PET scans can detect amyloid plaques and tau tangles in the brain, which are often early signs of Alzheimer’s disease. But they’re not enough on their own to make a diagnosis.

Alzheimer’s disease is the most common form of dementia, making up about 60–70% of cases. Dementia is a group of conditions that cause changes to your memory, behavior, and thinking.

One of the characteristic signs of Alzheimer’s is the irregular buildup of proteins in the brain called plaques and tangles. PET scans are under investigation for their potential to help identify these proteins.

Doctors can’t use PET scan results to diagnose Alzheimer’s, but the scans may provide supportive evidence along with other tests.

Keep reading to learn more about the role PET scans play in diagnosing Alzheimer’s.

Experts now recognize that you can detect the accumulation of plaques and tangles years before Alzheimer’s symptoms develop. PET scans may help identify these changes.

Three types of PET scans can potentially help detect Alzheimer’s disease:

  • Amyloid PET scans: These scans can potentially help diagnose Alzheimer’s by measuring levels of a protein called beta-amyloid that makes up plaques.
  • Tau PET scans: These scans detect the buildup of tau protein, which forms tangles characteristic of Alzheimer’s and other dementias. Tau PET scans can potentially allow doctors to monitor Alzheimer’s progression, but they’re most often used in research.
  • Fluorodeoxyglucose (FDG) PET scans: These scans measure your brain’s metabolism of glucose. They can help your doctor distinguish Alzheimer’s from other types of dementia, such as frontotemporal dementia (FTD).

The proposed draft 2023 guidelines from the National Institute on Aging and the Alzheimer’s Association suggest that doctors can diagnose Alzheimer’s with any core biomarker, such as:

  • beta-amyloid levels measured with a spinal tap
  • amyloid PET scan results
  • tau PET levels

PET scans may show reductions in brain activity in certain regions as well as the buildup of plaques and tangles in the brain.

FDG PET scans can reveal atypical glucose consumption by certain parts of your brain.

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An amyloid PET scan shows increased amyloid plaque deposits throughout the brain in a person with Alzheimer’s disease.
Science History Images / Alamy Stock Photo

Doctors can use other types of brain imaging to help in diagnosing Alzheimer’s, but mainly to exclude other causes of dementia.

CT scans were the first imaging technique doctors used to help in diagnosis. They can show shrinkage of brain areas that occurs with dementia. They can also help identify other causes of your symptoms, such as brain clots.

MRI scans can reveal whether certain parts of your brain have shrunk. They can help rule out other causes of symptoms, such as bleeding or fluid buildup in your brain.

Doctors can only confirm an Alzheimer’s diagnosis after death by analyzing samples of brain tissue. To make a diagnosis in living people, they use a combination of tests, including:

Learn more about Alzheimer’s tests.

PET scans may also help diagnose other types of dementia by identifying changes in your brain.

For example, fluorodeoxyglucose PET scans may help identify atypical patterns of glucose use in certain parts of your brain associated with FTD, which makes up about 5–10% of dementias.

You may be told not to eat for about 6 hours before a PET scan and to avoid exercise for about 24 hours. During your procedure:

  1. A healthcare professional will inject a radiotracer into a vein in your arm roughly 1 hour before your scan. This tracer will bind with plaques in your brain.
  2. During the scan, you’ll lie on a bed that will move through a large cylindrical scanner.
  3. You’ll need to lie still for about 30–60 minutes during the scan.

Here are some frequently asked questions people have about PET scans and Alzheimer’s disease:

How accurate is a PET scan for detecting Alzheimer’s?

PET scans seem to correctly identify Alzheimer’s in about 55–98% of people and correctly rule out Alzheimer’s in about 18–62%.

What is the best scan to detect Alzheimer’s?

Amyloid PET scans are one of the best types of imaging for identifying changes in the brain associated with Alzheimer’s. Amyloid scans are more accurate than FDG PET scans. MRI and CT scans may help rule out other conditions.

How much does an amyloid PET scan cost?

According to the Alzheimer’s Association, the out-of-pocket cost for an amyloid PET scan averages $3,000 or more. Medicare and private insurance do not typically cover the cost of amyloid PET scans.

PET scans may help doctors identify changes in your brain associated with Alzheimer’s. They may also help identify other types of dementia, such as FTD.

Researchers are examining how best to use PET scans to diagnose Alzheimer’s. Doctors sometimes use imaging to provide supportive evidence alongside other tests, such as neuropsychological tests, blood tests, and spinal taps.