PET scan

Positron emission tomography (PET) scans are used to produce detailed three-dimensional images of the inside of the body.

The images can clearly show the part of the body being investigated, including any abnormal areas, and can highlight how well certain functions of the body are working.

PET scans are often combined with computerised tomography (CT) scans to produce even more detailed images. This is known as a PET-CT scan.

They may also occasionally be combined with a magnetic resonance imaging (MRI) scan. This is known as a PET-MRI scan.

Why PET scans are used

An advantage of a PET scan is that it can show how well certain parts of your body are working, rather than showing what it looks like.

They're particularly helpful for investigating confirmed cases of cancer, to determine how far the cancer has spread and how well it's responding to treatment.

Sometimes PET scans are used to help plan operations, such as a coronary artery bypass graft or brain surgery for epilepsy. They can also help diagnose some conditions that affect the normal workings of the brain, such as dementia.

How PET scans work

PET scanners work by detecting the radiation given off by a substance called a radiotracer as it collects in different parts of your body.

In most PET scans a radiotracer called fluorodeoxyglucose (FDG) is used, which is similar to naturally occurring glucose (a type of sugar) so your body treats it in a similar way.

By analysing the areas where the radiotracer does and doesn't build up, it's possible to work out how well certain body functions are working and identify any abnormalities.

For example, a concentration of FDG in the body's tissues can help identify cancerous cells because cancer cells use glucose at a much faster rate than normal cells.

What happens during a PET scan

PET scans are usually carried out on an outpatient basis. This means you won't need to stay in hospital overnight.

It's important to arrive on time for your scan, as the radiotracer used has a short shelf-life and your scan may be cancelled if you're late.


Your appointment letter will mention anything you need to do to prepare for your scan.

You'll usually be advised not to eat anything for 6 hours beforehand. Drinking is allowed, but you should ideally just drink water. You should also avoid strenuous exercise for 24 hours before your appointment.

It's a good idea to wear loose comfortable clothes, as you may be able to wear these during the scan (although sometimes you may be asked to change into a hospital gown).

Try to avoid wearing jewellery and clothes containing metal (such as zips), as these will need to be removed.

Radiotracer injection

Before the scan, the radiotracer is injected into a vein in your arm or hand. You'll need to wait quietly for about an hour, to give it time to be absorbed by the cells in your body.

It's important to relax, keep as still as possible, and avoid talking while you wait, as moving and speaking can affect where the radiotracer goes in your body. You can go to the toilet if you need to at any point.

The scan

For the scan, you lie on a flat bed that will be moved into the centre of the large, circular scanner.

You should stay still and not talk while the scanner takes pictures of your body, which usually takes up to 30 minutes.

The scan is completely painless, but you may feel uncomfortable lying still for this long. If you feel unwell at any point there is a buzzer you can press to alert the medical team. The medical team will be able to see you throughout the scan.

For some people, being inside the scanner can be an unpleasant experience. Inform the hospital before the day of the scan if you think this may be a problem for you. They may be able to arrange for you to have a sedative to help you relax during the scan.


You should not experience any side effects after a PET scan and should usually be able to go home soon afterwards.

The results of your scan will not usually be available on the same day. They'll be sent to your specialist to be discussed at your next appointment.

Are there any risks of a PET scan?

Any exposure to radiation carries a very small risk of potential tissue damage that could lead to cancer at a later date.

However, in a standard PET scan the amount of radiation you're exposed to is small – about the same as the amount you get from natural sources, such as the sun, over 3 years.

The radiotracer becomes quickly less radioactive over time and will usually be passed out of your body naturally within a few hours. Drinking plenty of fluid after the scan can help flush it from your body.

As a precaution, you may be advised to avoid prolonged close contact with pregnant women, babies or young children for a few hours after a PET scan, as you will be slightly radioactive during this time.

The CT component of a PET-CT scan also involves exposure to a small amount of additional radiation, but the risk of this causing any problems in the future is still very small.

Last updated:
17 May 2023

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