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CT colonography

Find out what a CT colonography is, how you have it and what happens after it.

CT (computed tomography) colonography is a test that uses CT scans to check the large bowel (colon). 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 you might have this test

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

Photo of a CT scanner

Preparing for your CT colonography

Your bowel needs to be empty for the test. You'll have to take medication (laxatives) to empty your bowel the day before. Or you might drink a special liquid called gastrografin over 1 or 2 days.

Gastrografin is a type of dye (contrast medium) containing iodine, that also acts as a laxative. Gastrografin makes the scan pictures clearer by highlighting tissues.

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

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

The x-ray department should tell you the instructions that you need to follow.

What happens

Before the scan

At the hospital you change into a gown. Your radiographer answers any questions that you have. You lie on your left hand side.

The radiographer puts a small tube a few centimetres long into your back passage (rectum) to pump carbon dioxide or air inside. This opens the bowel, helping to get clear scans of the inside of your bowel. You might have some discomfort from the carbon dioxode or air. Rarely, people have pain. 

The radiographer then helps you to lie on your back.

You might have an injection of dye (contrast medium), to help show up your other organs outside of the bowel more clearly on the scan. And an injection of medicine to relax your bowel muscles. You usually have any injections through a fine tube (cannula) in your vein.

During the scan

The scanning table moves into the CT scanner. 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. 

You often have a second scan lying on your front. 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 the 2 scans and makes a virtual scan of the inside of your bowel. 

After your scan

Once the scan is finished, the radiographer removes the tube from your back passage, and shows you to the toilet. You might need to stay in the department for a short time after the test. 

The radiographer will remove the fine tube (cannula) from your vein and you can go home. You can usually eat and drink normally. 

Possible risks

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


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

Gastrografin can make people feel or be sick. It may cause a mild rash, or very rarely you can have an allergic reaction. 

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 tummy (abdomen) after the test. This is due to the carbon dioxide or air put into the bowel. This should go away after a few hours. 

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 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. 

Radiation exposure 

CT scanners use x-rays to make images. You will be exposed to a small amount of radiation. You should not have CT colonography if there is a risk that you are pregnant. 

Getting your results

It can take 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 freephone 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.

Last reviewed: 
11 Aug 2016
  • Bowel Cancer Screening: Having a CT colonography (CTC) scan

    NHS Bowel Cancer Screening Programme, 2013

  • Laxative-free CT colonography

    A Slater and others, 2012

    British Journal of Radiology, Volume 85


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