PET-CT scan

A PET-CT scan combines a CT scan and a PET scan. It gives detailed information about your cancer.

The CT scan takes a series of x-rays from all around your body and puts them together to create a 3 dimensional (3D) picture.

The PET scan uses a mildly radioactive liquid (radioactive tracer) to show up areas of your body where cells are more active than normal. 

You usually have a PET-CT scan in the radiology department as an outpatient. A radiographer operates the scanner. It usually takes between 30 and 60 minutes.

These scanners tend to be available only in the major cancer hospitals. So you might have to travel to another hospital to have one.

Photograph of a PET-CT scanner

Why do I need a PET-CT scan?

PET-CT scans are for many types of cancer. They are generally thought to be more accurate in diagnosing cancer than PET or CT scans alone. PET-CT scans can help to:

  • diagnose cancer
  • find out how big a cancer is and whether it has spread (stage a cancer)
  • decide whether you can have surgery to remove your cancer
  • decide which is the best treatment for your cancer
  • check whether your cancer has come back
  • plan radiotherapy treatment

A PET-CT scan can also show how well a cancer treatment is working.

After you have had treatment for cancer, a scan may show an area that looks like there might still be some cancer left. This might not be cancer but scar tissue left over from the cancer killed off by your treatment. A PET-CT scan can sometimes show whether this tissue is active cancer or not.

Preparing for your PET-CT scan

For most PET-CT scans, you need to stop eating 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 have diabetes. 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?

Your radiographer will ask you to change into a hospital gown. You have to remove any jewellery and other metal objects such as a belt, wired bra, hair clips and coins. Metal interferes with the images produced by the scanner. Wearing trousers or a skirt with an elasticated waistband and no zips might be useful, as you might not need to remove them.

You have the injection of the radioactive tracer about an hour before the scan. You have this injection through a small plastic tube in your arm (a 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.

In the scanning room

Your radiographer takes you into the scanning room. The PET-CT 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-CT 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 and in most places the radiographers 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-CT or PET scan.

After your PET-CT scan

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

You can then eat and drink normally. Drinking plenty of fluids after your scan helps to flush the radioactive tracer out of your system. 

Someone will need 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.

The radioactive tracer gives off very small levels of radiation. This goes away very quickly. For about 6 hours after your scan, keep any time you spend within arm's length of pregnant women, babies or young children as short as possible. Your radiographer will advise you about this.

If you are travelling abroad within a week of your scan, it may be a good idea to take your appointment letter with you. Most airports have sensitive radiation monitors which may pick up a trace of radiation left in your body following your test.

Possible risks

A PET-CT 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 are or think you might be pregnant.


If you are 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 during a PET-CT scan can slightly increase 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.

  • Evidence-based indications for the use of PET-CT in the United Kingdom 2022
    The Royal College of Radiologists, July 2022 

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

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

  • Principles and practice of oncology (11th edition)
    VT De Vita, S Hellman and SA Rosenberg
    Lippincott, Williams and Wilkins, 2019

  • Clinical Oncology Basic Principles and Practice (5th Edition)
    Peter Hoskin
    CRC Press Taylor and Francis Group, 2020

  • 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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