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PET scan


A positive emission tomography (PET) scan is used to produce a detailed, three-dimensional picture of the inside of the body. The images clearly show the part of the body that is being investigated and can also highlight how effectively certain functions of the body are working.

What are PET scans used for?

PET scans are most commonly used to help diagnose a range of different cancers and work out the best ways of treating them. The information provided by a PET scan can show how far a cancer has spread or how well it is responding to treatment.

PET scans are occasionally used to help plan complex heart surgery, such as a heart transplant. They are also used to help diagnose a number of conditions that affect the normal workings of the brain (neurological conditions), such as dementia.

Read more about why you might need a PET scan .

How it works

Before the scan takes place, a radioactive substance, known as a radiotracer, will be passed into your body either by injection, through an inhaler, or in the form of a small tablet or capsule that you swallow.

The tracer gives off particles called positrons that release a type of radiation known as gamma waves, which can be detected by the PET scanner.

By tracking the movement of the tracer, the scanner can build up a detailed image of a number of the body’s functions as well as highlighting areas of the body that have been affected by disease.

Read more about how PET scans work.

You will need to lie on a flat bed which will move you through a large circular scanner. 

The scan should not be painful but, for some people, being inside the scanner can be claustrophobic.

Read more about what happens during a PET scan.


PET scanners are expensive, so they are usually only found at larger hospitals and some specialised research centres.

Due to the lack of availability, PET scans are often only recommended for people with complex health problems. They are not routinely used to diagnose cancer, but they are often used in confirmed cancer cases to check how far the cancer has spread and whether treatment has been effective.

However, PET scans are becoming more widely used by the NHS, with an average of 40,000 PET scans being carried out by the NHS in Wales and  England each year.


Any exposure to radiation carries a very small risk of causing damage to tissues and the possibility of triggering a new cancer.

However, the amount of radiation you would be exposed to if you had a standard PET scan would be the same amount you would receive from natural sources, such as the sun (background radiation) over the course of three years.

Increasingly, PET scans are being combined with computerised tomography (CT) scans to provide more detailed images. See PET scan - how does it work for more information.

These new types of scanners, known as PET-CT scanners, use higher levels of radiation, which are equivalent to the recommended limit of radiation that anyone working in a nuclear power plant should be exposed to in any given year. However, this increased dose is still well within the acceptable safety limits for radiation exposure.

Read more about the possible risks of PET scans.

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Why is it necessary?

A positive emission tomography (PET) scan is usually used in combination with other tests, such as X-rays or magnetic resonance imaging (MRI) scans, to investigate or diagnose health conditions. PET scans can also be used to find out how well a treatment is working.

The main advantage of a PET scan compared with other types of scans, such as an X-ray or MRI scan, is that it can show how well a certain part of the body is working, rather than simply showing what it looks like.

This can show how certain conditions have affected the functions of the body and enable healthcare professionals to track accurately how far a condition, such as cancer, has spread.


One of the most common uses of a PET scan is to investigate confirmed cases of cancer. A PET scan can usually provide three important pieces of information:

  • how far a particular cancer has spread (this is known as staging - the higher the stage, the further the cancer has spread)
  • how well a particular cancer is responding to treatment
  • whether any cancerous cells remain after a course of treatment has been completed

Neurological conditions

A less common use of PET scans is to help diagnose a number of conditions that can affect the brain and nervous system (neurological conditions).

For example, a PET scan can be used to help plan brain surgery in cases of epilepsy that have not responded to medication.

The PET scanner can sometimes locate damage to the brain that is responsible for triggering seizures (seizures occur when the normal electrical activity of the brain becomes disrupted). The damaged areas of the brain can be removed using surgery.

PET scans can also help to diagnose dementia (a group of symptoms that are related to a decline in brain function) and Parkinson’s disease (a long-term neurological condition that affects around 120,000 people in the UK).

Heart disease

A PET scan is very effective in tracking the blood flow in and around the heart, so it is often used to determine whether someone could benefit from different types of heart surgery, such as a:

  • heart transplant - where a diseased heart is replaced with a healthy heart taken from a donor who has died
  • coronary artery bypass graft - where a blood vessel that is taken from another part of the body is used to bypass a blocked or narrowed section of artery
  • angioplasty - where a blocked or narrowed section of artery is widened using a small metal tube called a stent

Medical research

PET scans can also be used to look at how the body works and to help understand what happens when something goes wrong with the way the body functions.

For example, researchers are currently using PET scanners to study the brain function of people with autism.

When not to have a PET scan

A PET scan may not be suitable if you have chemical imbalances in your body. For example, the scan may give false results if you have poorly controlled diabetes.

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How does it work?

During a positive emission tomography (PET) scan, a substance known as a radiotracer is passed into your body. A radiotracer is a radioactive chemical that releases tiny particles called positrons.

There are several ways that a radiotracer can be introduced into the body. It can be injected into one of your blood vessels, it can be inhaled as a gas or it can be swallowed in the form of a tablet or capsule.

Most PET scanners use a radiotracer called fludeoxyglucose (FDG), which is similar to naturally occurring glucose except that it is radioactive. The advantage of using FDG is that your body will treat it in a similar way to normal glucose.

Studying how different parts of the body respond to glucose provides a great deal of information about the processes of the body. For example, cancerous tissue processes glucose in a different way from normal body tissue, so using FDG is an effective way of detecting cancers.

As FDG moves through your body, it will release a stream of positrons. The positrons quickly break down, releasing energy waves known as gamma waves. The PET scanner is designed to detect gamma waves, which show up as a three-dimensional image on a computer screen.

The image

The image produced during a PET scan highlights how certain parts of your body break down the radiotracer. Different concentrations of positrons show up as areas of a different colour and brightness on a PET scan image.

A radiologist, which is someone who is trained to interpret images of the inside of the body, will look at the images produced by your PET scan. They will report the findings to your specialist doctor.

PET-CT scanners

The newer generation of PET scanners, known as PET-CT scanners, also incorporate a computerised tomography (CT) scanner. A CT scanner takes a series of X-ray images and a computer is used to assemble each scan into a more detailed image.

The advantage of combining both types of scanner is that a PET scan can provide information about how well a certain part of the body is functioning and the CT scanner can provide information about the body part’s appearance (its anatomy).

Some conditions, such as cancer, can cause disruption to both the function and appearance of the affected body part. 

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How is it performed?

A positive emission tomography (PET) scan is usually performed on an outpatient basis, which means that you will not need to stay in hospital overnight.


Your doctor will give you instructions about how to prepare for your scan. You will not usually be able to eat anything for four to six hours before your scan and you will be advised to drink plenty of water. You may also be advised not to drink any caffeine during the 24 hours before your scan.

Eating food and drinking caffeine could change how your body responds to the radiotracer, which could make the result of your scan unreliable.

The PET scan procedure

A PET scanner is a machine that consists of a flat bed with a large, circular scanner at one end.

Before the scan can take place, a radiotracer will need to be introduced into your body. A radiotracer is a radioactive substance. See PET scan - how does it works for more information. Depending on which part of your body is to be scanned, this can be done by:

  • injecting the radiotracer into one of your blood vessels
  • using an inhaler to breathe in the radiotracer as a gas
  • swallowing a tablet or capsule

The radiotracer can take 30 to 90 minutes to travel around your body.

When you are ready for your scan, you will be taken to the examination room where the PET scanner is located. You will be asked to lie on an examination table which will be moved into the hole at the centre of the scanner.

During the scan, you will be asked to stay still and not to talk. Depending on the part of your body being scanned, the scan will usually take 30 to 60 minutes to complete.

The scan should not be painful, but if you feel unwell there is a buzzer that you can press to alert the medical team who are in charge of your care. The medical staff will be able to see you throughout the scan.

For some people, being inside the scanner can be a claustrophobic and unpleasant experience. If you have a previous history of claustrophobia, you should inform staff before the day of the scan. They may be able to arrange for you to have medication, known as a sedative, that will help you to relax during the scan.


As a PET scan involves exposure to only a very small amount of radiation, you will not experience any side effects and can usually go home soon after the scan has been completed. If you have had a sedative, you will need to arrange for someone to drive you home after the scan.

You should drink plenty of fluids after the scan to help flush the radioactive medication from your body. The radiotracer should leave your body naturally around three hours after it was given.

The results of your PET scan will be sent from the radiology department to your specialist. However, it may take a couple of weeks for your specialist to receive the results.

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During a positive emission tomography (PET) scan, your body’s tissue will be exposed to radiation. Any exposure to radiation carries a risk of damaging the body’s tissues and possibly triggering a cancer at a later date. However, the risk is very small.

Radiation is measured in units known as millisieverts, or mSv for short. A standard PET scan would expose you to about 7mSv, which is the same amount of radiation that you are exposed to from natural sources of radiation, such as the sun, over the course of three years.

A combined PET-CT scan would expose you to around 25mSv, which is just over the recommended annual limits that someone who works with radiation should be exposed to during the course of a year (20mSv).

Most experts believe that the risk of cancer developing only becomes significant in people who are exposed to 100mSv or more.

However, as a precaution, a PET scan is not recommended for women who are pregnant or breastfeeding unless there is a clear case that the benefits of the scan outweigh any risks to the baby. Close contact with pregnant women, babies or young children should be avoided for a few hours after having a PET scan.   

Read more about radiation

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