CT colonography

 CT (computed tomography) colonography is a test that uses CT scans to check the large bowel (colon) and back passage (rectum). It’s also called a virtual colonoscopy.

You have this test as an outpatient in the CT scanning (or radiology) department at the hospital. A radiographer or specialist doctor (radiologist) carries out the test.

It usually takes around 30 minutes but you should expect to be in the department for about an hour or so.

Why do I need a CT colonography?

You usually have this test to help find the cause of your symptoms.

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Before your test

You need to have an empty bowel for this test. This is so the radiographer can clearly see the inside of your bowel and rectum.

You'll get clear written information on what to do before you have a scan. Your preparation usually starts a couple of days before the test.

You need to take strong medication (laxatives) to empty your bowel the day before. Or you might need to drink a special liquid (contrast medium) called gastrografin over 1 or 2 days.

Gastrografin is a type of dye containing iodine. It helps to make scan pictures clearer. It also acts as a laxative and may give you diarrhoea.

Having gastrografin or laxatives will mean that you need to open your bowels often, and very suddenly. You might also have some cramping pains. It’s best to stay at home for a few hours after taking gastrografin or the laxatives, so that you are near a toilet. 

You might also need to follow a low fibre diet for 1 or 2 days before the test.

To prevent dehydration it’s important to drink plenty of clear fluids such as:

• water
• black tea or coffee
• squash (without red or purple colouring)
• clear soup

You may need to stop taking iron tablets or other medicines which can cause constipation. You usually stop these one week before the test. 

Contact the radiology department as soon as possible before your appointment if you take medicines to thin your blood or you are diabetic. You'll get other instructions to follow.