Brain scans: MRI, CT, PET

When someone has memory or thinking problems, the doctor will often order a picture of the brain. A brain scan cannot diagnose dementia by itself. But it can show other problems that need different care, such as a stroke, a tumor or extra fluid. Some newer scans can also show the proteins linked to Alzheimer's disease.^1,2

This page explains the main kinds of brain scans, what each one shows, what it is like to have one, and what Medicare pays for as of October 2026.

Why doctors order a brain scan

A brain scan is one piece of a bigger checkup. That checkup also includes a talk about symptoms, a physical exam, memory and thinking tests and blood work.¹ See getting a dementia diagnosis for the full process.

Scans help in three main ways:

1. Finding other causes. A scan can show a stroke, bleeding, a tumor, an infection or a buildup of fluid. Some of these can be treated.^1,5 Read about treatable conditions that look like dementia and normal pressure hydrocephalus.
2. Pointing to the type of dementia. The pattern of change can hint at vascular dementia, frontotemporal dementia or Alzheimer's disease.¹
3. Checking safety for new treatments. People on anti-amyloid drugs need MRI scans before and during treatment to look for brain swelling or tiny bleeds.²

Structural scans: MRI and CT

Structural scans show the shape and size of the brain. They are the scans most people with memory problems will have.

MRI

An MRI (magnetic resonance imaging) uses a strong magnet and radio waves to make detailed pictures. It does not use X-rays or radiation.⁴ A 2024 guideline from the Alzheimer's Association says an MRI without contrast dye is the first-choice brain scan for people being checked for memory problems.²

An MRI can show:^1,2

• Shrinkage (atrophy) of the brain, and where it is happening
• Old or new strokes, and damage to small blood vessels
• Tiny old bleeds, which matter if someone is thinking about anti-amyloid drugs
• Tumors, fluid buildup or bleeding near the brain

Shrinkage alone does not prove a person has dementia or tell which type.¹

CT

A CT (computed tomography) scan uses X-rays and a computer. It takes much less time than an MRI. It shows less detail but can still show strokes, bleeding, tumors and fluid.^1,5 Doctors use CT when a person cannot have an MRI, for example because of some pacemakers or other metal implants.^1,2

CT uses more radiation than a regular X-ray. The risk from one scan is small. Your doctor weighs it against the benefit of a clear answer.⁵

PET scans: looking at brain activity and proteins

A PET scan (positron emission tomography) uses a tiny amount of a radioactive tracer. The tracer is injected into a vein. It travels to the brain and sticks to a target. The scanner then makes a picture of where it collects.^6,8 There are three main kinds used for dementia.

Amyloid PET

Amyloid is a sticky protein that builds up in clumps called plaques between brain cells. Plaques are one of the main signs of Alzheimer's disease. An amyloid PET scan estimates how many plaques are in the brain.^1,6

• A negative scan (few or no plaques) means memory problems are probably not caused by Alzheimer's.^1,6
• A positive scan shows plaques are there. But plaques also show up in some older people with normal thinking, and in some other brain conditions. So a positive scan supports the diagnosis but does not prove it alone.^1,6

One tracer, florbetapir (Amyvid), was first approved by the FDA in 2012. Its label was updated in 2025. It now says the scan can also be used to pick people who may get amyloid-lowering drugs.⁶ Before a person starts lecanemab (Leqembi) or donanemab (Kisunla), amyloid must be confirmed.² An amyloid PET is one way to do that. A spinal fluid test is another, and blood tests are now part of the process too.

You lie still for about 10 minutes of scanning, starting 30 to 50 minutes after the injection.⁶ Side effects are rare. The most common in studies was a headache, in fewer than 2 in 100 people.⁶

Tau PET

Tau is another protein. In Alzheimer's it forms clumps called tangles. A tau PET scan shows how many tangles there are and where they are.⁸ The FDA approved the first tau tracer, flortaucipir (Tauvid), in 2020. It is for adults with memory or thinking problems who are being checked for Alzheimer's.⁸ It is not approved to check for brain damage from repeated head blows (CTE).⁸

Tau PET can help with the diagnosis when symptoms are unusual or start early. It can also help predict what may lie ahead.⁷ It is not common in everyday care yet. It is used more in specialty centers and research.¹

FDG-PET

FDG is a form of sugar with a tracer attached. An FDG-PET shows which parts of the brain are using less energy.¹ Different types of dementia leave different patterns. Doctors use it mostly when they suspect frontotemporal dementia instead of Alzheimer's.¹ The 2024 guideline says a dementia specialist may use it when the cause is still unclear after an MRI or CT.²

DaTscan for Lewy body dementia

A DaTscan uses a different kind of scanner, called SPECT. It was already used to help diagnose Parkinson's disease. In 2022, the FDA approved it for people with suspected Lewy body dementia. It helps tell Lewy body dementia apart from Alzheimer's and other types.¹¹

Who should get a PET scan, and who should not

PET scans are usually ordered by a specialist, such as a neurologist or a geriatric psychiatrist. See which specialist to see. In 2025, the Alzheimer's Association and a nuclear medicine society updated their advice on when these scans make sense.⁷

Source for the table: 2025 appropriate use criteria for amyloid and tau PET.⁷

The experts also say a PET scan should never replace a full exam by a doctor.⁷ If you have no symptoms but worry about your risk, read before symptoms: preclinical Alzheimer's and genes and dementia. A clinical trial may be a way to learn your status with support.

What it is like to have a scan

1. Tell the team about metal and health issues. Mention pacemakers, defibrillators, aneurysm clips, cochlear implants, heart valves, new joints or stents. Also mention kidney problems, diabetes medicine, or past reactions to contrast dye.^4,5
2. Leave metal at home or in the locker. Jewelry, watches, hairpins, glasses, hearing aids and removable dentures stay out of the MRI room.⁴
3. Ask about fear of closed spaces. If the person may panic, tell the doctor ahead of time. Options include a calming medicine or an open MRI machine.⁴
4. Lie still. Moving blurs the pictures. The MRI is loud, so ask for earplugs or headphones. Staff can talk to the person through a speaker.⁴
5. For a PET scan, expect a wait. After the tracer shot, there is a rest period before the scan starts.^6,8
6. Go home the same day. Most people return to normal activity right away, unless they were given a sedative.⁴

Is the radiation safe?

MRI uses no radiation.⁴ CT and PET do. The dose from an amyloid PET is about 7 mSv (millisieverts), and from a tau PET about 9 mSv.^6,8 Any radiation slightly raises cancer risk over a lifetime, and the risk grows with more scans.^5,8 For a person with real memory problems, the doctor weighs this small risk against the value of a clear answer.

What Medicare covers

These rules are current as of October 2026. Coverage details can change.

MRI and CT. Medicare Part B covers brain scans that your doctor orders to find or treat a problem. After the Part B deductible, you usually pay 20% of the Medicare-approved amount at a doctor's office or imaging center.¹² At a hospital outpatient department, your copay may be higher than 20%.¹² For advanced scans outside a hospital, check that the place is accredited by Medicare.¹²

Amyloid PET: the 2023 change. Starting in 2013, Medicare paid for amyloid PET only for people in approved research studies, and only one scan per lifetime.⁹ On October 13, 2023, Medicare ended that national rule.^9,10 Now:

• Coverage decisions are made by local Medicare contractors (called MACs), the companies that process Medicare claims in each region.^9,10
• The one-scan-per-lifetime limit is gone, so more than one scan can be covered when needed.¹⁰
• Medicare's reasons included the new anti-amyloid drugs. A scan can help decide who may get them, and may help with decisions during treatment.¹⁰

FDG-PET. Medicare's national rule covers FDG-PET mainly to help tell frontotemporal dementia from Alzheimer's. The person must have had a full workup first, including an MRI or CT, and the cause must still be unclear.¹³

Tau PET and DaTscan. Ask your doctor and your plan before scheduling, because coverage can vary.

Medicare Advantage and other plans. Your plan may need to approve the scan first. Call your plan before the scan. For help paying the 20%, see paying for new Alzheimer's drugs and what Medicare covers in dementia.

Questions to ask the doctor

Why do I need this scan?

• What are you looking for? What would change if the result is positive or negative?
• Is an MRI right for me, or should I have a CT?
• Have I already had a scan that could be used instead?
• Who will explain the results to us, and when?

Do I need a PET scan, a spinal fluid test or a blood test?

• Which test makes the most sense for me, and why?
• Am I thinking about lecanemab or donanemab? See are anti-amyloid treatments right for us?
• Is the PET scan done nearby, and who reads it?

What will it cost?

• Is the imaging center accredited and in my plan's network?
• Will this be done at a hospital outpatient department or an independent center?
• Does my plan need prior approval?

Keep scan dates and results in a memory notebook. After the visit, our page on the first 30 days after a diagnosis can help with next steps.

Education only. This page is general information written from the sources listed. It is not medical, legal or financial advice and does not replace a doctor, therapist or lawyer who knows your situation. How we write and check pages.