A hospital magnetic resonance imaging (MRI) room is no place for credit cards. The MRI magnets used to paint precision pictures of your innards are strong enough to yank a screwdriver out of your hands from a meter away. Cheaper, more portable devices may be in store, however. Researchers report in next week's Physical Review Letters that high-quality images of at least some parts of the body, such as the lungs, can be made using a magnet 10 times weaker than those that pin kindergarten art to refrigerators.
With traditional MRI, a person is slid into the bore of a giant superconducting magnet that generates a field some 20,000 times stronger than Earth's. The field aligns the nuclei of hydrogen atoms inside the body, leaving the atoms spinning like tiny bar magnets in synch. Then a brief electromagnetic pulse tips the nuclei so they wobble like off-kilter tops. The gyrating atoms emit an electromagnetic wave of their own that can be detected. By adding a second weak magnetic field to tweak the frequency of atoms at different locations, researchers can map the density of water molecules and construct a sharp picture of internal tissue and organs.
Ron Walsworth of the Harvard-Smithsonian Center for Astrophysics in Cambridge, Massachusetts, and colleagues took a slightly different approach. They used a technique pioneered several years ago by Princeton University physicist William Happer: First align the atomic nuclei in rubidium gas with a laser, then allow them to mingle with a helium-3 gas (helium minus a neutron). When the atoms collide, some of the alignment is gradually transferred to the helium atoms. The toxic rubidium atoms can then be removed. Because the helium atoms are an inert gas, they can remain aligned for minutes at a time even without a magnetic field.
In one test, the researchers inflated a rat's lung with polarized helium gas and made high-quality pictures of the cavities with a resolution of about a millimeter. The magnetic field used to tune their spinning rate was just 40 times stronger than Earth's. "It's embarrassingly simple," Walsworth says. "You and your brother can wind your own magnet and get as good an MRI as what you get in a hospital," he says, although ScienceNOW doesn't recommend it.
Researchers had known that such low-field imaging with tissue could be done, but no one had bothered to put all the pieces together, Walsworth says. Still, others are impressed: "It's a beautiful piece of work," says Alex Pines, a physicist at Lawrence Berkeley National Laboratory in California. "They're really sharp pictures."
Traditional MRI magnets can cost a million dollars, Pines points out. The technique could lead to portable, cheap MRIs for imaging lungs, he says. Happer adds that lungs are just the beginning. Replacing the helium with xenon gas, which is absorbed like oxygen into the bloodstream, will also allow researchers to take low-budget MRIs of arteries and the brain, he predicts.