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 you might have this test

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

Photo of a CT scanner

Before your test

You'll be given written information on what to do before you have a scan. Your preparation starts a couple of days before the test. It's important to follow the instructions so that your bowel is empty 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 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. 

Contact the CT department if you are pregnant or diabetic.

What happens

Before the scan

The radiographer will explain the procedure and you'll need to sign a consent form. They ask you questions about your medical history and any allergies. It's a good time to ask your radiographer any further questions that you may have. 

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

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

You lie on a couch 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 may feel uncomfortable and bloated. Rarely, people have pain. 

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. 

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 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 cannula from your vein and you can go home. You can usually eat and drink normally and take medicines as normal. 

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.

Possible risks

CT colonography is a very safe procedure but your radiographer 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 but this is uncommon. 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 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. 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. 

Last reviewed: 
07 Dec 2021
Next review due: 
06 Dec 2024
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