Micro-Tesla MRI

Micro-tesla MRI was reported at last week's APS March meeting in Indianapolis by Robert McDermott, a member of John Clarke's group at UC Berkeley. The principle behind MRI is nuclear magnetic resonance (NMR), a process in which a magnetic field (often a strong one), is used to orient atomic nuclei in space while a burst of radio waves explores the nuclear energy levels by charting the frequencies at which energy is absorbed resonantly.

In addition to establishing chemical identity, NMR can also be turned into an imaging method by carefully watching the timing and the location of the re-emitted radio waves. A tumor, say, will have a slightly different water density (as revealed, in this case, by the presence of protons in the NMR survey) from surrounding healthy tissue. Computer processing and contrast enhancement will disclose the tumor's position to a trained observer.

Generally large magnets are required to produce sharp NMR images, and the development of a low-field version would benefit medical and scientific studies. McDermott reported an experiment in which an array of four columns of fluid were imaged with a field of 10 micro-Tesla over the period of several hours. (See also McDermott et al., Science, 22 March 2002.)

Also at the APS meeting, Mark Haacke of the MRI Institute for Biomedical Imaging in St. Louis (314-961-9105, nmrimaging@aol.com) discussed a new MRI technique called susceptibility weighted imaging (SWI). The technique measures differences among brain tissue in its magnetic susceptibility, essentially its magnetic response to the applied magnetic field of the MRI machine.

Yielding unique information from veins and blood products, SWI has already provided more sharply detailed MRI images of blood vessels in the brain than previously possible and the presence of small hemorrhages in heretofore unavailable detail. SWI can potentially detect angiogenesis, the growth of blood vessels caused by cancer, and may improve diagnosis of Parkinson's and Alzheimer's diseases, through its ability to monitor iron deposits in the brain.