Laboratory MR device highlights potential of alternative technologies

A laser-based technique in development at the U.S. Department of Energy's Lawrence Berkeley National Laboratory may lead to a highly compact form of MRI that is portable and relatively cheap. The technique is unlikely to replace conventional MRI, but its development exemplifies the potential in alternative means for performing magnetic resonance imaging.

Developers say that optical atomic magnetometry is better suited to the study of cell biology, microfluidics, or even geology. The technique, which requires neither high-field magnets nor cryogenics, depends on two technological advances. One physically separates the two basic steps of MRI: signal encoding and detection. This separation optimizes those steps for sensitivity. The other advance is the development of 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.

"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," said Alexander Pines, who holds a joint appointment as a chemist with Berkeley Lab's materials sciences division and with the University of California, Berkeley, where he is as a professor of chemistry.

Pines led the development of this new technique along with Shoujun Xu, a member of Pines' research group, and Dmitry Budker, who holds a joint appointment with Berkeley Lab's nuclear science division and UC Berkeley's physics department. The three described the technique in a paper that appeared in the Aug. 22 issue of the Proceedings of the National Academy of Science.

The alternative MRI technology is noteworthy as much for what it doesn't have as what it does. Unlike mainstream superconducting MR systems, it is highly sensitive to low-field magnetic signals and operates at room temperatures.

"The fact that it does not require superconducting magnets or cryogens significantly reduces the cost and maintenance of the apparatus, and opens the technology up to a broad range of applications," Xu said. "Furthermore, our technique has simple electronics that can be easily integrated into detector arrays."

This alternative MRI technology could cost only a few thousand dollars to implement, according to Budker.

"Our system is fundamentally simple and does not involve any single expensive component," he said. "We anticipate that the whole apparatus will become quite compact and deployable as a battery-powered portable device."

The MRI system developed by the Berkeley researchers is a long way from the medical mainstream. The prototype was tested using water, which was passed through two small cells for signal encoding, then transported to a U-shaped detection area to be probed by a pair of magnetometers. This configuration enabled the researchers to detect an MRI signal from microliters of water in 0.1 seconds 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 realm," Xu said. "Further consolidation of the apparatus is under way 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."

The significance of the underlying concept technology takes on added weight when one realizes that modern MRI evolved from nuclear magnetic resonance. The spectra from this laboratory technique were used for decades to determine the composition of test tube samples until the technology developed enough for application to clinical studies.