Laboratory. “Magnetic signals detected outside the head can create
images with much higher spatial resolution, but the signals are
exceedingly faint.”
That is why Knappe’s team is at work on a new
kind of magnetometer that can detect fields as weak as a few femtotesla
(10-15 T, or quadrillionths of a tesla -- far smaller than the ion
currents generate), potentially at a fraction of the cost of
conventional systems.
The typical instrument used in
magnetoencephalography (MEG) is an array of a few hundred highly
sensitive field detectors called SQUIDs.* These devices operate near
absolute zero, and have to be immersed in liquid helium.
“It’s
like a gigantic helmet that’s largely filled with helium,” Knappe says.
“So it requires thermal insulation that keep the sensors a couple of
centimeters from the head, and it can’t be adjusted to accommodate
different head shapes and sizes. The prototypes we are building --
called microfabricated optically pumped magnetometers (µOPMs) -- do not
require cooling, and can be placed within a few millimeters of the
scalp.”
The devices measure the effect of magnetic fields on a
trapped population of rubidium atoms enclosed in a glass cell about a
cubic millimeter in size. A laser raises the temperature to about 150
°C, resulting in a vapor of a hundred trillion atoms. A specially
polarized, “pump” laser beam is directed through the vapor, aligning the
spins of all the atoms in the same direction. The same laser is used as
a “probe” beam: It shines through the vapor, out of a window in the
cell, and into a special detector that measures the polarization of the
arriving light.
In the absence of a magnetic field, the atoms’
spins would retain their original orientation, and the polarization of
the probe beam would remain unchanged. But when a field is present, it
twists the atoms’ spin orientation slightly. That, in turn, changes the
amount of the light entering the detector.
In their present form,
the Knappe team’s µOPMs have been tested against SQUID-based detectors
and have been shown to approach the limits of sensitivity of commercial
SQUIDs. But there is much more to be done on many fronts.
In the
near term, “we want to increase the dynamic range of those sensors and
improve the noise rejection,” says project scientist Abigail Perry. “A
screwdriver or some other metal implement in the neighborhood will have
much larger fields than we’re trying to measure -- even when they’re
three meters away. A car driving by outside we would see and hear on the
detectors.”
The principal noise-reduction scheme is to use two
highly sensitive instruments -- one at the measurement site and one far
away. The on-site sensor will hear the desired signal, but both will
hear the magnetic noise. So it should be possible to subtract out much
of the magnetic interference, raising the signal-to-noise ratio.
Another problem is designing a system that will be flexible enough to be
widely applicable. “Every head is different,” says project scientist
Dong Sheng. “So we have to have a mechanism for assessing exactly where
the sensors are with respect to the brain.” A lot of peripheral
questions come into play. For example, the team is experimenting with
different combinations of plastics to reduce the amount of conductive
material in the µOPM’s enclosure.
“Eventually, of course, we’d
like to reduce the amount of shielding that’s needed for MEG -- to take
magnetometry out of the shield,” Knappe says. “We hope that future
systems could work in the field, where there’s no liquid helium around
for a SQUID-based detector. This could maybe help to diagnose traumatic
brain injuries (TBIs) where they happen.”
Major TBIs cause
structural damage to brain tissue that can be seen using conventional
MRI. However, lower-intensity injuries -- which may still be very
dangerous -- do not cause the same damage, and therefore don’t show up
on conventional MRI.
“But some of the naturally occurring waves
in the brain do seem to be affected by such injuries,” Knappe says. “The
electrophysiology seems to be altered, but there is no apparent
structural damage. So medical personnel are looking for a biomarker that
says, ‘Oh, here’s something!’ MEG could contribute potentially to
providing that marker, and help diagnosticians decide whether to send a
soldier back into combat or a football player back onto the field. It’s
especially important in the case of concussions, because if you have a
second one, it just multiplies the severity.”
A mobile MEG system
might not be that far in the future, Knappe says. “We’ve built
prototype arrays, we’ve put them on people, there is a nascent industry
that is starting to take these procedures over, and the companies are
selling prototype systems. So there has been a lot of progress. They are
starting to be used in pilot studies, though not so far in clinical
studies.
“If we want to go beyond what the current system can do,
get higher spatial resolution, that hasn’t been demonstrated yet. We
think we have the technology, but we don’t quite know. Still, we could
be as much as halfway there.”
Meanwhile, Knappe’s project, part
of PML’s Atomic Devices and Instrumentation Group, is finishing up work
on the latest generation of µOPM prototypes. They will be evaluated by
team’s medical collaborators Yoshio Okada and Matti Hamalainen at the
Boston Children’s’ Hospital and Massachusetts General Hospital in coming
months.
“This is one of several important applications for our
magnetometer technology,” says NIST Fellow John Kitching. “We hope these
sensors will begin to make an impact in a variety of areas, including
nuclear magnetic resonance, magnetic anomaly detection, and navigation,
in the coming years. It’s an exciting time to be involved in this
field!”
* A superconducting quantum interference device (SQUID)
is a highly sensitive detector of magnetic fields. It senses the effect
of fields on the current in a superconducting loop containing Josephson
junctions
Image : A helmet-like array of chip-scale atomic magnetometers being developed in NIST's Physical Measurement Laboratory.
Courtesy/NIST
Courtesy/NIST
Source: National Institute of Standards and Technology (NIST)
http://hyperphysics.phy-astr.gsu.edu/hbase/solids/squid.html
http://whatis.techtarget.com/…/superconducting-quantum-inte…
http://whatis.techtarget.com/definition/Josephson-junction
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