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PET scan

PET scans are a type of test that create 3 dimensional (3D) pictures of the inside of your body. PET stands for positron emission tomography.

This type of scan can show how body tissues are working, as well as what they look like. It's used to help diagnose some conditions including cancer. It can also help to find out where and whether cancer has spread.   

You’ll usually have a PET scan in the radiology 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 30 and 60 minutes. You'll be in the department for a least an hour beforehand as you need dye for the scan. 

PET scans are often combined with CT scans to produce more detailed images. These are called PET-CT scans.

Why you might have 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 water during this time.

You might have instructions not to do any strenuous exercise for 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. And 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. Metal interferes with the images created by the scanner.

The radiographer puts a thin plastic tube (cannula) into a vein in the back of your hand. You have an injection of a dye called a radiotracer about an hour before the scan. 

The radiotracer 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.

You need to rest and avoid moving too much during this hour. You usually lie down. This allows the drug to spread through your body and into your tissues.

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.

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

After your PET scan

Your radiographer removes the cannula from your hand 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 drug out of your system.

You’ll still have some radiotracer left in your body after the test. This will go away very quickly. As a precaution, you should avoid contact with pregnant women, babies or young children for at least 6 hours after your scan.

You need someone to take you home and stay overnight if you’ve had medicine to help you relax (sedative). You also shouldn’t drive, drink alcohol, operate heavy machinery or sign any legally binding documents for 24hours.

If you are travelling abroad within 3 days of your scan, it may be a good idea to take your appointment letter with you to show that you have had a scan. Most airports have sensitive radiation monitors which may pick up the trace of radiation 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.


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 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 drug. You might need to store enough expressed milk for at least one feed.


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

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. 

Bruising and swelling

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

There's 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.

More information

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

Last reviewed: 
19 Dec 2018
  • Clinical Commissioning Policy Statement: Positron Emission Tomography – Computed Tomography (PET – CT) Guidelines (all ages)
    NHS England, 2015

  • Cancer and its management (7th edition)
    J Tobias and D Hochhauser 
    Blackwell, 2015

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

  • The Role of PET Scan in Diagnosis, Staging, and Management of Non-Small Cell Lung Cancer
    L Schrevens and others
    The Oncologust, 2004. Volume 9, Pages 633 – 643

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