In Rosen's imager, the magnetic field is generated by two coils mounted on what look like two metal garden trellises. Wire grids and rings in these structures direct the field towards a person lying, standing, or sitting up in the center. (In conventional MRI machines, a cylindrical magnet that surrounds the supine patient produces the strong magnetic field.) The subject wears an antenna over his or her chest that consists of a cardboard tube wrapped with a coil of wire and coated in rubber. After inhaling a mixture of polarized helium and air through a tube, the subject must sit still and hold his or her breath for up to 30 seconds while the antenna picks up the magnetic spin of the helium in the subject's lungs. Rosen says the system cost less than $100,000 to build.

In their imager, Rosen and Walsworth can see the position of helium atoms in a subject's lungs. Oxygen molecules influence the spin of the polarized helium, so the Harvard system can also visualize the concentration of oxygen in the lungs. If it's high in one region of the lung, oxygen isn't being absorbed well and the subject may have poor lung function.

There are several other groups using polarized gas to create MRI images of the lungs, says Driehuys. But all the other groups use expensive, commercially available MRI systems. What makes Rosen and Walworth's work unique, Driehuys says, is that they've built their own cheap, low-power MRI scanner.

The Harvard-Smithsonian researchers are now adapting their system to shorten the time needed for imaging. "You do have to stay still and hold your breath," says Rosen, which can be difficult for people with compromised lung function.