Brain Imaging: MRI, CT and PET

What MRI, CT and PET brain scans can and can't show when doctors evaluate memory problems, how amyloid, tau and FDG PET scans work, and what to expect on the day of a scan.

Last reviewed: October 2026

When someone is being checked for memory problems, the doctor will often order a brain scan. It's natural to hope the scan will give a clear yes-or-no answer about dementia. In reality, brain imaging is one important piece of a bigger puzzle. As the Alzheimer's Association explains, there is no single test that can determine whether a person has Alzheimer's or another dementia. Doctors combine scans with a medical history, a physical and neurological exam, Cognitive Screening Tests (MMSE, MoCA), and sometimes blood or spinal fluid tests.

Why doctors order a brain scan

Brain scans do two main jobs:

Rule out other causes. According to the Alzheimer's Association, the standard workup often includes an MRI or CT mainly to rule out conditions that can cause similar symptoms but need different treatment. These include tumors, small or large strokes, damage from a serious head injury, and a buildup of fluid in the brain (see Normal Pressure Hydrocephalus).
Support a diagnosis. The National Institute on Aging (NIA) says scans can support a diagnosis of Alzheimer's or another dementia, or point toward a different cause.

Scans may also be repeated over time to see how the brain is changing (NIA).

Structural scans: MRI and CT

These scans show the brain's structure—its size, shape and any damage.

MRI (magnetic resonance imaging)

MRI uses strong magnetic fields and radio waves, not X-rays, to make detailed pictures of the brain. NIA notes that MRI can:

Show whether parts of the brain have shrunk (atrophied)
Give a detailed picture of the brain's blood vessels, helpful for spotting signs of strokes and Vascular Dementia
Help rule out bleeding or a buildup of fluid
Show changes over time when scans are repeated

An important limit: brain shrinkage may support a diagnosis of Alzheimer's or another dementia, but it cannot point to one specific diagnosis on its own (NIA).

MRI is also used to monitor safety in people taking the newer anti-amyloid medicines, such as Lecanemab (Leqembi) and Donanemab (Kisunla), which can cause brain swelling or small bleeds called ARIA. See ARIA: Side Effects of Anti-Amyloid Drugs.

CT (computed tomography)

A CT scan is a type of X-ray that uses radiation to make images. NIA explains that a head CT can show shrinkage of brain regions and signs of other problems, such as an infection or a blood clot. Doctors may compare the size of brain areas with earlier scans or with what's expected for a person's age. CT is often used when someone can't have an MRI because of metal in the body, such as a pacemaker.

Functional and molecular scans: PET

PET (positron emission tomography) uses a small amount of a radioactive substance called a tracer, given by injection. Different tracers show different things. According to NIA:

Amyloid PET measures deposits of beta-amyloid, the protein that forms the plaques seen in Alzheimer's. A positive scan may mean symptoms are due to Alzheimer's. But some people have amyloid plaques and never develop symptoms, so doctors weigh the result with other tests. A scan showing few or no plaques usually means Alzheimer's is not the cause.
Tau PET detects tangles of the tau protein. It may be used to track Alzheimer's, but it isn't common in everyday practice and is used more in research.
FDG PET measures how brain areas use sugar for energy. People with dementia often show reduced use in certain areas. In clinical care, it may be used when a doctor strongly suspects Frontotemporal Dementia rather than Alzheimer's.

Checking for amyloid—by PET, spinal fluid or blood—is also part of deciding whether someone may be a candidate for the newer anti-amyloid treatments. Your doctor can explain whether that applies. See Amyloid and Tau Explained and Dementia Treatments: An Overview.

What to expect on the day

Every center is different, so follow the instructions you're given. For a head MRI, MedlinePlus describes:

Time: usually 30 to 60 minutes, sometimes longer.
Lying still on a narrow table that slides into a tunnel-shaped scanner. Movement blurs the pictures.
Noise: loud thumping and humming. Ask for earplugs or headphones.
Contrast dye is sometimes given through a vein.
No pain, and usually no recovery time.
No metal. Jewelry, watches, hearing aids, eyeglasses, hairpins and removable dental work stay outside the room.

Tell the team beforehand if the person has a pacemaker or defibrillator, aneurysm clips, cochlear implants, a heart valve, a stent, a recent joint replacement, kidney disease, or a history of working with sheet metal (MedlinePlus).

Tips for someone with memory problems

A scan can be stressful for a person who is confused or anxious. Some ideas:

Mention claustrophobia or anxiety when booking. MedlinePlus notes that medicine to help relax, or an open MRI, may be options.
Explain simply just before the scan rather than days ahead, which can cause worry. See Repetitive Questions and Behaviors.
Go along. Ask whether a family member can stay nearby, and bring a list of medicines and implants.
Schedule for their best time of day, often the morning.
Arrange a ride if relaxing medicine will be given.

See Preparing for a Memory Appointment for more.

Understanding the results

Scan reports are written for doctors and can be full of terms like "volume loss" or "white matter changes." Some changes are common with age. Don't try to interpret the report on your own; ask the doctor:

What did the scan show, and what does it rule out?
How does it fit with the other test results?
Do we need more tests, or a repeat scan later?
Will insurance cover any further imaging? (See What Testing Costs and Insurance Covers.)

When to talk to a doctor

If you or someone you love has memory or thinking changes, start with a primary care doctor, who can decide whether a scan or specialist referral is needed. See Getting a Diagnosis and Which Doctor Diagnoses Dementia?. Sudden confusion, weakness, trouble speaking or a severe headache is different: call 911, because these can be signs of a stroke. This article is general information; your doctor can explain which tests are right for you.

Sources

Not medical advice. This article is general information, not a diagnosis or treatment plan for any individual. Talk with a doctor about your situation.

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