By Peter Weiss
People undergoing magnetic resonance imaging (MRI) scans must remain within the coils of the scanner’s electromagnet. Otherwise, the patient isn’t in a magnetic field uniform enough to give a good image. Unfortunately, patients often find this situation uncomfortable.
Maybe there’s a better way, say Carlos A. Meriles, Alexander Pines, and their colleagues at Lawrence Berkeley (Calif.) National Laboratory and the University of California, Berkeley.
These researchers have developed a means for recovering useful magnetic resonance signals from materials within a nonuniform magnetic field. The method, described in the July 6 Science, may benefit both medical imaging and the widely used chemical-analysis technique known as nuclear magnetic resonance (NMR) spectroscopy, Meriles says.
Ultimately, it may enable doctors, scientists, and others to analyze samples placed beside a magnetic coil, not in it. For patients, that could translate into a less claustrophobic experience.
Both MRI and NMR depend on the spin of atomic nuclei, a property that makes those nuclei act like tiny bar magnets. In a magnetic field, the nuclear spins line up either with or against the field.
Today’s magnetic-resonance devices use a uniform field and also transmit radio pulses at the sample or person inside. The pulses knock out of alignment some nuclear spins, which then wobble and emit informative radio echoes.
For example, researchers can easily discern atoms’ identities from those radio responses. Moreover, slight variations from atoms’ signature frequencies reveal interatomic bonding patterns.
Rather than relying on a constant magnetic field, the Berkeley scientists created a field that decreased from one end of the sample to the other, as though the sample were outside the scanner. They also varied the radio pulses beamed into the sample in a way that compensates for the field’s decline.
By zapping the chemical trans-2-pentenal with those pulses, the team found it could detect the molecule’s atoms and bonds nearly as well in the nonuniform field as in a uniform one. This result gives the researchers confidence that their method may offer an alternative to the wraparound scanner.
