What is amyloid PET, and when is it used?

What it is

Amyloid PET - short for amyloid positron emission tomography - is a brain scan that uses a small amount of an injected, FDA-approved imaging agent (a radiotracer) that binds to a protein called amyloid-beta in the brain. The scan produces an image that shows how much amyloid has accumulated and where.

In Alzheimer's disease, amyloid is one of the proteins that builds up abnormally in the brain. An amyloid PET scan does not tell you that someone has Alzheimer's disease. It tells you whether the amyloid pathology associated with Alzheimer's disease is present.

Why it matters

Two patients can have very similar memory symptoms, and one of them can have Alzheimer's disease while the other does not. Cognitive symptoms have many possible causes, including other neurodegenerative diseases, sleep disorders, medication effects, and depression. Amyloid PET is one of the tools clinicians use to figure out whether what they are seeing is consistent with Alzheimer's disease specifically.

When the answer to that question changes treatment decisions - for example, deciding whether someone is a candidate for an anti-amyloid antibody - amyloid PET is one of the most useful tests a clinician can order.

How the scan works

The procedure usually takes about ninety minutes from start to finish:

• The patient receives a small injection of the radiotracer through an IV.
• After a wait - usually 30 to 90 minutes, depending on the tracer - the patient lies still on a scanner bed for a roughly 20-minute scan.
• A radiologist or nuclear medicine specialist interprets the images, often supported by quantitative analysis.

The amount of radiation involved is small and comparable to other diagnostic imaging studies.

What the scan can and cannot tell you

It can tell you:

• Whether amyloid pathology is present, in a way that is well-validated against autopsy findings.
• How much amyloid is present and where, in a way that is increasingly quantitative.
• Whether amyloid pathology has changed between two scans done at different points in time - this is how amyloid clearance from anti-amyloid therapy is measured.

It cannot tell you:

• Whether someone definitely has Alzheimer's disease as a clinical condition. Many people without symptoms have positive amyloid scans, and not all of them go on to develop dementia in their lifetime.
• The cause of any specific symptom in any specific patient. The clinical picture has to be put together with the scan, not from the scan alone.
• How fast someone's symptoms will progress.

When it is used

Amyloid PET is most often used in two situations:

• When a clinician needs to confirm or rule out Alzheimer's pathology in a patient with cognitive symptoms, particularly when the picture is unclear or other causes are also possible.
• When a patient is being considered for anti-amyloid antibody therapy and amyloid status needs to be confirmed before starting treatment.

In US clinical practice, amyloid PET is increasingly used after a less expensive plasma biomarker test has suggested that amyloid pathology is likely. We cover plasma biomarkers in our explainer on plasma biomarkers and our Insight on how blood biomarkers will reshape diagnosis.

Coverage

In the US, Medicare coverage of amyloid PET for diagnostic purposes is in place under defined indications. Commercial coverage varies by plan. International coverage varies by country.

What this means in practice

If a clinician has recommended an amyloid PET scan, it is usually because the answer to the question "is this Alzheimer's pathology" will change what they do next. Knowing what the scan does and does not tell you can help patients and families read the result for what it is - a piece of the diagnostic puzzle, not the whole picture.

This page is a plain-language primer. It is not medical advice. Decisions about diagnostic testing belong with the patient and their clinician.