CT scan

A CT scan is a test that uses x-rays and a computer to create detailed pictures of the inside of your body. It takes pictures from different angles. The computer puts them together to make a 3 dimensional (3D) image.  

CT (or CAT) stands for computed (axial) tomography.

Photograph of a CT scanner

You usually have a CT scan in the x-ray (radiology) department as an outpatient Open a glossary item. A radiographer operates the scanner. The whole appointment can take up to an hour and a half depending on which part of your body they are scanning. 

Why you might have a CT scan

You might have a CT scan:

  • to diagnose a range of conditions including cancer
  • to help work out where the cancer is, how close it is to nearby organs and how big it is - this can help your doctors decide about whether you need further tests or what treatment you need
  • to check how well treatment is working
  • as part of your follow up after treatment

Preparation for a CT scan

Some CT scans need special preparation beforehand.

For most scans, you have a drink or an injection of contrast medium, or both. This is a dye that shows up body tissues more clearly on the scan. You have the injection through a small thin tube (cannula) in your arm. The tube is left in place until after your scan, in case you have any problems after having the injection.

CT scans of the abdomen

If you are having a CT scan of your abdomen, you might be asked:

  • to drink a liquid contrast medium some time before the scan
  • to drink more of the liquid contrast or water in the x-ray department
  • not to eat or drink after midnight the night before the scan (this is for a CT scan of the inside of the large bowel, called CT colonography)

You usually have the contrast medium by injection and also as a drink. This helps to show up the gut (digestive system) more clearly in the scan.

CT scans of the head

For some brain scans, you might have an injection of the contrast medium dye beforehand to make the scan clearer.

CT scans of the chest

You might have an injection of the contrast medium during the scan. This is to help show up the tissues close to the area containing cancer, for example, blood vessels. It may help to show whether cancer can be removed with surgery or not.

Pelvic CT scans

If you are having a CT scan of the pelvis, you might be asked:

  • not to eat or drink for some time before the scan
  • to have an injection of contrast medium

Occasionally, for a rectal scan, you need to have an enema of contrast medium. This shows up on the x-ray and makes the outline of the bowel show up more on the scan. It might make you constipated. Your first couple of bowel motions will be white, but there are no other side effects.

CT colonography

You might have a very detailed scan of the bowel called a CT colonography (or virtual colonoscopy). 

If you're having one of these, you will be asked to clear your bowel by taking strong laxatives, drinking a special liquid with meals and following a special diet about 2 days before the test. Your doctor or nurse will tell you about this. 

You might also have a medicine to slow down the normal movement of your bowel. This movement (called peristalsis) can change the scan and make it more difficult to read.

What happens?

When you arrive the radiographer might ask you to change into a hospital gown. You should remove jewellery and other metal objects, bra, hair clips around the area being scanned. Metal interferes with the images produced by the scanner.

In the scanning room

When you’re ready, your radiographer or an assistant takes you into the scanning room. A CT scanning machine is large and shaped like a doughnut.

You might have an injection of a type of dye called a contrast medium through a small tube (cannula) in your arm. You may:

  • feel hot and flushed for a minute or two
  • have a metallic taste in your mouth
  • feel like you’re passing urine but you aren’t – this feeling is common and passes quickly

Tell your radiographer if you feel anxious or claustrophobic about having a scan. 

Having the CT scan

You usually lie 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 hole of 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.

During the scan

You’ll hear a whirring noise from the scanner.

Your radiographer might ask you to hold your breath at times.

The scan usually lasts between 20 and 30 minutes. 

When the scan is over, your radiographer comes back into the room and lowers the couch so you can get up.

This 2 minute video shows what happens when you have a CT scan.

After your CT scan

You stay in the department for about 15 to 30 minutes if you had an injection of the dye. This is in case it makes you feel unwell, which is rare.

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

You should be able to go home, back to work or the ward soon afterwards. You can eat and drink normally.  

Possible risks

A CT scan is a safe test for most people but like all medical tests it has some possible risks. Your doctor and radiographer make sure the benefits of having the test outweigh these risks.

Allergic reaction

Rarely, people have an allergic reaction to the dye (contrast medium). This most often starts with weakness, sweating and difficulty breathing. Your radiographer will ask if you have any allergies before you have the contrast medium. Tell them immediately if you feel unwell after having it.

Bruising and swelling

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

There's a risk that the contrast medium will leak outside the vein. This can cause swelling and pain in your hand or arm but it’s rare.

Kidney problems

There is a small risk that the contrast medium can affect your kidneys. Your radiographer checks your most recent blood test results before your scan to make sure your kidneys are working well. 


Exposure to radiation during a CT scan can slightly increase your risk of developing cancer in the future. Talk to your doctor if this worries you.


Pregnant women should only have CT scans in emergencies. Contact the department as soon as you can before the scan if you are pregnant or think that you might be.

Getting your results

You should get your results within 1 or 2 weeks. 

Waiting for results can make you anxious. Ask your doctor 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.

You might have contact details for a specialist nurse who you can contact for information 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 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.

Last reviewed: 
05 Mar 2019
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