PET Scans

Positron Emission Tomography (PET) is used to look at how your body uses substances such as glucose, ammonia, water and oxygen. Seeing how these molecules move through your body, and where they are being used, allows your doctor to check for anything unusual that might suggest the presence of disease.

How PET Scanning Works

Positron Emission Tomography (PET) is used to look at how your body uses substances such as glucose, ammonia, water and oxygen. Seeing how these molecules move through your body, and where they are being used, allows your doctor to check for anything unusual that might suggest the presence of disease.

By tracking how your body uses glucose, a PET scan can produce images of cancerous tumours and help your doctor work out what treatment is best. Your body uses glucose - a type of sugar - for energy so cancerous tissue, which uses more glucose than normal body tissue, will show up as a bright area on the PET image.

If oxygen is used as the tracking molecule, PET scans can be used to image brain activity and look at blood flow in the heart to detect coronary heart disease and other heart problems.

To allow the molecules to be tracked, radioactive isotopes are attached to them before they are injected into your body. A radioactive isotope is an unstable atom with too many or too few neutrons in its nucleus. In PET the isotopes used have too few neutrons. This makes them unstable and eventually their nuclei will split, releasing positrons - positively charged electrons- in the process. When these positrons come into contact with electrons in neighbouring atoms they generate gamma rays. The PET scanner detects these gamma rays and works out where in your body they have come from, and hence the location of the molecules being tracked.

This information is then used to generate a series of images that use different colours or degrees of brightness to show the changing concentration and location of the molecule over time.

The radioactivity of the isotope decreases with time, so to get a good series of images the PET scan has to be performed as soon as possible after you've been injected.

Although having a PET scan means you are injected with a radioactive substance, the dose of radiation involved is about the same as you would be exposed to naturally over two or three years. This means that your body isn't damaged, although you wouldn't want to have lots of scans in a short period of time.

PET Scan Images

Below you will see three sets of PET scans of three people's brains.

Healthy Patient

Set of 6 PET images, showing healthy patient's brain

This scan is from a healthy 70-year-old control patient. The FDG scan looks typical for the age with a good cortical signal (that is in the outer rim of this transverse section through the brain).

Patient with Alzheimer's Disease

 

Set of 6 PET images, showing brain of patient with Alzheimer's Disease

Alzheimer's disease (AD) is a condition where the brain gradually deteriorates, leading to changes in a person's behaviour, as well as severe short-term memory loss and cognitive impairment.

AD is usually diagnosed by assessing the patient's memory and intellectual function through verbal tests. Brain imaging using PET can confirm a diagnosis as patients with Alzheimer's will typically show a reduction in glucose use in the cerebral cortex - the thin, outermost layer of the brain responsible for many complex brain functions including memory, language and consciousness.

Patient with Brain Tumour

 

Set of 6 PET images, showing brain tumour

Brain tumours are areas of tissue where the cells have mutated and are replicating uncontrollably. Mutation can occur as a result of normal brain cells coming into contact with radiation or harmful chemicals, which damage the DNA of the cells.

PET scans are useful for identifying tumours because they use more energy, and therefore glucose, than normal tissue. The FDG tracer, which is a form of glucose, occurs in higher quantities at the tumour site and this shows up as a bright area on the PET image.