PET Imaging

PET imaging is a way of taking a picture of what the living brain is actually doing, not just what it looks like. Before the scan, a person is given a tiny, harmless dose of a special tracer — a familiar substance like a sugar, labeled with a faintly radioactive tag. The brain pulls this tracer to wherever it is busiest or wherever a particular target sits, and a ring-shaped scanner watches where the tracer piles up. The result is a colorful map showing which regions are most active or which chemicals are most present, glowing like lights in a dark city seen from above.

The trick behind it is positron emission. The radioactive tag decays and throws off a positron — a sort of mirror-image of an electron — which almost instantly bumps into an ordinary electron, and the two annihilate, vanishing into two bursts of energy that fly off in exactly opposite directions. The scanner's detectors catch both bursts at the same moment and draw a straight line between them; the tracer must have been somewhere along that line. Stack up millions of these lines and a computer reconstructs a three-dimensional image. Depending on which tracer is used, PET can map the brain's energy use (metabolism), its blood flow, or the docking sites (receptors) for messenger chemicals such as dopamine — which is why it is prized for studying disorders like Alzheimer's, Parkinson's, and addiction.