About a CT virtual colonoscopy
The large bowel (colon and rectum) is the last part of the gut, where water is absorbed and stools form. A CT virtual colonoscopy, also called CT colonography (CTC), is a CT scan of the large bowel taken after it has been cleaned out and gently inflated with gas. A computer then turns the pictures into 2D slices and a 3D “fly-through” view of the inside of the bowel, similar to the view from a camera.
CT colonography is used to look for bowel cancer and polyps (small growths, some of which can turn into cancer over time). It is about as good as a standard colonoscopy at finding bowel cancers and larger polyps. It may be used:
- to investigate symptoms such as a change in bowel habit, bleeding from the bottom or anaemia, particularly in people who are frail or would find a colonoscopy difficult
- when a colonoscopy could not reach the whole bowel
- for people who prefer not to have, or cannot have, sedation or a camera test
Unlike a colonoscopy, it cannot take samples (biopsies) or remove polyps. If anything is found, you will usually need a colonoscopy afterwards. Very small, flat polyps can be missed. Because the scan also covers the rest of the tummy, it can show findings outside the bowel, most of which turn out to be harmless but some of which may need further tests. You can read more in our article on CT virtual colonoscopy.
What happens during the scan
You will be given an injection of Buscopan to relax the bowel, unless there is a reason you cannot have it. You lie on your side, and a thin, soft tube is placed a short way into your back passage. Carbon dioxide gas is gently pumped in to inflate the bowel, which can cause a feeling of fullness or cramping. Scans are then taken with you lying on your back and on your front (or side), each needing a short breath-hold. Contrast dye may also be injected into a vein. Afterwards the tube is removed, and the gas is absorbed quickly, so most people feel back to normal within an hour.
Is CT safe?
CT uses X-rays, which are a type of ionising radiation. CT colonography uses a low-dose technique. The dose is lower than for a standard CT of the abdomen and pelvis (about 10 mSv). The risk from a single scan is very small, and every CT scan is checked by a radiologist or radiographer to make sure the benefit of the scan outweighs the risk, as required by the Ionising Radiation (Medical Exposure) Regulations. If you have had other CT scans recently, tell us when you book.
What the scan can show
A CT virtual colonoscopy can help to look at:
- bowel cancer
- polyps in the large bowel
- diverticular disease
- narrowing of the bowel
- parts of the bowel a colonoscopy could not reach
- other organs in the tummy and pelvis
See a GP if you have bleeding from your bottom, blood in your poo, a change in your bowel habit lasting 3 weeks or more, unexplained weight loss or tiredness, or a lump in your tummy. Go to A&E if you are bleeding heavily from your bottom or have severe tummy pain.
Preparing for your scan
Your bowel must be as empty as possible for clear pictures. The scanning centre will send you a bowel preparation pack and instructions, usually involving a low-fibre diet for a day or two, then clear fluids only and a laxative the day before. Many centres also give you a contrast drink to “tag” any remaining stool, so it can be told apart from polyps. The laxative will make you go to the toilet often, so plan to stay at home that day.
Tell us when booking if you have kidney problems, diabetes, heart problems, glaucoma, inflammatory bowel disease, or have had bowel surgery, as these can affect the bowel preparation or the use of Buscopan. Buscopan can cause short-lived blurred vision, so do not drive until your vision is clear. Very rarely it can cause painful red eyes with blurred vision within 24 hours; if this happens, go to A&E straight away.
Wear loose, comfortable clothing without metal zips, poppers or buttons, and leave jewellery at home, as metal in the scan area can affect the pictures. You may be asked to change into a gown. Unlike MRI, CT is not affected by pacemakers or most metal implants, and the scanner is a short open ring rather than a tunnel, so it is usually easier for people who feel anxious in enclosed spaces.
Tell the team before your scan if you are, or might be, pregnant or breastfeeding, if you have kidney problems, diabetes (especially if you take metformin), asthma, thyroid problems or allergies, or if you have had a reaction to contrast dye before. If you need contrast, you may be asked for a recent kidney blood test result.
Frequently asked questions
Is a virtual colonoscopy as good as a normal colonoscopy?
It is about as good at finding bowel cancers and larger polyps. A colonoscopy is better at finding very small or flat polyps, and can take samples and remove polyps during the same test.
Does it hurt?
Most people find it uncomfortable rather than painful. The gas can cause a feeling of fullness and cramping, which settles quickly once the tube is removed. No sedation is needed, so you can usually go home straight away.
What happens if a polyp is found?
You will usually be referred for a colonoscopy, so the polyp can be removed or sampled. Your doctor will explain the results and next steps.
Do I need a referral?
Because CT uses radiation, each scan has to be justified by a qualified professional before it can go ahead. Upload a referral from your GP or consultant when you book if you have one. If you do not, select “no referral” and the scanning centre will review your request and confirm the next steps with you, which may include arranging a referral.
Sources
- NHS – Bowel cancer: diagnosis
- Colorectal cancer: CT colonography and colonoscopy for detection – systematic review and meta-analysis (PMC)
- Computed tomography colonography for the practicing radiologist: current recommendations on methodology and clinical indications (PMC)
- RadiologyInfo.org – CT Colonography
- NHS – CT scan
- UKHSA – Patient dose information: guidance
The information on this page has been gathered from various sources and is provided for general information only. It is not medical advice. TOP MRI is not liable for the accuracy or completeness of this information. Please speak to your doctor or the scanning centre about your own situation.