Understanding tests after your visit.

PET is not a single test

PET imaging allows us to look at a selected biological process in the brain. A small amount of a radioactive tracer is given through an IV. After a waiting period, you lie still while the scanner collects images. The preparation, timing, and meaning of the scan depend on which tracer is used.

Amyloid PET asks whether amyloid plaques are present. Tau PET shows the amount and regional pattern of a form of tau associated with Alzheimer's disease. FDG PET shows patterns of glucose use, an indicator of how different brain regions are using energy. These scans are related, but they do not answer the same question.

What a PET result means

An amyloid-positive scan shows that amyloid is present. It does not prove that amyloid is the only cause of the symptoms, and it does not tell us how impaired a person is. Amyloid can be present in people who do not have cognitive symptoms, particularly with increasing age.

Tau PET provides different information about the location and amount of Alzheimer's-associated tau, but it does not capture every disease in which tau is involved. FDG PET shows a pattern of brain metabolism. It does not directly identify a particular protein.

PET scans involve radiation, can be expensive, and may not be covered in every situation. Each tracer answers a narrow question, and mixed brain diseases are common. A PET result adds evidence. It does not replace the clinical evaluation.