The micromagnets shift the frequency of only those radio waves emitted by water traveling between their constituent discs. If this space is blocked off, says Zabow, the particles have no effect on the MRI signal. Consequently, the micromagnets could act as miniature chemical sensors. "You could purposely block the space with a material that melts at a certain temperature or that is in some way reactive, expanding or shrinking" under specific conditions inside the body, says Zabow.

Contrast agents for MRI already exist, and some of them can even be targeted to particular tissues or cell types. Like the NIST micromagnets, these agents make part of the image look brighter or darker by shifting the radio frequencies emitted by protons. Unlike the NIST particles, however, they're made using chemical techniques, so the dimensions of their particles can't be carefully controlled. As a consequence, they shift the frequencies of the protons' signals unpredictably. On average, they provide a contrast to the standard MRI signal, but they can't offer more-precise, local distinctions.

"If you were to put in two [such agents], you couldn't tell the difference between them," says Richard Bowtell, a professor of physics at the Institute of Neuroscience at the University of Nottingham, in England. Each NIST particle, though, produces a distinct signature. "If you put them all in simultaneously, you could see which signal belongs to which one," Bowtell says.

The micromagnets, described this week in the journal Nature, have so far been made from nickel, which is toxic. But Zabow says that they could easily be made from iron, which is nontoxic and magnetic. The researchers are exploring the idea of using the particles as "sensors of physiological conditions inside the body," says Zabow, but he cautions that they are only now taking the first step in that direction, planning tests in cells.