CT colonography

CT (computed tomography) colonography is a test that uses CT scans to check the large bowel (colon) and back passage (rectum). It’s also called a virtual colonoscopy.

You have this test as an outpatient in the CT scanning (or radiology) department at the hospital. A radiographer or specialist doctor (radiologist) carries out the test.

It usually takes around 30 minutes but you should expect to be in the department for about an hour or so.

Why do I need a CT colonography?

You usually have this test to help find the cause of your symptoms.

Photo of a CT scanner

Before your test

You need to have an empty bowel for this test. This is so the radiographer can clearly see the inside of your bowel and rectum.

You'll get clear written information on what to do before you have a scan. Your preparation usually starts a couple of days before the test.

You need to take strong medication (laxatives) to empty your bowel the day before. Or you might need to drink a special liquid (contrast medium) called gastrografin over 1 or 2 days.

Gastrografin is a type of dye containing iodine. It helps to make scan pictures clearer. It also acts as a laxative and may give you diarrhoea.

Having gastrografin or laxatives will mean that you need to open your bowels often, and very suddenly. You might also have some cramping pains. It’s best to stay at home for a few hours after taking gastrografin or the laxatives, so that you are near a toilet. 

You might also need to follow a low fibre diet for 1 or 2 days before the test.

To prevent dehydration it’s important to drink plenty of clear fluids such as:

  • water
  • black tea or coffee
  • squash (without red or purple colouring)
  • clear soup

You may need to stop taking iron tablets or other medicines which can cause constipation. You usually stop these one week before the test. 

Contact the radiology department as soon as possible before your appointment if you take medicines to thin your blood or you are diabetic. You'll get other instructions to follow. 

What happens

When you arrive you usually fill out a checklist or questionnaire with the radiographer. They will explain the procedure and ask you to sign a consent form. They ask you questions about your medical history and any allergies. It's a good time to ask them any further questions that you may have. 

They'll ask you to change into a hospital gown.

You might have a medicine to relax your bowel muscles. This medicine is Buscopan. You have it as an injection through a fine tube (cannula) into your bloodstream.

You lie on the couch on your side.

The radiographer puts a small tube a few centimetres long into your rectum. They then pump carbon dioxide or air inside. This opens the bowel, helping to get clear scans of the inside of your bowel. You may feel uncomfortable and bloated. Rarely, people have pain. 

This 2 minute video shows how you have a CT colonography. 

During the scan

The radiographer will then ask you to lie on your front and the scanning table moves into the CT scanner. The middle part of your body is in the centre of the ring. You will be alone in the room while you have the scan. But they can still see you and talk to you through an intercom. 

Midway through your scan you might have an injection of contrast medium Open a glossary item. This helps show up the bowel more clearly on the scans. You have this through your cannula. 

You often have a second scan lying on your back. At certain times the radiographer may ask you to hold your breath for a few seconds. It is important that you lie as still as you can during the scan.

A computer matches all the scans you've had to make a virtual scan of the inside of your bowel. 

After your scan

After the scan the radiographer removes the tube from your back passage, and shows you to the toilet. You usually stay in the department for a short time after the test. This is to check you don't have an allergic reaction to the Buscopan or the contrast medium. 

The radiographer will remove the cannula from your vein when its time for you to go home. You can eat, drink and take your medicines as normal after the scan. 

Possible risks

CT colonography is a very safe procedure. Your radiographer will tell you who to contact if you have any problems after your test. Your doctor will make sure the benefits of having a CT colonography outweigh the possible risks.

Some of the possible risks include:

Loss of body fluid (dehydration) 

Clearing the bowel can cause dehydration in some people. It is very important to drink plenty of fluids before and after your scan.

Effects of gastrografin

Gastrografin can make people feel or be sick. It may cause a mild rash but this is uncommon. Very rarely you can have an allergic reaction. If you feel ill or have problems breathing during the test, tell the radiographer straight away. 

Effects of contrast medium 

The injection of contrast medium might make you hot and flushed for a few minutes. You might get a metallic taste in your mouth. It is common to feel warmth like you are passing urine, but you are not. This feeling goes away quickly. 

Very rarely, people have a reaction to the contrast medium. If you feel ill or have problems breathing during the test, tell the radiographer straight away. 

Tummy (abdominal) pain 

You may have some bloating or pain in your abdomen after the test. This is due to the carbon dioxide or air put into the bowel. This should go away shortly afterwards. 

Tear in your bowel 

There is a small risk of a tear (perforation) in your bowel. This is very rare. If it happened you would often need surgery to repair the tear. 

Blurred vision 

The medicine used to relax your bowel wall might cause temporary blurred vision. You shouldn't drive for an hour or so or until your vision has returned to normal. Tell your radiographer if you have glaucoma or heart problems before you have medicine to relax your bowels. 

Radiation exposure 

CT scanners use x-rays to make images which use a small amount of radiation. 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.

You should not have CT colonography if there is a risk that you are pregnant. 

Getting your results

It can take up to 1 to 2 weeks to get your results. You usually get your results from your specialist. It is important to check with your doctor how long you should expect to wait for your results. 

Waiting for results can be an anxious time. It might be helpful to talk to someone close to you. 

If you have not had your results a few weeks after your test, you could contact your doctor to chase your results for you.

You can also contact the Cancer Research UK nurses on a freephone number 0808 800 4040. The lines are open from 9 to 5, from Monday to Friday.

More information

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

  • Standards of practice for computed tomography colonography (CTC)
    British Society of Gastrointestinal and Abdominal Radiology and The Royal College of Radiologists, January 2021

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

  • Guidance on the use of CT colonography for suspected colorectal cancer
    British Society of Gastrointestinal and Abdominal Radiology (BSGAR) and The Royal College of Radiologists, published 2014. Last updated 2017

  • Oxford handbook of clinical medicine (10th edition)
    M Longmore and others
    Oxford University Press, 2017

  • Computed tomographic colonography (virtual colonoscopy). Interventional procedure guidance
    National Institute of Health and Care Excellence, 2005

Last reviewed: 
04 Aug 2022
Next review due: 
04 Aug 2025

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