Dialing Down the MRI

Magnetic Resonance Imaging (MRI) machines' knack for peering at soft tissue deep within the body has made them one of the most popular imaging tools. But MRI isn't perfect. It works by beaming radiofrequency pulses into a patient and tracking how this radiation affects the magnetic behavior of tissues. But those pulses must be carefully controlled to prevent them from overheating tissue and injuring patients. Now, a new study could pave the way to a new form of radiofrequency-free MRI scans that would offer several advantages. MRI owes its success to the magnetic behavior of the protons in hydrogen atoms within the body. Those protons have a magnetic moment, which makes them behave essentially like compass needles. To create an image, MRI machines place patients in a strong magnetic field, which causes the protons in the body to align their magnetic compasses with that field. Technicians then send in precisely tuned pulses of radiofrequency energy that knocks some of those compasses out of alignment. By tracking how the needles return to equilibrium, researchers can infer their distribution and thus the makeup of the tissue. But Norbert Müller of Johannes Kepler University in Linz, Austria, and Alexej Jerschow of New York University in New York City wanted to see if they could do away with the need for radiofrequency pulses. The chemists relied on the fact that even in a strong magnetic field the magnetic orientation of protons fluctuates. Müller and Jerschow tracked the magnetic behavior of elements for a few milliseconds and then repeated the process over and over. The multiple readings yielded a strong enough signal to reveal the magnetic signature of protons. Next, to determine how those protons are distributed, Müller and Jerschow applied an external magnetic field that varied in strength across the sample. Because the degree of alignment of a proton's magnetic moment is related to how strong the magnetic field is, that enabled them to locate those protons. The researchers report in the 2 May issue of the Proceedings of the National Academy of Sciences that by integrating some 30 scans of four water-filled glass capillaries, one of which was spiked with proton-rich deuterium, they could create a 2-dimentional image revealing the location of the spiked capillary. "This is fantastic, novel work," says Alexander Pines, a nuclear magnetic resonance expert at the University of California, Berkeley. Pines and Jerschow note that the new technique isn't ready for human imaging yet, because for now it works best on small samples. But Jerschow says it probably could be improved by using more sensitive magnetic detectors already on the market. As an added benefit, he says, better detectors would also likely make the technique work with smaller magnets than the large expensive superconducting magnets used in conventional MRI machines--an improvement that could make future MRI's both cheaper and safer. "Source":[ http://sciencenow.sciencemag.org/cgi/content/full/2006/505/2]

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