Authors: Dr Helen Moore*
                            Prof Parm Naidoo *

What is a CT colonography?

(Also known as CT colonoscopy, virtual colonography or virtual colonoscopy)

A computed tomography colonography (CTC), is a CT scan to look at the colon, or large bowel. A CT scan is a specialised X-ray test during which the patient lies on a moving table/bed as it passes through a circular X-ray machine. In order to study the large bowel, dietary preparation is required, usually for 1–3 days before the scan. This is to cleanse the bowel, and involves taking a prescribed laxative, similar to having other large bowel tests, such as barium enema or colonoscopy.

Why would my doctor refer me to have this procedure?

The main purpose of CTC is to look for cancer, or polyps, in the large bowel. Polyps are growths arising from the surface of the colon that may grow into cancers of the colon. If a cancer or significant sized polyp is found, you will need to have another test, such as a colonoscopy to biopsy (take a small tissue sample) or remove a polyp.

CTC is also excellent for diagnosing diverticulitis (where pouches develop in the wall of the colon) or as an investigation if you have symptoms suggestive of bowel problems.

Another reason to have a CTC is if you have had a difficult or an incomplete conventional colonoscopy, when the whole bowel may not have been completely or confidently assessed.

You must inform your doctor if you are pregnant. You should not have a CTC if you are pregnant.

How do I prepare for a CT colonography?

You should be given clear instructions by the hospital or radiology clinic carrying out the CTC. The important thing is to have a clean bowel before the test, so that any polyp or cancer can be detected. The exact instructions may differ depending on any other medical problems you may have, such as diabetes, kidney disease or some heart problems. It is important to tell your referring doctor as well as the radiologist (specialist doctor) supervising the CTC about these. Pregnant women should not have this test.

The instructions will be provided as a package comprising written information, instructions and some laxative pills. It may also contain a chalky substance to take for highlighting or ‘tagging’ any residual faeces or fluid in the bowel, and a suppository to encourage bowel movement. In most cases, you will need to go on a low-fibre diet between 1–3 days before the test. The evening before the test, the laxative pills are taken, along with several drinks of water, and usually this means you may need to go to the toilet several times during the evening. In the morning, you must not eat breakfast, and before the test the suppository is inserted in your back passage or rectum. It is important to read and follow the instructions exactly.

What happens during a CT colonography?

When you arrive at the clinic or radiology department, you will be guided to a change room to remove your clothes and put on a simple gown. Then you will be taken to the CT scanner room, and asked to lie on the CT table/bed. A nurse, radiographer (who operates the CT scanner) or radiologist (specialist doctor) will explain the procedure as you go, and you can ask questions. A small soft tube is placed in the back passage, or rectum, through which air or carbon dioxide is passed into your large bowel. This causes it to expand so that the inside wall of the bowel can be clearly seen on the CT scan images or pictures. You may or may not be given a small injection of a bowel muscle relaxant to help this process. As the air or carbon dioxide passes in, you will feel your abdomen become quite tight and bloated, this is often described as ‘uncomfortable fullness’, and you may feel like burping or passing wind. Occasionally, people may experience mild nausea, which usually passes quickly.

Two scans (usually taking between 3–10 seconds) will be taken; one while you are lying on your back and one while you are lying on your front. You will have to hold your breath for this short scan. Occasionally, the scan will be carried out while you lie on your side. The images will be checked, and you can relax and the tube will be removed from the anus. You will be able to go to a nearby toilet straight away. You can then get dressed and will usually be offered a drink and biscuit before leaving.

Are there any after effects of a CT colonography?

After the test, your abdomen may feel uncomfortable, a bit ‘crampy’ or ‘bloated’ for a few hours. If this occurs, it usually passes quickly.

You may be hungry or feel a little light-headed from not eating breakfast, and the staff should offer you a drink and biscuit afterwards. You can eat as usual afterwards.

If you have had the small injection to relax the bowel, it is unlikely, but possible, that your eyesight might be a little blurry for half an hour or so, and that your mouth might feel dry.

How long does a CT colonography take?

The actual time in the CT scanner room is usually approximately 10–20 minutes. You may spend a variable time getting changed and toileting, depending on how you feel, before and after the test. If the radiologist is available to check your images straight away, this may require you to wait longer after the test.

What are the risks of a CT colonography?

A CTC is the safest way of examining the large bowel. There is a tiny chance of a bowel perforation (making a hole in the bowel wall) from having a CTC. This would show up by having worsening abdominal pain.

In the unlikely event you are concerned after your test, it is important that you notify your doctor or the radiologist who carried out the test immediately, or go to the nearest hospital emergency department immediately. The rate of this rare complication is reported to range from 0–0.03% of patients having this procedure1.

Because CTC uses X-rays, there is an extremely small radiation risk; for more information see:
Radiation Risk of Medical Imaging for Adults and Children.

If you are pregnant, you should tell your doctor, and should not have this test.

What are the benefits of a CT colonography?

CTC is the safest and least invasive way of examining the whole large bowel. It does not require sedation (receiving an injected medicine to cause sleepiness or drowsiness), hence you can continue a normal day after having the test.

Who does a CT colonography?

The CTC is carried out by a medical radiation or imaging technologist (MRT/MIT or radiographer) trained in operating the CT machine and a radiologist (specialist doctor), often with the help of a nurse. The radiologist will interpret the images and give a written report to your referring doctor.

Where is a CT colonography done?

The test is carried out in the CT scanning room, with toilet facilities close by, of a radiology department in a hospital, clinic or private radiology practice.

When can I expect the results of my CT colonography?

The time that it takes your doctor to receive a written report on the test or procedure you have had will vary, depending on:

  • the urgency with which the result is needed;
  • the complexity of the examination;
  • whether more information is needed from your doctor before the examination can be interpreted by the radiologist;
  • whether you have had previous X-rays or other medical imaging that need to be compared with this new test or procedure (this is commonly the case if you have a disease or condition that is being followed to assess your progress);
  • how the report is conveyed from the practice or hospital to your doctor (i.e. phone, email, fax or mail).

Please feel free to ask the private practice, clinic or hospital where you are having your test or procedure when your doctor is likely to have the written report.

It is important that you discuss the results with the doctor who referred you, either in person or on the telephone, so that they can explain what the results mean for you.

Further information about CT colonography:

The alternative tests used to screen for bowel cancer are optical colonoscopy and faecal occult blood test.

Optical colonoscopy is the preferred screening test for people at higher risk, such as those with:

  • Family history of bowel cancer.
  • Previous bowel polyps or bowel cancer.
  • If you have symptoms that might be caused by bowel cancer, such as:
  • Bleeding from the back passage or rectum.
  • Weight loss.
  • Optical colonoscopy is the best test, because it allows biopsy of any suspicious abnormalities in the bowel.
  • An optical colonoscopy is carried out by a doctor who examines the whole bowel by inserting a small, flexible telescope into your rectum (back passage).

The faecal occult blood test (FOBT) is a test that can detect tiny traces of blood in the stool — such small traces that they can’t be seen with the naked eye.

Useful websites about CT colonography

American College of Radiology Site:


  1. Berrington de Gonzalez A, Kim KP, Yee J. CT colonography: perforation rates and potential radiation risks. Gastrointestinal endoscopy clinics of North America. [Research Support, N.I.H., Intramural Review]. 2010 Apr;20(2):279-91.
  2. *The author has no conflict of interest with this topic.

    Page last modified on 18/8/2017.

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