Magnetic resonance imaging (MRI) is a powerful scanning technology used to probe brain function and mental illness, including depression. It may also have a role in treating that disease. "We were using MRI to investigate the effectiveness of certain medications in bipolar patients," says Michael Rohan of McLean Hospital in Belmont, Mass., "and noticed that many came out of the MRI feeling much better than when they went in." A controlled study found that 23 of 30 bipolar subjects who had scans showed mood improvement. If larger studies confirm the effect, a tabletop version of the scanner could someday be used in doctors' offices.