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All you need to know about brain scans and dementia

Explainer

Emma Taylor

On 10 June 2022

MRI scan on screen with person in scanner visible through window

When someone goes to the doctor with dementia symptoms, a brain scan can be used alongside other tests like blood tests and memory tests to find out the cause. Scans are not always used but can help a doctor to see what type of dementia someone has, and to rule out other conditions.

Dementia is caused by different diseases, like Alzheimer’s disease and vascular dementia. These diseases damage different areas of the brain and cause different symptoms. Brain imaging allows doctors to see where damage has occurred.

Brain scanning and imaging has led to more accurate ways to diagnose dementia. It has also allowed scientists to understand more about what happens in the brain as diseases like Alzheimer’s start and progress. We’ll give some information about the commonly used scans in dementia diagnosis and dementia research which we hope you find helpful.

Doctor showing patient a brain scan

Computerised tomography (CT)

A CT scan involves combining a series of X-rays taken from different angles of the head to produce images of a person’s brain. CT scans are the most common type of brain scan used in dementia diagnosis.

They are useful for ruling out other conditions that cause similar symptoms to dementia and at showing changes to brain structure that occur in diseases like Alzheimer’s.

CT scans can show:

stroke related damage.
brain shrinkage (atrophy) and blood vessel changes seen in dementia.
signs of a brain tumour.
build-up of fluid or blood in the brain.

A CT scan involves lying in a scanner for 10-20 minutes. You may be asked to breathe in our out, or hold your breath for a short time during the scan.

For more information: https://www.nhs.uk/conditions/ct-scan/

Magnetic resonance imaging (MRI)

MRI scans use a strong magnet and radio waves to produce detailed images of inside the body. Using MRI, we can get a good idea of what a person’s brain looks like.

A brain MRI can help doctors look for:

brain shrinking (atrophy).
damage following a stroke.
problems with blood vessels.
inflammation.
tumours.
damage following an injury.

A CT scan showing the structure of a healthy brain (left) and a brain with Alzheimer’s disease (right). Image credit Professor John O’Brien, University of Cambridge and Newcastle University.

In Alzheimer’s disease, the hippocampi (circled in the image below) are often affected first. A doctor will use an MRI to see if there are visible changes to these structures, which can help to diagnose Alzheimer’s.

Healthy brain scan vs a brain scan of someone with Alzheimer's. The memory centres of the person with Alzheimer's have shrunk.

Images credit: Professor John O’Brien, University of Cambridge and Newcastle University

An MRI involves lying still for 15-90 minutes. Because MRI scans use magnets, people with metal in their bodies or a pacemaker cannot have this type of scan and you’ll be asked to remove metal you are wearing, such as belts or jewellery.

Below you can hear from Larry, who has been diagnosed with mild cognitive impairment. He is taking part in a clinical trial for the drug lecanemab, so has an MRI scan every few months to monitor for side effects.

Everyone’s experience is different. If you’re worried about your own memory and thinking, or a loved one’s, speak to your doctor.

For more information: https://www.nhs.uk/conditions/mri-scan/

Functional magnetic resonance imaging (fMRI)

fMRI is used to look at activity in the brain, like blood flow.

fMRI is more commonly used in research rather than dementia diagnosis, to compare brain activity in people with dementia against people without dementia. Usually, the person is given a task to complete during the scan, like a thinking test. Researchers observe increases and decreases in brain activity during the task and compare results between the two groups. This gives us clues about how dementia affects our brain’s ability to do things like solve puzzles.

An fMRI usually takes longer than an MRI. They can involve lying still for around an hour.

Electroencephalography (EEG)

An EEG is a recording of overall brain activity. It involves attaching small sensors called electrodes to the top of the head (scalp) to measure electrical signals in the brain. They are usually used to diagnose conditions like epilepsy which causes abnormal electrical patterns in the brain.

EEG is sometimes used to diagnose different types of dementia. This is because the pattern of electrical activity in a brain with dementia is different to a healthy brain. Using this scan to look for brain activity patterns typical of dementia can improve the accuracy of diagnosis compared to using traditional memory and thinking tests. As seizures can be a symptom in some types of dementia, EEGs can also investigate this.

EEGs require you to either sit or lie down during the scan, which should take around 20-40 minutes.

For more information: https://www.nhs.uk/conditions/electroencephalogram/

Single-photon emission computerised tomography (SPECT)

Where an MRI or CT scan tend to produce 2D images of the brain, SPECT imaging can provide true 3D images. This scanning method involves injecting a substance that emits low level radioactive rays attached to a dye. This substance binds to cells in the brain that doctors or scientists are trying to investigate, and builds a 3D image based on the rays released by the dye whilst the scan rotates around the patient.

SPECT imaging can be used to study the flow of blood throughout the brain. As each type of dementia is associated with different patterns of blood flow, these scans can help doctors and scientists to distinguish between them. This allows doctors to diagnose which disease is causing a patient’s dementia and decide on the right course of treatment for them.

Positron emission tomography (PET)

A PET scan showing build of up amyloid in a healthy brain (left) and a brain with Alzheimer’s (right). Image credit: Dr Manja Lehmann, University College London & University of California SF.

PET scans detect specific molecules in the brain. They are more commonly used in research than for diagnosis. By injecting or swallowing safe radioactive dyes which bind to molecules in the brain, the scanner can produce an image of where the target molecules are by detecting the dyes.

PET scans are used to show:

abnormal build-up of proteins that cause diseases like Alzheimer’s.
how much glucose is present in the brain, to look at brain activity.

The images below are from a PET scan showing a protein called amyloid, a key feature of Alzheimer’s disease. Warmer colours (red) represent areas with more amyloid. The brain on the right is typical of someone with Alzheimer’s disease. This type of scan allows researchers to understand more about changes that occur in a brain with dementia and to see if new treatments are removing or slowing down amyloid build-up.

A PET scan of a healthy brain and a brain with Alzheimer's disease. The healthy brain has very little amyloid protein (shown in red), and the Alzheimer's brain has lots, particularly around the front and sides of the brain.

Healthy brain (left) Alzheimer’s disease (right) Image credit: Dr Manja Lehmann, University College London & University of California SF

The dye is injected or swallowed at least an hour before the scan to let it reach the target, then you will lie in the scanner for 30-60 minutes. You should not wear anything with metal such as a watch or jewellery during the scan.

Different methods of imaging the brain have allowed doctors and researchers to understand more about the diseases that cause dementia. As research and technology continues to make advances, this understanding will increase and will lead to improved accuracy in dementia diagnosis and treatments for people with dementia. This progress brings us closer to a world where people are free from the fear, harm and heartbreak of dementia.

Our Dementia Research Infoline can answer any questions you may have about dementia, diagnosis, dementia research or how to get involved in research. You can contact them at infoline@alzheimersresearchuk.org or call 0300 111 5 111 (Monday-Friday, 9am-5pm).

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