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Magnetic Resonance Images Obtained in Ultralow Magnetic Fields
New Techniques Eliminate Need for Large Magnets in MRI
Alexander Pines

A collaborative research effort led by Alex Pines and John Clarke has developed a series of new techniques that allow nuclear magnetic resonance (NMR) spectra and magnetic resonance images (MRI) to be obtained in extremely low magnetic fields. The team has obtained NMR spectra at fields as low as 0.06 mT (milli-Tesla), equivalent to the earth's magnetic field, and MRI images at a field only a factor of 10 larger. These advances may eliminate the need for large magnets in MRI instruments used for some medical imaging applications.

Magnetic resonance occurs in atomic nuclei that contain at least one "unpaired" proton or neutron. These nuclei act as if they were bar magnets and, when they are exposed to an external magnetic field, they align their axes along the lines of magnetic field. If, while exposed to the magnetic field, the nuclei are bombarded with radio waves, they absorb the energy and are knocked out of alignment. As the nuclei relax back into alignment, they emit energy which can then be detected by the NMR spectrometer. The frequency of this emission is sensitive to the chemical environment of the nucleus involved. For this reason, NMR spectroscopy is a powerful tool for chemical characterization. Magnetic resonance is also used in an imaging mode (MRI) in which the density of nuclei are measured as a function of position.

In general, NMR and MRI are relatively insensitive measurement techniques. The overall magnetic signal is proportional to the small population difference between nuclei that align themselves "up" and those aligned "down." This difference (and hence the NMR signal) increases with applied magnetic field. As a consequence, magnetic fields as large as 8 T are employed to get useful signals. Even so, the natural population difference between up and down nuclear spins is no more than one in 10,000.

Considerations of cost, bulk of the magnets, hazards of high fields, and resolution make it desirable to use lower magnetic fields but this has not been practical to date due to the loss in sensitivity. The MSD team solved this problem by employing various combinations of two techniques: (1) "optical pumping" (see MSD Highlight 97-4) which creates an artificially large population of unpaired spins ("hyperpolarization") in an inert gas (3He or 129Xe) and (2) the use of ultra sensitive Superconducting QUantum Interference Devices (SQUIDs, see MSD Highlight 98-5) to detect directly the magnetic flux produced by the spinning nuclei.

In their first demonstration, the team introduced hyperpolarized Xe into the sample tube of their MRI instrument. The signal was detected with a SQUID employing a low-TC material and the sample and detector were both held at 4 K. Using "two-pulse spin echo" mode, the researchers obtained a 1-D profile of the density of spins in the tube at a magnetic field of 0.5 mT. A 2-D cross section (see figure) was obtained by rotating the sample. In their second demonstration, the team used room temperature samples. A SQUID based on high Tc material was cooled to 77 K and placed 1.5 mm from the sample. A sapphire window provided thermal insulation. MRI images of room temperature liquid samples including mineral oil have been generated at 2.0 mT. They also demonstrated that NMR spectra can be obtained from mineral oil at a field of 0.06 mT, a field equivalent to the magnetic field of the earth.

Alex Pines (510 486-6097) and John Clarke (510 642-3069), Materials Sciences Division (510 486-4755), E. O. Lawrence Berkeley National Laboratory.

M. P. Augustine, A. Wong-Foy, J. L. Yarger, M. Tomaselli, A. Pine, D. M. TonThat, and J. Clarke, "Low field magnetic resonance images of polarized noble gases obtained with a dc superconducting quantum interference device," Applied Physics Letters 72, 1908 (1998).

Research funding from the Division of Materials Sciences (DMS) at the U.S. Department of Energy (DOE).