This article was a feature story in Technology Review's December 2005/January 2006 print issue. It has been divided into three parts for presentation online. This is part 2; part 1 appeared on Monday, January 23, and part 3 will appear on Wednesday, January 25.

Part 1 discussed the work of John Port, a neuroradiologist at the Mayo Clinic who is using MRI to explore the parts of the brain that may be involved in bipolar disorder, also known as manic-depression."I'm dedicating the rest of my career to coming up with an imaging test that will help psychiatrists diagnose" bipolar disorder and other illnesses, Port told Technology Review.

Port is one of many researchers now experimenting with MRI spectroscopy, in which software produces an image of the brain based on a spectroscopic scan. The image is made up of individual data points called voxels, cubes analogous to the pixels in a 2-D computer image. Each corresponds to a volume about the size of a kidney bean. For each voxel, Port gets a reading on the presence or absence of certain chemicals that are indicators of brain function.

To understand how MRI spectroscopy works, it's necessary to understand a bit about how magnetic resonance imaging works more generally. MRI scanners pick up extremely faint electromagnetic signals coming from protons in the atoms of molecules that make up the body's tissues -- in this case, brain tissue.

"Think of it like listening for a pin drop in a thunderstorm," Port says. Each proton has a magnetic field that points in a certain direction, as the earth's does. When the MRI is turned on, its magnet aligns the protons' magnetic fields in the same direction. Bursts of radio frequency energy temporarily knock some of the protons out of alignment. When the protons snap back into place, they release energy, generating a minuscule signal that the MRI's detectors can pick up. By flipping the protons different ways and measuring various properties of those flips, including the time they take, researchers can identify various tissues and chemicals in the brain.