Research

What can PET scans tell us about dementia?

Our series of research videos show what it’s like to have five different types of assessment for dementia.

This post shows how one of them – the amyloid PET scan – is becoming a vital part of research by revealing the microscopic signs of Alzheimer’s disease. 

What can brain scans tell us about dementia?

The two characteristic changes in the brain of people who have Alzheimer’s disease are clumps of toxic proteins called amyloid and tau. Build-up of these proteins along with the death of brain cells defines the disease and leads to people’s symptoms.

The first account of these changes in the brain being related to the symptoms of Alzheimer’s disease is attributed to Alois Alzheimer in the early 1900s. The disease was later named after him. Seventy years later,  the first computerised tomography (CT) brain scans were developed that revealed shrinkage of the brain due to loss of brain cells in people living with Alzheimer’s.

Since then, tools for measuring the size and shape of the brain have improved dramatically. Magnetic resonance imaging (MRI) brain scans acquire beautifully detailed images of the structure of the brain, and can pinpoint more subtle areas of shrinkage as groups of brain cells die.

Until recently, spotting the toxic proteins was impossible except under the microscope, making it difficult to be certain whether the shrinkage was caused by Alzheimer’s disease, or one of the other less common causes of dementia.

Why have a PET scan?

In the early 2000s, there was an exciting development – it became possible to see amyloid in the brains of living people.

Scientists developed a special compound that could be injected into the bloodstream and attaches to clumps of amyloid protein in the brain. The compound emits a small radioactive signal that can be detected by a Positron Emission Tomography (PET) scanner.

This breakthrough made waves in the scientific community as it allowed researchers to be much more confident about the microscopic elements that were causing people’s symptoms.

‘This is vitally important because new drugs being tested are designed to target amyloid in the brain.’

Including people without amyloid pathology in previous tests of these drugs could be one reason why early trials failed.

With this new technology, researchers can be more confident that the right patients are being given the right treatments, improving the chance of finding a successful new treatment.

What happens during a PET scan?

When having a PET scan a small dose of a radioactive compound is injected, and after about an hour the scan is carried out to detect the signal that is emitted. The amount of radiation used is small but as a precaution people having a scan are advised to avoid prolonged close contact with pregnant women, babies or young children for a few hours afterwards.

The tools for detecting these microscopic signs of Alzheimer’s disease are constantly improving. Most recently, compounds have been developed that allow abnormal clumps of tau to be detected by PET scans. Precise measures of these proteins can also now be obtained using cerebrospinal fluid from a lumbar puncture.

These developments will continue to push forward our understanding of how Alzheimer’s develops and help us to test whether new drugs are successful, bringing us one step closer to a world without dementia.