a recent experiment the letters CAL, carved through the
end of a plastic tube placed in an MRI encoding coil, were
reconstructed from 10 batches of spin-polarized xenon carried
to a detection coil at a separate location. Although the
batches arrived at different times, the spatial arrangement
of the letters was accurately reproduced. (image by Alex
Paul Preuss, LBNL
Chemistry professor Alexander Pines and his colleagues have
discovered a remarkable new way to improve the versatility and sensitivity
of magnetic resonance imaging (MRI) and the technology upon which
it is based, nuclear magnetic resonance (NMR).
latest details of the new technique, known as remote detection,
are reported by Pines, postdoctoral fellow Song-I Han, and
doctoral candidate Juliette A. Seeley in the Journal of
detection depends on physically separating the two basic steps of
NMR, signal encoding and detection -- normally carried out in the
same instrument -- in order to customize each step for the best
results. Using laser-polarized xenon gas as the medium for "remembering"
the encoded information and carrying it to the remote detection
site, Pines and his group have achieved orders-of-magnitude improvement
in MRI image resolution, plus manifold increases in NMR sensitivity.
encoding is exceptional at recovering chemical, biological, and
physical information from samples, including living organisms, without
disrupting them," says Pines, noting that MRI, a closely related
technology, is equally adept at nondestructively picturing the insides
of things. "The problem with this versatile technique is low
says Han, "by separating the encoding and detection phases
of NMR/MRI, we can gain data about physical, chemical, and biological
properties of samples that we simply could not have gotten previously."
conceptual foundation and feasibility of remote detection were earlier
established in Pines's laboratory by Adam Moulé, Megan
Spence, Kimberly Pierce, and Sunil Saxena, in addition
to Han and Seeley. The group reported their results in the August
5, 2003 issue of Proceedings of the National Academy of Sciences.
now an assistant professor of chemistry at the University of Pittsburgh,
remarks that while using two distinct pieces of apparatus "might
seem counterintuitive at first glance, in many circumstances the
use of one set-up in NMR leads to an uneasy balance between effective
signal encoding and sensitive signal detection. By separating the
two, not only can the signal fidelity be vastly improved, but also
many new schemes that use much more powerful and sensitive detection
and coding methods and principles can be envisaged."
and MRI work because many atomic nuclei have magnetic moments, acting
like toy bar magnets with north and south poles. In a magnetic field
these spinning nuclei orient themselves along the field lines, with
spins up or down. Slightly more energy is required to maintain the
the encoding phase, a radiofrequency (RF) pulse matched to the energy
difference between the two states knocks the target nuclei atilt,
causing their spin axes to precess around the field lines like off-center
toy gyroscopes. The exact precession rate is characteristic of each
chemical species --ubiquitous hydrogen is the most commonly used
species in NMR and MRI -- and is also affected by the chemical and
MRI, an extra step encodes additional data. In addition to the uniform
magnetic field, additional magnets are turned on briefly to superimpose
fields that are stronger in one direction than the other. When the
target nuclei are subjected to RF pulses, the differences in field
strength are reflected in changed angles and speeds of precession.
Together with the timing of the pulses, the gradient fields give
each spinning nucleus a unique set of coordinates corresponding
to its position.
the detection phase, the net magnetic moment of the spinning nuclei
is measured and analyzed for information about the chemical environment.
In MRI the magnetization of each batch of spins formed by a train
of radiofrequency pulses is measured, which yields the spatial characteristics
of the sample.
factors affect the best ways to encode and detect information about
a particular sample, whether it's a living organism, a sample of
tissue on a slide, a gas, liquid, or mineral sample, or even a solid
encoding, the RF coil has to be of the same dimension as the sample
and often surrounds it; in addition, the main magnet has to be big
enough to bathe the sample in a magnetic field that is typically
very strong. In hospitals, for example, MRI equipment big enough
to examine the head or lungs is bulky and expensive.
both encoding and detection, the proportion of target nuclei in
the sample is another important consideration. If the sample is
large but the proportion of target nuclei is small, this small "filling
factor" makes for a weak signal.
-- the difference in the number of spin-up versus spin-down nuclei
-- is also vital. Even in a strong magnetic field, the excess of
spin-up hydrogen is at best 1 in 100,000.
unlike hydrogen, can be optically "hyperpolarized" before
it is introduced into the sample, where its nuclei interact with
the surroundings to encode NMR and MRI information. Because xenon
is a noble gas, chemically inert and nontoxic, it is ideal for many
biological applications. In hyperpolarized xenon some 20 percent
of nuclei are spin-up, "so we don't waste all those spins,"
optimum signal encoding, then, an NMR/MRI set-up may include a big
RF coil and a strong magnetic field, while the best detection set-up
for the same sample might require a more sensitive magnetic field
and a smaller RF coil -- or even a supersensitive, non-MRI detector
like a superconducting quantum interference device (SQUID) or a
weak magnetic fields might be an advantage for encoding some subjects,
for example patients with pacemakers or metal implants. Signals
encoded in a weak field can only be recovered by a high-field detector.
detection allows us to combine the ability to obtain rich information
about a variety of interesting samples with sensitive detection,"
Seeley says. "This is possible because nuclear spins have the
ability to retain memory of their prior surroundings. They remember
the information that was encoded in an environment not optimized
for detection, and later they can be detected more sensitively."
NMR/MRI to do its best:
describes the successful first attempt to prove the remote-detection
principle: "We were able to demonstrate the rather fantastic
notion that a picture of a sample cell could be obtained by saturating
it with xenon gas in one spectrometer and then moving the gas --
by as much as 15 feet -- to another spectrometer for signal detection.
Despite the long separation and travel time, the xenon gas faithfully
remembered the shape of the sample cell."
Seeley, Han, and Pines have demonstrated remote detection of MRI
of porous samples at higher resolution, again using xenon gas to
carry the signal to a detector of optimum design. The xenon is optically
hyperpolarized, then introduced into the encoding chamber, where
it flows through the voids in the sample. Because the sample is
surrounded by a large radiofrequency coil, the filling factor is
poor. Bearing the encoded information, the gas flows on to the detection
chamber. Since xenon has a long spin-relaxation time, no spin-polarization
or pulse-timing information is lost during transport.
the recent experiment, the different encoding and detection coils
were near each other but physically separated, and both were subjected
to virtually the same strong magnetic field. With a much smaller
detector coil the filling factor was greatly enhanced: the coil
surrounds only the target nuclei, not the entire sample.
and detecting NMR/MRI signals separately makes many otherwise difficult
or impossible applications possible. For example, xenon can be dissolved
in chemical solutions or in the metabolic pathways of biological
systems, then concentrated for more sensitive detection. Since xenon
is not normally present in biological or geological samples, its
signal stands out clearly against a noiseless background.
signal carriers can also be used for remote detection, including
hyperpolarized helium gas for medical imaging or liquid oil or water
for geological analysis. Since only the carrier reaches the detector,
alternate detection methods, incompatible with the sample because
they may be intrusive or require transparency, can also be used
-- for example, optical methods that can detect the miniscule NMR
signals from living cells.
freeing the detection phase from the confines of the sample chamber,
remote detection liberates NMR/MRI technology from its restraints
opens a new realm of possibilities -- from targets as big as geological
core samples and human bodies down to microstructures and single
No other spectroscopic or imaging tool has such a rich combination