Tests and scans
A PET scan is a test that creates 3 dimensional (3D) pictures of the inside of your body. PET stands for positron emission tomography.
A PET scan uses a mildly radioactive liquid called a radioactive tracer. It shows areas of your body where cells are more active than normal. It's used to help diagnose some conditions, including cancer. It can help to find out:
where the cancer is
the size of the cancer
whether the 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. The scan usually takes between 20 and 30 minutes. But you will be in the department for around 2 hours. This is because you need to arrive about an hour before the scan to have the injection of the radioactive tracer. Your appointment letters will usually tell you how long your appointment will be.
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.
You can read more about PET-CT scans and PET-MRI scans using our A-Z list
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
help doctors 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. A PET scan can help show whether this tissue is active cancer or not.
Check your appointment letter for instructions on how to prepare for this test. You should follow the information given in your appointment letter. Some hospitals may have slightly different instructions.
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. They will tell you if you need to stop any regular medication you are taking.
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 your appointment time could change.
It is important to tell the scanning staff beforehand if you are pregnant or think you might be pregnant. Or you are breastfeeding.
You might have instructions not to do any strenuous exercise for around 12 to 24 hours before the scan.
Some people feel claustrophobic or closed in when they are having a scan. Contact the PET scanning 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.
Your radiographer will explain the test to you. This is a good time to ask any questions you may have.
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 about an hour before the scan. You have the injection through a small plastic tube called 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.
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.
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.
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.
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 the rest of the day. Your radiographer will give you more information about this. Speak to them if you have any questions. Some hospitals may have different instructions.
If you’ve had medicine to help you relax (sedation), you need someone to take you home and stay overnight. If you've had sedation, for the next 24 hours, you also shouldn’t:
drive
drink alcohol
operate heavy machinery
sign any legally binding documents
If you are 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.
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.
You might get a small bruise around the area where they put the in. There is a risk that the radioactive tracer will leak outside the vein, this is rare. If this does happen, it can cause swelling and pain in your arm.
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.
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.
You should get your results within 1 or 2 weeks.
Ask your doctor, radiographer, or nurse how long it will take to get them. Contact the doctor who arranged the test if you haven’t heard anything after a couple of weeks.
Waiting for test results can make you anxious. You might have the contact details of a specialist nurse who you can speak to for information and support if you need to. It may also help to talk to a close friend or relative about how you feel.
We have more information on tests, treatment and support if you have been diagnosed with cancer.
Last reviewed: 29 Apr 2025
Next review due: 29 Apr 2028
Find out about tests to diagnose cancer and monitor it during and after treatment, including what each test can show, how you have it and how to prepare.
A CT scan is a test that uses x-rays and a computer to create detailed pictures of the inside of your body. Find out how you have it and what happens afterwards.
A PET-CT scan combines a CT scan and a PET scan. It gives detailed information about your cancer.
A PET-MRI scan combines a PET scan and an MRI scan into one to give detailed information about your cancer.
Search for the cancer type you want to find out about. Each section has detailed information about symptoms, diagnosis, treatment, research and coping with cancer.
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