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 will be given clear instructions by Canberra Imaging Group leading up to the CTC. The important thing is to have a clean bowel before the test, so that any polyp or cancer can be detected. It is important to tell your referring doctor as well as the radiologist (specialist doctor) supervising the CTC about any pre-existing medical conditions, such as diabetes, or diverticulitis. Pregnant women should not have this test.
The instructions will be provided as a package comprising written information, instructions and a letter to be taken to a pharmacy concerning the laxative you will need to use. It will also contain a chalky substance to take for highlighting or ‘tagging’ any residual faeces or fluid in the bowel, and a liquid called “Ioscan”. In most cases, you will need to go on a low-fibre diet between 1–3 days before the test. 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 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 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 be given a small injection of a bowel muscle relaxant called Buscopan to help this process. As the 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 about 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 rectum. You will be able to go to a nearby toilet straight away. You can then get dressed.
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, please alert the radiographer if you are feeling unwell. You can eat as usual afterwards.
If you have had the small injection of Buscopan 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, depending on how easily the gas can move through your bowel. You may spend a variable time getting changed and toileting, depending on how you feel, before and 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 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 procedure*.
Because CTC uses X-rays, there is an extremely small radiation risk. For more information, please refer to:
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.
* 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.