Virtual colonoscopy

Published by Bupa's Health Information Team, May 2011.

This factsheet is for people who are having a virtual colonoscopy, or who would like information about it.

Virtual colonoscopy is a procedure used to examine the large bowel for polyps and signs of cancer. It’s also called computerised tomography (CT) colonography.

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 examine your large bowel (the colon and rectum) for polyps or signs of cancer. It’s done using a CT scanner, which uses X-rays to produce three-dimensional images of your bowel.

It can help your doctor to find out what is causing symptoms such as:

• abdominal (tummy) pain
• weight loss
• blood or mucus in your faeces (stools)
• a change in your usual bowel habits

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. Polyps are usually harmless, but they can sometimes develop into cancer if left to grow over a number of years.

If you’re at high risk of developing bowel cancer, you may have a virtual colonoscopy to screen for the disease. Alternatively, it can be used if you have already had a different bowel cancer screening test known as a faecal occult blood test that has shown blood in your faeces. A virtual colonoscopy can help your doctor get a better look at your large bowel for signs of cancer.

What are the alternatives?

There are other investigations that can be used to examine 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, telescopic camera called a colonoscope.
• Flexible sigmoidoscopy. This is a similar procedure to colonoscopy, except that a shorter telescopic camera called a sigmoidoscope is used to look inside your rectum and the lower part of your bowel.
• Rigid sigmoidoscopy. This procedure uses an even shorter telescopic camera than in flexible sigmoidoscopy to look at your rectum.
• Barium enema. This test involves placing a fluid containing barium (a substance which shows up on X-rays) into your bowel via your rectum. X-ray images of your abdomen then show the inside of your bowel more clearly.

Your doctor will talk about the options available to you.

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 usually 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 two days before your test. You might also be asked to take laxatives and drink plenty of fluids immediately before the procedure to empty your bowel.

Your doctor may ask you to swallow an iodine-based fluid two days before your test. The fluid helps to show your bowel more clearly on the scan. If you have an allergy to iodine, you must let your doctor know beforehand.

Before your procedure, tell your doctor about any medicines you're taking and if you have any allergies. You should also tell your doctor 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 20 minutes.

You may be asked to change into a hospital gown and lie on the scanning table. 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, depending on the reason for your test. This will be given through a drip placed in the back of your hand.

Your doctor will gently pass a thin tube a few centimetres into your rectum. A small amount of carbon dioxide or air is pumped into your bowel to expand it and make the wall easier to see. When this happens, you may briefly feel some discomfort or pain similar to trapped wind.

The table will move into the ring of the CT scanner so that the middle part of your body is lying in the centre of the scanner. The scanner will rotate around you to produce images from every direction.

The scanner is operated from behind a window by a radiographer (a health professional trained to perform imaging procedures). He or she will be able to see, hear and speak to you throughout the procedure.

You will have scans taken when you’re lying on your back and others taken when you’re lying on your stomach. 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 to 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. The report can take a few days to reach your doctor.

Virtual colonoscopy is used to help diagnose your condition rather than to treat it. If the virtual colonoscopy shows that you have polyps, you will need to have another procedure, for example a conventional colonoscopy, to remove them. Large polyps or cancer may need surgery.

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. The amount of radiation used during virtual colonoscopy is more than an ordinary X-ray. You will be exposed to about the same amount of radiation 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 child. 

Side-effects

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

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 previous reaction to contrast medium. If you feel unwell or have problems breathing during the test, tell your radiologist 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 can lead to bleeding and infection, which may need treatment with medicines or surgery, but this is rare.

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

For sources and links to further information, see Resources.