A virtual colonoscopy uses a CT scan to take a series of x-rays that build up a three-dimensional picture of your bowel.

In a virtual colonoscopy, a computerised tomography (CT) scanner takes a series of x-rays. This builds up a three-dimensional picture of your bowel. It is done in the hospital CT department and you can usually have it as an outpatient.

This test may be done instead of a colonoscopy. Or it may be done if the colonoscopy did not give a clear enough picture.

Your bowel must be completely empty for a virtual colonoscopy. You will need to follow a special diet for a few days and take a laxative before the test. Your hospital will tell you what to do.

Your doctor may give you an injection to help the muscles in your bowel relax. You may also have an injection of a dye called a contrast medium at the same time. Your doctor will tell you if you are going to have this.

Just before the scan, your doctor passes a tube into your back passage (rectum) and pumps in some air and gas (carbon dioxide). This expands the bowel and helps to give a clearer picture. You will have two CT scans – one lying on your back and one lying on your front.