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.
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.
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.
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.
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.
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.
What will happen if a polyp is found during my virtual colonoscopy?
Polyps are small growths that can develop inside your bowel. They are quite common and don’t usually cause any problems. However, in some people polyps can grow into cancer over time. So, if the results of your virtual colonoscopy show you have polyps, your doctor will discuss with you whether they need to be removed.
Polyps are growths that can occur inside your bowel. They are small, fleshy lumps, usually less than 1cm in size, but they can grow to be bigger. Polyps are common – about two to three people out of 10 will develop a polyp before the age of 50.
Some polyps are attached to the side of your bowel on a stalk and look a bit like a mushroom, whereas others are flatter and have no stalk. You may have just one polyp or several, but it’s unlikely that you will have more than five.
Most polyps don’t cause any problems. However, there is a small chance that over a long period of time a certain type of polyp called an adenoma will grow into a cancer. An adenoma that is 1cm in size has about a one in 10 risk of developing into a cancer.
If your virtual colonoscopy shows that you have a polyp, your doctor may suggest that you have it removed. This is usually done during a conventional colonoscopy or a sigmoidoscopy depending on where the polyp is in your bowel.
After your polyp is removed it will be sent to a laboratory. This is to find out exactly what type it is and whether any of the cells show signs of cancer or look as though they might develop into cancer in the future. Depending on the results you may need to go back to hospital for more treatment or further tests.
Some people who have a polyp are at a higher risk of getting more polyps. If you develop polyps a number of times, your doctor may advise you to have regular check-ups with conventional colonoscopy.
What is the difference between a virtual colonoscopy and a conventional colonoscopy?
Conventional colonoscopy and virtual colonoscopy are both investigations that enable your doctor to look at your large bowel (your colon and rectum). One important difference between them is that during a conventional colonoscopy your doctor can remove any polyps that he or she finds, or take a biopsy.
There are a number of 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. The camera lens at the end of the colonoscope sends images to a monitor, which allows your doctor to look at your bowel. You will usually have a sedative during the procedure to relieve any anxiety and help you to relax.
If you have a conventional colonoscopy, your doctor can pass instruments through the colonoscope to remove polyps or to take biopsies if this is necessary. This isn't possible during a virtual colonoscopy so if your doctor sees any polyps or possible tumours when you have this test, you may need to go back to hospital at a later date for a conventional colonoscopy to have them removed.
Because instruments are inserted into your body during a conventional colonoscopy, you're slightly more at risk of some complications that aren't usually associated with a virtual colonoscopy.
Because there are sharp bends in your bowel, it isn’t always possible to see all of it with a conventional colonoscopy. This isn’t a problem with virtual colonoscopy. Other advantages of virtual colonoscopy are that it’s less invasive and may be safer if you’re frail, your bowel is blocked or if you’re taking a medicine to thin your blood, such as warfarin.
It’s important to remember that virtual colonoscopy is carried out using a CT scanner. This uses X-rays to produce images, so you will be exposed to some radiation. This doesn’t happen during a conventional colonoscopy.
The investigation that is best for you will depend on your general health, your symptoms and what your doctor thinks may be causing them. Your doctor will discuss the different types of tests available with you.
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.
Research has been carried out to find out how accurate a virtual colonoscopy is at detecting bowel cancer or polyps compared with a barium enema and a conventional colonoscopy.
The research shows that a virtual colonoscopy may be more accurate than a barium enema at finding large polyps. However, it may not be as accurate as a conventional colonoscopy at finding both large and small (less than 1cm) polyps. Therefore, if your doctor thinks you have cancer, he or she may suggest that you have a conventional colonoscopy rather than a virtual colonoscopy. This is because conventional colonoscopy is likely to be more accurate at picking up the early signs of cancer and also means your doctor can take a biopsy if he or she needs to.
If you need to have repeat check-ups for polyps, this will be done with conventional colonoscopy.
All of the investigations for bowel cancer and polyps have benefits and risks. Your doctor will discuss these so that you can find out which one is most suitable for you.
- Computed tomographic colonography (virtual colonoscopy). National Institute for Health and Clinical Excellence (NICE), 2005. www.nice.org.uk
- Virtual colonoscopy. National Digestive Diseases Information Clearinghouse (NDDIC). www.digestive.niddk.nih.gov, published January 2012
- Virtual colonoscopy – overview. University of Maryland Medical Center. www.umm.edu, published December 2010
- CT colonography. Radiological Society of North America. www.radiologyinfo.org, published June 2012
- Radiation dose in X-ray and CT exams. Radiological Society of North America. www.radiologyinfo.org, published April 2012
- Polyps in the bowel. Core. www.corecharity.org.uk, published October 2012
- Colonic polyps. eMedicine. www.emedicine.medscape.com, published June 2012
- Side effects of colonoscopy. healthtalkonline.org. www.healthtalkonline.org, published October 2010
- Colonoscopy tests for bowel cancer. Macmillan Cancer Support. www.macmillan.org.uk, published January 2013
- CT colonography (virtual colonoscopy): a noninvasive display of the state of the colon. Radiological Society of North America. www.radiologyinfo.org, accessed 5 March 2013
- When a screening test is abnormal: preparing for a colonoscopy. healthtalkonline.org. www.healthtalkonline.org, published October 2012
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