Magnetic resonance imaging (MRI) is a clinical workhorse, producing exquisitely detailed 3-D pictures of tumors, blood vessels, bones, and structures deep inside the body. MRI images are in shades of gray, and their contrast is based on how much water is in the part of the body under study. Now physicists have fabricated miniature magnetic particles that could brighten MRI with a rainbow of colors that convey a wealth of information about the disease states and behavior of tissues in the body.

Research on these particles, under way at the National Institute of Standards and Technology, in Boulder, CO, is in its early stages, and the particles haven't been tested in animals. But if multicolor MRI lives up to its promise, it could provide visual information at the level of genes, proteins, and other molecules. Researchers hope that such "molecular imaging" will eventually become part of personalized medicine, allowing doctors to literally see the processes underlying an individual patient's inflammation or tumor growth and then prescribe the right therapy with less guesswork. Most molecular-imaging techniques are optical and involve fluorescent tags such as the tiny particles of semiconductor material known as quantum dots. But the light emitted by these tags can travel through only about a centimeter of tissue, so they're not very useful for imaging organs. MRI provides a noninvasive look below the surface.

Magnetic resonance images are generated from radio frequency signals emitted by water molecules inside the body. When the strong circular magnet that surrounds the patient is turned on, the nuclei of hydrogen atoms inside the patient's body align with the magnetic field. A radio frequency pulse knocks them out of alignment, and as they snap back into position, they release their excess energy as radio waves.

The particles designed by the National Institute of Standards and Technology (NIST) act like miniature magnets, causing a predictable shift in the frequency of the radio waves emitted by water flowing through them. The magnitude of this shift is directly related to the size and shape of the particles, which consist of two disc-shaped nickel magnets held together by nonmagnetic posts. The varied radio frequency shifts can be mapped onto the spectrum of colors of visible light.

"We can engineer whatever color we want," says Gary Zabow, a physicist in NIST's electromagnetics division who is leading the particles' development. Using microfabrication techniques that are standard in the computer industry, he says, "we get these colors by controlling the particles' exact shape."