PET scan

A PET scan is a test that creates 3 dimensional (3D) pictures of the inside of your body. PET stands for positron emission tomography.

The PET scan uses a mildly radioactive liquid (radioactive tracer) to show up areas of your body where cells are more active than normal. It's used to help diagnose some conditions, including cancer. It can also help to find out where and whether cancer has spread.   

You usually have a PET scan in the radiology or nuclear medicine department as an outpatient. These scanners tend to be only in the major cancer hospitals. So you might have to travel to another hospital to have one.

A radiographer operates the scanner. It usually takes between 45 and 60 minutes. You will be in the department for at least an hour beforehand. This is because you have the injection of the radioactive tracer before the scan. 

PET scans are often combined with CT scans to produce more detailed images. These are called PET-CT scans. You can also have an MRI scan with a PET scan. These are called PET-MRI scans. 

Why do I need a PET scan?

A PET scan can help to:

  • show up a cancer
  • find out how big it is and whether it has spread (stage a cancer)
  • show whether a lump is cancer or not
  • decide the best treatment for your cancer
  • show how well a treatment is working

After you have had treatment for cancer, a CT scan may show that there are still some signs of the cancer left. But this may not be active disease. It could be scar tissue left over from cancer killed off by your treatment. A PET scan can show whether this tissue is active cancer or not.

PET scans are sometimes used to look for cancer in the lymph nodes in the centre of the chest.

Preparing for your PET scan

For most PET scans, you need to stop eating for about 4 to 6 hours beforehand. You can usually drink unflavoured water during this time.

You might have instructions not to do any strenuous exercise for between 12 and 24 hours before the scan. 

Call the number on your appointment letter if not eating is a problem for you, for example if you’re diabetic. You might need to adapt your diet and sugar control and your appointment time could change.

Some people feel claustrophobic when they‘re having a scan. Contact the department staff before your test if you’re likely to feel like this. They can take extra care to make sure you’re comfortable and that you understand what’s going on. Your doctor can arrange to give you medicine to help you relax, if needed.

What happens?

At the hospital

Your radiographer might ask you to change into a hospital gown. You have to remove any jewellery and other metal objects such as hair clips, coins, belts and a wired bra. Metal interferes with the images created by the scanner.

You have the injection of the radioactive tracer about an hour before the scan. You have the injection through a small plastic tube in your arm (cannula). It's only a small amount of radiation.

You need to rest and avoid moving too much during this hour. This allows the radioactive tracer to spread through your body and into your tissues.

The radioactive tracer is a radioactive sugar. The one commonly used is called FDG (fluorodeoxyglucose). Cancer cells are very active when they are growing and reproducing in a specific area. They need energy to grow. So, active cancer cells take up the FDG which then shows up brighter on the scan.

In the scanning room

Your radiographer takes you into the scanning room. The PET machine is large and shaped like a doughnut.

You have most scans lying down on the machine couch on your back.

Once you’re in the right position, your radiographer leaves the room. They can see you on a TV screen or through a window from the control room. You can talk to each other through an intercom.

Having the PET scan

The couch slowly slides backwards and forwards through the scanner. The machine takes pictures as you move through it. 

The scan is painless but can be uncomfortable because you have to stay still. Tell your radiographer if you’re getting stiff and need to move.

It’s not particularly noisy but you’ll hear a constant background noise. In most places the radiographer will be able to play music for you. 

When it’s over, your radiographer will come back into the room and lower the couch so you can get up.

This 3-minute video shows you what happens when you have a PET scan or PET-CT scan.

After your PET scan

Your radiographer removes the cannula from your arm before you go home.

You can then eat and drink normally.

The radiation in the radioactive tracer is very small. Drinking plenty of fluids after your scan helps to flush the radioactive tracer out of your system.

The radioactive tracer gives off very small levels of radiation that go away very quickly. As a precaution, you should avoid close contact with pregnant women, babies and young children for 6 hours after the scan.

You need someone to take you home and stay overnight if you’ve had medicine to help you relax (sedative). For the next 24hours you also shouldn’t:

  • drive
  • drink alcohol
  • operate heavy machinery
  • sign any legally binding documents

If travelling abroad within a week of your PET scan, it's a good idea to take your appointment letter with you. This is because most airports have sensitive radiation monitors. This may pick up a trace of radiation left in your body following your test.

Possible risks

A PET scan is a safe test for most people. But like all medical tests it has some risks. Your doctor and radiographer make sure the benefits of having the test outweigh these risks.

Some of the possible risks include:


Pregnant women should only have the scan in an emergency. There’s a risk that the radiation could harm the developing baby. Contact the department beforehand if you're pregnant or think you might be pregnant.


If you're breastfeeding, let the department know a few days before your appointment. They will let you know if you need to stop breastfeeding for a length of time after having the radioactive tracer. You might need to store enough expressed milk for at least one feed.


Exposure to radiation from the radioactive tracer during a PET scan slightly increases your risk of developing cancer in the future. Talk to your doctor if this worries you. 

Bruising and swelling

You might get a small bruise around the area where they put the needle in. 

There is a risk that the radioactive tracer will leak outside the vein. This can cause swelling and pain in your arm but it's rare.

Allergic reaction

Rarely, people have an allergic reaction to the radioactive tracer. This most often starts with weakness, sweating and difficulty breathing. Tell your radiographer immediately if you feel unwell.

Getting your results

Your scan will be looked at by a specialist doctor and you should get your results within 1 or 2 weeks. You won't get any results at the time of the scan. 

Waiting for results can make you anxious. Ask your doctor or nurse how long it will take to get them. Contact them if you haven’t heard anything after a couple of weeks.

You might have the contact details for a specialist nurse. You can contact them for information and support if you need to. It may help to talk to a close friend or relative about how you feel. 

For information and support, you can also call the Cancer Research UK nurses on freephone 0808 800 4040. The lines are open from 9am to 5pm, Monday to Friday.

More information

We have more information on tests, treatment and support if you have been diagnosed with cancer.

  • Recommendations for cross-sectional imaging in cancer management (3rd Edition)
    The Royal College of Radiologists, April 2022

  • FDG PET Scan
    British Nuclear Medicine Society (BNMS) Website. Accessed November 2022

  • The role of PET in first-line treatment of Hodgkin lymphoma
    J Trotman and S F Barrington
    The Lancet, January 2021. Volume 8, Issue 1, Pages E67 to E97

  • PET/CT Imaging Basics and Practice
    K Agrawal and others (Editors)
    Springer, 2022

  • Oxford handbook of clinical medicine (10th edition)
    M Longmore and others
    Oxford University Press, 2017

  • The information on this page is based on literature searches and specialist checking. We used many references and there are too many to list here. Please contact with details of the particular issue you are interested in if you need additional references for this information.

Last reviewed: 
03 Nov 2022
Next review due: 
03 Nov 2025

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