PET and SPECT

Imaging the brain's chemistry with radioactive tracers: glucose metabolism, dopamine and serotonin systems, amyloid and tau in Alzheimer's disease, and the trade-offs of nuclear imaging.

How PET works

Positron emission tomography (PET) uses a molecule labelled with a short-lived radioactive isotope (such as fluorine-18 or carbon-11). When the isotope decays, it emits a positron, which almost immediately meets an electron and annihilates, producing two gamma rays travelling in opposite directions. Rings of detectors around the head record these paired gamma rays and reconstruct where the tracer is concentrated. Because many isotopes decay within minutes to hours, some must be made in a cyclotron near the scanner.

Measuring metabolism

The most widely used tracer is FDG (fluorodeoxyglucose), a glucose analogue that active cells take up but can't fully break down, so it accumulates. FDG-PET maps glucose metabolism, revealing reduced activity in particular patterns in different dementias, the seizure focus between seizures in epilepsy, and the metabolic activity of tumours. Early functional brain-mapping studies in the 1980s used PET measures of blood flow with oxygen-15-labelled water.

Imaging chemistry

PET's unique strength is imaging specific molecules. Tracers that bind dopamine receptors or transporters show dopamine systems — revealing, for example, reduced dopamine transporter in Parkinson's disease and dopamine release in response to drugs. Other tracers image serotonin receptors, opioid receptors and synaptic density.

Amyloid and tau PET

In 2004 researchers in Pittsburgh reported Pittsburgh compound B (PiB), a tracer that binds amyloid plaques, showing Alzheimer's pathology in living people for the first time. Approved amyloid tracers and newer tau PET tracers now help diagnose Alzheimer's, select patients for anti-amyloid drugs and track their effects.

SPECT and trade-offs

Single-photon emission computed tomography (SPECT) uses tracers emitting single gamma rays; it's cheaper and more widely available but with lower resolution. A common clinical SPECT scan images the dopamine transporter (DaTscan) to help distinguish Parkinsonian conditions from other tremors. Both PET and SPECT involve a small radiation dose, are relatively expensive, and have coarser spatial and temporal resolution than MRI.