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Virtual colonoscopy


Expert reviewer, Dr Matthew Brown, Clinical Fellow, Bupa & Radiology Registrar, St Bartholomew’s and the Royal London
Next review due January 2023

A virtual colonoscopy, also known as CT colonography, is a procedure used to take images of the inside of your large bowel, using a CT scan. It’s used to check your large bowel for polyps (growths on the bowel lining) or signs of cancer, and to investigate symptoms affecting your bowel.


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Why should I have a virtual colonoscopy?

A virtual colonoscopy is an alternative procedure to a conventional colonoscopy – the usual test for examining your large bowel, where a camera is inserted into your back passage.

Reasons for needing a procedure to look at your bowel include the following.

  • As part of the national bowel cancer screening programme in England – if initial tests have found blood in your poo.
  • Investigating symptoms that could indicate bowel cancer, such as bleeding from your bottom or a change in your bowel habits (eg needing to poo more often, diarrhoea or constipation).
  • You have a strong family history of bowel cancer (eg two or more first-degree relatives have had bowel cancer, or one relative under the age of 45 has had it).

You may be offered a virtual colonoscopy if you’ve previously had a colonoscopy that couldn’t be completed. Or you might have one if you have health issues that make a conventional colonoscopy difficult – for instance, because you can’t have sedation.

Not everyone can have a virtual colonoscopy. You won’t be able to have one if you have an active flare-up of inflammatory bowel disease – such as Crohn’s disease or ulcerative colitis, or you have diverticulitis. Other situations where it might not be appropriate include if you’ve recently had abdominal surgery, and if you have particular types of hernia or severe abdominal pain.

Preparing for a virtual colonoscopy

You’ll have a virtual colonoscopy in the CT scanning department at a hospital, usually as an out-patient. This means you have the test and go home the same day. It will usually be done by a radiographer – a health professional trained in performing imaging tests. Your hospital will give you detailed information about how to prepare for the test.

You may need to stop certain medicines before having a virtual colonoscopy. These may include iron supplements and medicines that cause constipation. Let your doctor know if you’re taking anything like this before your virtual colonoscopy. They can check if you need to stop taking them, and for how long.

Clearing your bowel

Your bowel needs to be as empty as possible for the procedure, so the CT scanner can produce clear pictures of your bowel. Your hospital may give you a strong laxative that you’ll usually need to take the day or evening before your virtual colonoscopy. It usually comes as a powder that you mix with water.

Or, you may need to drink a special liquid called gastrografin for a day or two before your test. This is a dye that helps to show your bowel more clearly on the CT scan; but it also acts as a laxative. It’s important to follow the instructions given to you by your hospital on exactly how to take your medicine. In some areas, your hospital may ask you to take a type of dye (contrast) on the day of the test, to show up the different parts of your bowel.

The laxative or gastrografin can quickly give you diarrhoea, so make sure you stay close to a toilet after you take it. You might feel some discomfort or bloating in your tummy (abdomen) when you take a laxative, and it can taste unpleasant. You might find it tastes better if you put the mixture in the fridge before you drink it. Remember that although taking a laxative can be unpleasant, it’s really important for your test to be done effectively.

Your hospital will also give you advice about what you can eat or drink before the procedure. You might be asked to only eat low-fibre foods for a day or two beforehand. You may also be asked to stick to clear fluids like water, squash or black tea or coffee.

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What happens during a virtual colonoscopy?

When you arrive at the hospital, you’ll meet the person performing your test. They’ll explain exactly what will happen and what you can expect afterwards. You’ll be able to ask any questions you may have. It's important that you feel fully informed so you can give your consent to go ahead. When you’re ready, you’ll be given a hospital gown to change into.

A virtual colonoscopy may take up to 30 minutes, but you’ll be in the department for longer – usually an hour or so.

You won’t need an anaesthetic or sedative for a virtual colonoscopy. Your radiographer will ask you to lie on your side on the scanning table. You’ll have a fine tube (a cannula) placed into a vein in the back of your hand. You might be given a medicine to relax the muscles of your bowel wall through the cannula. You may also have a dye (contrast medium) through the cannula, which will show up your bowel more clearly on the scan. This might make you feel hot for a few minutes and give you a strange taste in your mouth, but it should pass quickly.

Your radiographer will gently pass a small tube into your back passage. They’ll use this to pump gas into your bowel, which will inflate it so it can be seen more clearly on the scan. You may feel some slight discomfort when this happens, but it isn’t usually painful.

Your radiographer will then 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 some more. They might also ask you to hold your breath for a few seconds at certain points during the scan. It’s important to try to lie very still during the scan.

What to expect after a virtual colonoscopy

After your virtual colonoscopy, you can usually go home as soon as you feel ready. If you had a medicine to relax your bowel, it can sometimes cause blurry vision. If this happens, you’ll need to wait until your vision has returned to normal – normally an hour or so – before you can drive.

Your scans will be analysed by a specialist doctor, and they’ll send the results to whoever referred you for the virtual colonoscopy. This may take a week or two.

  • If the test shows that you have polyps, you’ll need to have a procedure to directly visualise them and treat them as necessary. This might be 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 further investigation and an operation.
  • If the virtual colonoscopy didn’t find what’s causing your symptoms, you may need to have further tests, such as a conventional colonoscopy.

Recovering from a virtual colonoscopy

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

You might feel a bit bloated and have pain in your tummy for a short time. This is due to the air that was pumped into your bowel during the procedure. It doesn’t usually last long, but you can take over-the-counter painkillers if you need to. You can go back to eating a normal diet after the procedure.

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 straightaway. 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 during the procedure. It’s been estimated that this happens in fewer than one in 10,000 people who have a virtual colonoscopy, so it’s very rare. You may need an operation to repair the tear if this happens.

CT scanners use X-rays to produce images, so you’ll be exposed to a small amount of radiation. But if you’re pregnant or think you could be, let your doctor know. Your doctor will often want to avoid a CT scan if you’re pregnant as your baby is more sensitive to the potential effects of radiation. You might need to have a conventional colonoscopy instead.

What are the alternatives to virtual colonoscopy?

The main alternative to virtual colonoscopy, if you’re unable to have a conventional colonoscopy, is to have a flexible sigmoidoscopy followed by a barium enema. This involves the following tests.

  • Flexible sigmoidoscopy. This is similar to a colonoscopy, but uses a shorter instrument to look inside the lower part of your large bowel only.
  • Barium enema. This involves taking X-ray images of your large bowel, using a contrast or dye (barium) to show up the detail and give clearer X-ray images.

Which test you are offered will depend on your own circumstances, as well as what is available at your local hospital. Your doctor will talk to you about the options available and which is best for you.

Frequently asked questions

  • Polyps are small growths that can develop in the lining of your bowel. They’re quite common and usually 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 your doctor finds any polyps during your virtual colonoscopy, you’ll usually have further treatment to remove them. This is usually done in a conventional colonoscopy.

    After you’ve had a polyp removed, you may be advised to have regular colonoscopies to check whether you’ve developed any more. How often you have these will depend on how many polyps you’ve had and when the last one was found.

  • 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. This is inserted into your bowel via your back passage. Your doctor will need to put a tube into your back passage for a virtual colonoscopy too, to inflate your bowel with air. But it won’t go as far in (just a few centimetres).
    • You’ll usually need to have a sedative during a colonoscopy to help you to relax, as it can be uncomfortable. You don’t usually need a sedative for a virtual colonoscopy, which means there’s no recovery period.
    • 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.
    • There’s more risk of your bowel being damaged during a conventional colonoscopy, compared to a virtual colonoscopy, although this is rare for both procedures.
    • 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. In virtual colonoscopy, accurate assessment can be difficult if your bowel isn’t completely empty.
    • 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.

    Conventional colonoscopy is usually considered the best test for diagnosing bowel cancer; but there may be circumstances where your doctor considers virtual colonoscopy more appropriate. These include if:

    • you’re elderly or frail
    • it’s not possible to view the whole of your large bowel through conventional colonoscopy, for instance if your bowel is blocked
    • you’re taking a medicine to thin your blood, such as warfarin

    Your doctor will talk to you about which test is best for you.

  • There’s enough evidence to say that virtual colonoscopy seems to be effective enough, and safe enough for doctors to recommend its use.

    It seems to be just as good as conventional colonoscopy at detecting bowel cancer in adults at average risk of the disease, who don’t currently have any symptoms. It also appears to be more effective than a barium enema. It may not be as good at detecting smaller or flat polyps, though. All of the investigations for polyps and bowel cancer have benefits and risks. And not all the procedures may be available in your area. Your doctor will discuss the different tests available so you can decide which one is best for you.


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  • Reviewed by Pippa Coulter, Freelance Health Editor, January 2020
    Expert reviewer Dr Matthew Brown, Clinical Fellow, Bupa & Radiology Registrar, St Bartholomew’s and the Royal London
    Next review due January 2023



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