PET scan

Technician carrying out a PET scan with a patient at Guy's and St Thomas' Hospitals

A positron emission tomography (PET) scan produces detailed, three-dimensional pictures that show how well certain parts of your body are working.

It does this by tracking the movement through your body of a radioactive substance known as a radiotracer, which gives off a type of radiation that the PET scanner can pick up. You have the radiotracer as an injection, through an inhaler, or by swallowing a small tablet or capsule.

Most PET scans at done at King's, but a few patients may have them at University College Hospital.

What is it used for?

A PET scan is usually used to help diagnose a range of different cancers, to find out how far they have spread and to decide the best ways of treating them. It gives detailed information about how well parts of your body are working as well as showing the areas that have been affected by disease.

Do I need to prepare?

You will not usually be able to eat anything for about four to six hours before you scan. You will be advised to drink plenty of water. You may also be advised not to have any food or drink containing caffeine for the 24 hours before you scan. This is because caffeine can affect your scan results.

A member of the team providing your care will explain what to do if you do need to prepare.

What happens during the procedure?

You will be given a radioactive substance known as a radiotracer. You may have this as an injection into one of your veins, by breathing it in through an inhaler, or as a small tablet or capsule that you swallow.

The scan itself usually takes between 30 and 60 minutes, depending on which part of your body is being scanned. You lie still on a flat bed which will move you through the large circular scanner. It is safe and should not be painful, but if you feel unwell there is a buzzer that you can press to alert the medical staff caring for you. They will be able to see you throughout your scan.

Some people feel slightly claustrophobic ('closed in') when they are inside the scanner. If you are worried this may happen, please tell us beforehand and we may give you a sedative to help you relax during the test.

Drink plenty of fluids after the scan to help flush the radioactive medication from your body. It should leave your body naturally around three hours after you were given it.

Go to for more information about PET scans.