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Virtual colonoscopy

Key points

  • Virtual colonoscopy is usually used to screen for polyps or the early signs of bowel cancer.
  • It involves inserting a thin tube into your rectum and using gas or air to expand your bowel before a CT scanner produces images of it.
  • Virtual colonoscopy is less invasive than conventional colonoscopy and enables all of your bowel to be seen.
  • It’s not possible to take a biopsy or remove polyps during a virtual colonoscopy.



How accurate is a virtual colonoscopy compared with other investigations?


Virtual colonoscopy is a relatively new procedure and so there is less information available about how accurate it is compared with other investigations. The test is being reviewed all the time to check on its safety and reliability.

See all our FAQs on virtual colonoscopy

Virtual colonoscopy is a procedure used to check your large bowel for polyps and signs of cancer and other diseases. It’s also known as computed tomography (CT) colonography or CT pneumocolon.

You will meet the doctor carrying out your procedure to discuss your care. It may differ from what is described here as it will be designed to meet your individual needs.

About virtual colonoscopy

Virtual colonoscopy is a procedure used to look at your large bowel (your colon and rectum), usually to screen for polyps or the early signs of bowel cancer. It’s done using a CT scanner, which uses X-rays to produce three-dimensional images of your bowel.

It can also be used to try to find out more about symptoms such as bleeding or mucus in your faeces, but usually other tests, such as a conventional colonoscopy, are used for this. See our frequently asked questions for more information.

Virtual colonoscopy can show if you have polyps (small growths on the inside of your bowel) or other abnormalities on the wall of your bowel. Small polyps are usually harmless and stay that way, but they can develop into cancer if they are large or if they are left to grow over a number of years.

If you don’t have any symptoms but are at an increased risk of developing bowel cancer or polyps, for example because one or more family members have had it or if you have had polyps before, you may have a virtual colonoscopy.

What are the alternatives to virtual colonoscopy?

There are other investigations that can be used to look at your bowel. The main ones are listed below.

  • Conventional colonoscopy. A colonoscopy is a procedure that allows your doctor to look inside your large bowel using a narrow, flexible, tube-like telescopic camera called a colonoscope. 
  • Flexible sigmoidoscopy. This is a similar procedure to colonoscopy, but uses a shorter telescopic camera called a sigmoidoscope. It looks inside your rectum and the lower part of your bowel only (because most bowel cancers develop in this area).
  • Rigid sigmoidoscopy. This procedure uses a shorter, straight camera than flexible sigmoidoscopy and looks at your rectum only.
  • Barium enema. This test involves placing a fluid containing barium (a substance which shows up on X-rays) into your bowel via your anus. X-ray images of your abdomen (tummy) then show the inside of your bowel more clearly.

Discuss the options available to you with your doctor.

Preparing for a virtual colonoscopy

Virtual colonoscopy is usually carried out in the X-ray department of your hospital. You will have the test and go home the same day. A radiologist (a doctor who specialises in using imaging methods to diagnose medical conditions) will carry out the procedure.

You will be given information about how to prepare for your scan. Your bowel needs to be empty during the test so that the CT scanner can produce clear pictures of the inside of your bowel. You may need to follow a special diet for up to three days before your test. This is likely to involve cutting down on solid food and drinking enough clear liquids. You will also be asked to take a laxative the day before the procedure to completely empty your bowel.

Your doctor may ask you to take an iodine-based fluid a couple of days before your test. The fluid helps to show your bowel more clearly on the scan. If you have an allergy to iodine or to anything else, you must let your doctor know beforehand. It’s also important that you tell him or her about any medicines you're taking and if you are, or could be, pregnant. Virtual colonoscopy, like all X-ray tests, isn’t recommended for pregnant women, unless there is an urgent medical reason.

Your doctor will discuss with you what will happen before, during and after your procedure, and any pain you might have. This is your opportunity to understand what will happen, and you can help yourself by preparing questions to ask about the risks, benefits and any alternatives to the procedure. This will help you to be informed, so you can give your consent for the procedure to go ahead, which you may be asked to do by signing a consent form.

What happens during a virtual colonoscopy?

The procedure usually takes about 10 to 20 minutes.

You will be asked to change into a hospital gown. You will then be asked to lie on the scanning table, and straps and pillows may be put in place to help you stay comfortably in one position. Your doctor may give you an injection of a medicine that will help to relax the muscles of your bowel wall. You may also have an injection of a dye (contrast medium) at the same time. This helps to show up your bowel more clearly on the X-rays. You will be given this through a drip placed into a vein in the back of your hand.

Your doctor will gently pass a thin tube a few centimetres into your rectum. He or she will then pump a small amount of carbon dioxide or air into your bowel to expand it so that it can be seen more easily on the X-ray. You may feel some slight discomfort similar to trapped wind when this happens, but this doesn’t usually last very long.

The table will move into the ring of the CT scanner so that the middle part of your body is lying in the centre. A radiographer (a health professional trained to carry out imaging procedures) will operate the scanner from behind a window. He or she will be able to see, hear and speak to you throughout the procedure. Your radiographer will rotate the scanner around you to produce images from every direction.

You will have scans taken when you’re lying on your back and others taken when you’re lying on your front or side. At certain points during the scan you may be asked to hold your breath for a few seconds. It can take several minutes for each image to form so it's important that you lie very still during the scan.

What to expect afterwards

You will be able to go home when you feel ready.

If you have had an injection of contrast medium, you will need to wait for an hour before you can drive. Your results are usually sent in a report to the doctor who requested your scan – this can take a few days.

Virtual colonoscopy is used to help diagnose your condition rather than to treat it. If the test shows that you have polyps, you will need to have another procedure, for example a conventional colonoscopy, to remove them. See our frequently asked questions for more information.

If you’re found to have large polyps or cancer, you may need to have surgery. You may also need to have another procedure if the virtual colonoscopy wasn’t able to find the cause of your symptoms.

Recovering from a virtual colonoscopy

You will be able to return to your usual activities straight away because you don’t need to have painkillers or sedation for a virtual colonoscopy.

It’s possible that you will have a feeling of ‘fullness’ and as though you have trapped wind for a short while after having a virtual colonoscopy because of the air that is pumped into your bowel. You may have some abdominal pain but this is rare and doesn’t usually last long.

What are the risks?

As with every procedure, there are some risks associated with virtual colonoscopy. We have not included the chance of these happening as they are specific to you and differ for every person. Ask your doctor to explain how these risks apply to you.

CT scanners use X-rays to produce images, so you will be exposed to some radiation. This is about the same amount as the natural radiation that you get from the atmosphere over about three years.

If you are, or think you could be, pregnant, you shouldn't have a CT scan as there is a risk that the radiation may harm your unborn baby.


Side-effects are the unwanted but mostly temporary effects you may get after having the procedure. The main side-effects of virtual colonoscopy are:

  • pain or discomfort when the air or carbon dioxide is put into your bowel
  • a feeling of warmth or needing to pass urine if you have an injection of contrast medium


Complications are when problems occur during or after the procedure. Complications from virtual colonoscopy are uncommon.

Very rarely, it's possible to have an allergic reaction to the contrast medium. Tell your doctor if you have had a reaction to contrast medium in the past. If you feel unwell or have problems breathing during the test, tell your doctor straight away.

There is a very small chance that your bowel may be damaged or torn when air or carbon dioxide is put in during the procedure. This is very rare and happens to fewer than one in 10,000 people who have a virtual colonoscopy. If your bowel is damaged, it can lead to bleeding and infection and you may need treatment with medicines or surgery.


Produced by Polly Kerr, Bupa Health Information Team, March 2013.

For answers to frequently asked questions on this topic, see FAQs.

For sources and links to further information, see Resources.

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  • This information was published by Bupa's Health Information Team and is based on reputable sources of medical evidence. It has been reviewed by appropriate medical or clinical professionals. Photos are only for illustrative purposes and do not reflect every presentation of a condition. The content is intended only for general information and does not replace the need for personal advice from a qualified health professional. For more details on how we produce our content and its sources, visit the about our health information page.

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