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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 is also called virtual colonography.

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

You have this test as an outpatient in the CT scanning (or radiology) department at the hospital. The test is done by a senior radiographer or specialist doctor (radiologist).

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

Photo of a CT scanner

Before your test

Your bowel needs to be empty before the test. You need to take strong medications (laxatives) to empty your bowel the day before the test. 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 the 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 perhaps very suddenly. You might also have some cramping pains. It is sensible to stay at home for a few hours after taking gastrografin or laxatives so you are near a toilet. 

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

What happens

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

You might have an injection of medicine to relax the bowel muscles. You also have an injection of dye (contrast medium) at the same time. This helps to show up the bowel more clearly on the scans. You have the injections through a fine tube (cannula) in your vein. 

Just before the test the radiographer puts a small tube a few centimetres 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 helps you lie on your back 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. 

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 very still during the scan. 

A computer matches the 2 scans and makes a virtual scan of the inside of your bowel. 

After your test

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

The radiographer will remove the fine tube in the 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 these 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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