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PET scan

Find out about having a PET scan what it is, how you have it and what happens afterwards.

PET stands for positron emission tomography. This type of scan can show how body tissues are working, as well as what they look like.  

Not everybody who has cancer will need to have a PET scan. Other types of tests and scans may be more suitable.

You’ll usually have a PET scan in the x-ray 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.

Why you might have a PET scan

A PET scan can help to:

  • show up a cancer
  • find out the stage of a cancer, so doctors know how big it is and whether it has spread
  • show whether a lump is cancer or not
  • show whether a cancer has spread to other parts of the body
  • decide the best treatment for your cancer

PET scans can also show how well a cancer drug 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 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 produced by the scanner.

You have an injection of a dye called a radiotracer about an hour before the scan. You have this into your arm through a small plastic tube called a cannula.

Then you rest and avoid moving too much. This allows the drug to spread through your body and travel to places where glucose is used for energy, like your brain.

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 tube from your arm before you go home.

You can then eat and drink normally.

You need someone to take you home if you’ve had medicine to help you relax. You won’t be able to drive for the rest of the day, as you might be drowsy.

If you are travelling abroad within a few 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 baby. Contact the department as soon as you can before the scan if you are, or think you might be, pregnant.

Contact the department if you are breastfeeding. They will let you know if you need to stop breastfeeding for a length of time after the radioactive injection. You might need to store enough milk for one feed.

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.

More information

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

Last reviewed: 
28 Apr 2015
  • A Framework for the Development of Positron Emission Tomography (PET) Services in England

    Department of Health, 2005

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

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