Tests and scans
A PET-CT scan combines a CT scan and a PET scan. It gives detailed information about your cancer.
The PET scan uses a mildly radioactive liquid called a radioactive tracer. This shows up areas of your body where cells are more active than normal. PET stands for positron emission tomography.
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. CT stands for computed tomography.
You usually have a PET-CT scan in the radiology or nuclear medicine department as an outpatient. A operates the scanner. It usually takes between 30 and 45 minutes. But you will be in the department for around 2 hours.
These scanners tend to be available only in the major cancer hospitals. So you might have to travel to another hospital to have one.
PET-CT scans are for many types of cancer. They 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 from your treatment. A PET-CT scan can sometimes show whether this tissue is active cancer or not.
You should follow the information given by the scan team and in your appointment letter. Your appointment letter will have written instructions on what to do before your appointment. This information varies between hospitals.
For most PET-CT scans, you need to stop eating about 4 to 6 hours beforehand. You can usually drink water during this time. 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 your appointment time may change.
You can usually take regular medication, unless you are told otherwise by the staff in the scanning department.
You might have instructions not to do any strenuous exercise for 24 hours before the scan.
It is important to tell the scanning staff before your appointment if you are pregnant or think you might be pregnant. Or you are breastfeeding.
Some people feel claustrophobic when they're having a scan. Let the department staff know 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 what to expect during your scan. This is a good time to ask any questions you may have.
They will then 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 may also have a blood test to look at your blood sugar levels.
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. The plastic tube is 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.
Just before your scan, your radiographer may ask you to have a pee to empty your .
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.
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
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.
The radioactive tracer gives off very small levels of radiation. This goes 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.
If you’ve had to help you relax, 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-CT scan, it's a good idea to take your appointment letter with you. This is because most airports and ferry ports have sensitive radiation monitors. This may pick up a trace of radiation left in your body following your test.
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.
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.
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 during a PET-CT scan can slightly increase your risk of developing cancer in the future. Talk to your doctor if this worries you.
You should get your PET-CT scan 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 be a worrying time. You might have contact details for a specialist nurse. You can get in touch with them for information and support if you need to. It may 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: 06 May 2025
Next review due: 06 May 2028
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 scan is a test that creates 3 dimensional (3D) pictures of the inside of your body. PET stands for positron emission tomography.
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.
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