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Virtual colonoscopy


Expert reviewer, Mr Stephen Pollard, Consultant Surgeon
Next review due June 2020

Virtual colonoscopy, also known as computed tomography (CT) colonography or CT colonoscopy, is a procedure to look at the lining of your large bowel. Your large bowel is the last part of your digestive system, and consists of your bowel and rectum. Your rectum connects your bowel to your anus. Waste material empties from your rectum through your anus. A virtual colonoscopy can show any irritated and swollen tissue, ulcers or growths such as polyps. It helps to diagnose health conditions such as bowel cancer.


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Preparing for a virtual colonoscopy

You’ll meet the radiologist who’s going to do your colonoscopy before it goes ahead so they can make sure you know what’s going to happen. A radiologist is a doctor who specialises in using imaging methods to diagnose medical conditions. For simplicity, we’ll refer to your ‘doctor’ throughout. Your care may be different from what’s described here as it will be designed to meet your individual needs.

You’ll have a virtual colonoscopy in the CT scanning department of your local hospital, usually as a day case. This means you have the test and go home the same day. Your hospital will send you some information about how to prepare for your virtual colonoscopy. It may be possible to choose where you have your procedure. You might be told about units with a JAG accreditation, if this is an option for you. This comes from The Joint Advisory Group on Gastrointestinal Endoscopy (JAG), which is part of the Royal College of Physicians. If you’d like to find out more, visit their website for further information.

Your bowel may need to be empty for the procedure to go ahead so the CT scanner can produce clear pictures of your bowel. You might need to stop eating and just drink clear fluids like water, squash or black tea or coffee the day before your procedure. Your hospital will confirm how long you need to do this for.

Your hospital may also give you a strong laxative that you’ll usually need to take the evening before your virtual colonoscopy – but check the instructions. The laxative comes as a pill or a powder that you mix with water. The laxative will give you diarrhoea, so make sure you stay close to a toilet after you take it. Drink plenty of clear fluids to make sure you don’t get dehydrated. You might feel some discomfort or bloating in your tummy (abdomen) when you take a laxative, and it can taste unpleasant. It might taste better if you put the mixture in the fridge before you drink it. Remember that although taking a laxative isn’t very nice, it’s really important for the test to go well.

Instead of taking a laxative, you might be able to take a contrast medium or an iodine-based fluid before you have a virtual colonoscopy. You usually take contrast medium in the form of a drink the night before your procedure. And you can take an iodine-based fluid for a couple of days before your test with meals. These show your bowel more clearly on X-rays and can help your doctor tell the difference between any faeces remaining in your bowel and polyps.

Whether you have a laxative or contrast medium or an iodine-based fluid will depend on where you have the investigation done. So follow the instructions that your hospital gives you.

If you take any medicines or iron supplements, let your hospital know before you have the virtual colonoscopy. They can check if you need to stop taking them, and for how long. If you have a health condition, such as diabetes, let your hospital know as the preparation might be different for you. Your hospital will give you all the information you need.

Your doctor will already have gone through everything in detail before you get to the point where you go to hospital for your virtual colonoscopy. They’ll also have given you the opportunity to ask questions about the risks, benefits and alternatives to the procedure. Once you get to hospital, they’ll go over what will happen before, during and after your procedure, and any pain you might have. So you should have all the information you need to give your consent for the virtual colonoscopy to go ahead. You’ll be asked to sign a consent form beforehand.

What are the alternatives to virtual colonoscopy?

Virtual colonoscopy uses X-rays to produce two- or three-dimensional images of your bowel. There are other procedures that your doctor can use to look at your bowel. The main ones are listed below.

  • Conventional colonoscopy. With conventional colonoscopy, your doctor looks inside your large bowel using a long, flexible, illuminated, telescopic camera. They pass the camera, called a colonoscope, through your anus and up into your bowel. Unlike a virtual colonoscopy, your doctor can remove any polyps they find, or take a biopsy.
  • Flexible sigmoidoscopy. This is a similar procedure to colonoscopy, but uses a shorter telescopic camera called a sigmoidoscope. It enables your doctor to look inside your rectum and the lower part of your bowel only.
  • Rigid sigmoidoscopy. As the name suggests, this uses a rigid instead of a flexible sigmoidoscope, and your doctor can look at your rectum and lower bowel only. But it can’t go as far up your bowel as a flexible sigmoidoscopy.
  • Barium enema. This test involves placing a fluid containing barium (a substance that shows up on X-rays) into your bowel via your anus. The fluid coats the inside of your bowel, which gives clearer X-ray images.

If you are, or think you could be, pregnant, let your doctor know. You shouldn't have a CT scan as there’s a risk that the radiation may harm your baby. You might need to have a conventional colonoscopy instead.

Have a chat with your doctor about the options available and which is best for you.

What happens during a virtual colonoscopy?

A virtual colonoscopy usually takes about 10 to 15 minutes.

A nurse will ask you to change into a hospital gown and lie on your back on the scanning table. You’ll have a drip placed into a vein in the back of your hand. To help to relax the muscles of your bowel wall, your doctor may give you an injection of a medicine through the drip. They might also inject a dye (contrast medium) at the same time to show up your bowel more clearly on the X-rays.

Your doctor will gently pass a thin, flexible, tube through your anus and into your rectum. They’ll then pump a small amount of air into your bowel, which will inflate it so it can be seen more clearly on the X-ray. You may feel some slight discomfort when this happens, but it shouldn’t be painful.

Your doctor will move the scanning table into the tunnel of the CT scanner. They’ll take scans while you’re lying on your back and then ask you to lie on your front or side and take more scans. They might also ask you to hold your breath for up to 15 seconds at certain points during the scan. It can take several minutes for each image to form so try to lie very still.

What to expect after a virtual colonoscopy

After your virtual colonoscopy, you can usually go home when you feel ready. But if your doctor gave you a medicine to relax your bowel, you’ll need to wait for an hour before you can drive. This is because it can blur your vision.

Your doctor will send the results to whoever referred you for the virtual colonoscopy. This may take anything from a few days to two weeks.

  • If the test shows that you have polyps, you’ll need to have a procedure to remove them, such as a conventional colonoscopy. For more information, see our FAQ: Next steps if polyps are found.
  • If the test finds you have bowel cancer, you might need to have an operation.
  • If the virtual colonoscopy didn’t find what’s causing your symptoms, you may need to have another type of procedure to try and find out what’s happening.

Recovering from a virtual colonoscopy

After your virtual colonoscopy, you’ll be able to return to your usual activities straight away.

You might feel a bit bloated and have trapped wind for a short time. This is due to the air that was pumped into your bowel during the procedure. You might pass wind and possibly have some tummy pain but this doesn’t usually last long. You can go back to eating a normal diet after the procedure.

Side-effects of a virtual colonoscopy

As with every procedure, there are some risks associated with virtual colonoscopy. We haven’t 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.

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 when you have an injection of contrast medium
  • blurred vision from the medicine that’s used to relax your bowel

Complications of a virtual colonoscopy

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

  • Allergic reaction to the contrast medium. This is very rare but if you feel unwell or find it difficult to breathe during the test, tell your doctor straight away. Let your doctor know if this has ever happened to you before prior to having the virtual colonoscopy.
  • Damage to your bowel. There’s a very small chance that your bowel may be damaged or torn when air or carbon dioxide is put in during the procedure. This happens to 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 an operation to repair it.

CT scanners use X-rays to produce images, so you’ll 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.

Frequently asked questions

  • Polyps are small, fleshy growths that can develop in the lining of your bowel. They’re quite common and usually don’t cause any problems. But some types of polyp can grow into cancer over time. If the results of your virtual colonoscopy show you have polyps, your doctor will probably suggest you have a conventional colonoscopy to remove them.

    About 35 in every 100 people develop a polyp in their lifetime, most often when they’re over 60. Men are slightly more likely to get them than women.

    Some polyps are attached to the side of your bowel on a stalk and look a bit like a mushroom. Others are flatter and have no stalk. You may have just one polyp, or several.

    Most polyps don’t cause any problems. But there’s a small chance that over time a type of polyp called an adenoma will grow into a cancer. Most bowel cancers are thought to start as a polyp. So if the virtual colonoscopy finds that you have any, you’ll usually have further treatment to remove them. This is usually done in a conventional colonoscopy.

    Once your polyp is removed it doesn’t usually come back. But because you may develop new polyps, you may be advised to have regular colonoscopies. How often you have these will depend on how many polyps you have had and when a polyp was last found. It’s usually one to three years after a polyp is found, depending on the type of polyp.

  • Conventional colonoscopy and virtual colonoscopy are both procedures that enable your doctor to look at your large bowel. A key difference is that during a conventional colonoscopy your doctor can remove any polyps they find or take a biopsy.

    Here are some of the differences between conventional and virtual colonoscopy.

    • During a conventional colonoscopy, your doctor looks at the inside of your bowel using a narrow, flexible, tube-like, telescopic camera called a colonoscope. Your doctor will need to put a tube in your anus for a virtual colonoscopy too to pass air to inflate your bowel, but it won’t go as far in (just a couple of inches).
    • You’ll usually need to have a sedative during a colonoscopy to relieve anxiety and help you to relax. In contrast, you don’t usually need a sedative for a virtual colonoscopy.
    • Virtual colonoscopy can be quicker than a conventional colonoscopy and you may find it more comfortable, but this will depend on your circumstances.
    • If you have a conventional colonoscopy, your doctor can pass thin instruments through the colonoscope to remove polyps or to take biopsies if necessary. This isn't possible during a virtual colonoscopy.
    • With conventional colonoscopy, there’s slightly more risk of your bowel being damaged during the procedure. This risk is higher if your doctor takes a biopsy or removes a polyp.
    • It isn’t always possible to see all of your bowel with a conventional colonoscopy (for example, if the camera cannot pass through the whole bowel). Your doctor might not have been able to see your bowel clearly if it was blocked by a growth of tissue, for example. This isn’t a problem with virtual colonoscopy.
    • Virtual colonoscopy uses a CT scanner that uses X-rays to produce images, so you’ll be exposed to some radiation. This doesn’t happen during a conventional colonoscopy.

    Another advantage of virtual colonoscopy is that it may be safer if:

    • you’re elderly or frail
    • your bowel is blocked
    • you’re taking a medicine to thin your blood, such as warfarin

    Ask your doctor which test is best for you.

  • Virtual colonoscopy is a relatively new procedure so there’s less information available about how accurate it is compared with some other procedures. But experts believe it’s safe and works well.

    Research has looked at how accurate a virtual colonoscopy is at detecting bowel cancer or polyps compared with a barium enema and a conventional colonoscopy. This research shows that a virtual colonoscopy is comparable to a conventional colonoscopy at detecting larger polyps (over 6mm). But it isn’t as good at detecting smaller or flat polyps.

    Compared to a barium enema, it’s much more accurate at finding polyps and also signs of bowel cancer.

    All of the investigations for polyps and bowel cancer have benefits and risks. Your doctor will discuss these with you so you decide together which one is best for you.


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Related information

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    • Personal communication, Mr Stephen Pollard, Consultant Surgeon, Spire Leeds Hospital, 10 May 2017
  • Reviewed by Rachael Mayfield-Blake, Freelance Health Editor, June 2017
    Expert reviewer, Mr Stephen Pollard, Consultant Surgeon
    Next review due June 2020



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