Your CT virtual colonoscopy

A CT scanner machine uses ionising radiation (X-rays), and a computer to produce very detailed cross-sectional images of any part of the body. The information is built up into a series of pictures for the radiologist to view.

CT virtual colonoscopy is different from other CT scans because carbon dioxide is used to inflate the colon (large bowel), via a thin flexible tube that is inserted into the back passage (anus). The CT scanner the puts together a detailed 3D model of the colon, which the radiologist uses to view the bowel in a way that simulates travelling through the bowel. This is why the procedure is called a virtual colonoscopy.

At the same time, 2D images of the colon, abdomen, and pelvis are taken without any additional radiation.

The alternative way of looking at the large bowel is endoscopy. Endoscopy involves using a tube with a camera on the end (colonoscope) which is passed into the back passage, an moved up and around the bowel. Although it doesn‘t use radiation, it is more invasive than a virtual colonoscopy and usually requires sedation. However, it does allow for a biopsy or removal of small polyps at the same time.

• We will send you a special medicine before the scan. Please mix this with water and follow the instructions given to you on a separate leaflet. This medicine may cause diarrhoea, which is required to empty the bowel. It also coats the lining of the bowel so we can see it on the scan. 
• You will also be given a separate leaflet with what foods you can and cannot eat. Please follow these instructions carefully.
• Please drink plenty of water over the 2 days before the scan. 
• If you are diabetic and have received an afternoon appointment, please let us know. Morning appointments are available to ensure you can follow the preparation safely.

• The Radiology department assistant will explain the process and ask you to change into a hospital gown. You will then be taken into the scan room where the radiographer will again explain and answer any questions.
• A small cannula will be put into your arm so that a muscle relaxant can be injected. This drug is used to relax the involuntary muscles of your bowel to reduce discomfort and decrease bowel movement on the scan.
• You will be asked to roll onto your side and a small flexible tube (around the width of a pencil and 2 inches in length) will be inserted into your back passage. A lubricant is used to ease this process.
• Carbon dioxide gas will then be passed through the tube and into the large bowel for the duration of the examination to maintain inflation. You will be asked to move into different positions on the scanning table (on your front, back and sides) and several scans will be taken.
• The radiographers will check the quality of the images and ensure they have all the information required from each position. Occasionally, more information may be required, and the scan will also include your chest. A contrast dye will be inserted into your cannula during the scan if this is required.
• During the scan you will be asked to hold your breath and retain the gas as much as possible whilst the table moves slowly. You will move in and out several times. Occasionally, a contrast dye may be given through the cannula and the scan to provide more information.
• The whole examination will take around 30 minutes and is very clean. A small balloon on the tube keeps the tube in place and any faeces (poo) left inside you will go down the tube. You will not be given sedation for this examination.
• Once all the images are taken the tube will be gently removed. You will then be taken into a joining room where there is a toilet and your clothes to get dressed. The team will talk to you before you go to ensure you feel ok.

You may eat and drink as normal. Please drink plenty of water as the bowel preparation medicine may dehydrate you. 

The carbon dioxide gas will naturally be absorbed by the body and the bloated feeling will subside quickly once you have passed wind and used the toilet. 

The radiologist will examine the images in detail and send the written results to the consultant or GP that referred you for the scan. You will get your results from that doctor.

A CT scan involves the use of ionising radiation (X-rays); we are all exposed to natural background radiation every day from radioactivity in the air, food that we eat and even from space.

Exposure to ionising radiation from a CT scan carries a small risk, however, the main benefit of having the scan is to get an accurate diagnosis so that you can get the right treatment for you. A specialist will have agreed that the benefit, of the scan, outweighs the risk of the ionising radiation and we can assure you that all safeguarding to minimise ionising radiation exposure is taken.

If there is a possibility that you may be pregnant, then please contact the radiology department prior to the scan.

During the CT virtual colonoscopy, you may experience abdominal discomfort and bloating, this is normal but if it becomes too painful then please let the radiographer know. Once the scan is complete you could become faint when sitting up, however, the team follow several steps to limit.

There is a very small risk that inflating your bowel with air could cause injury to or perforation of the bowel, however, this occurs in fewer than 1 in 10,000 patients. This will, in most cases, heal itself, however, may require medical assistance or antibiotics.

If you have had the injection of muscle relaxant, you may experience a dry mouth and blurred vision. The latter is very rare, but if it does happen, it will wear off within 30 minutes but please do not drive until your vision is back to normal. Please let the radiographer know if you suffer from angina, any heart problems or glaucoma.

Some patients may require an injection of contrast medium, also described as X-ray dye, for their scan which increases the amount of information seen on the scan. There is a small chance of an allergic reaction to the injection of contrast medium, however, the Imaging team are trained to deal with any complication, and again the risk is very small. The Imaging team will make sure you are feeling alright and have recovered before letting you leave the department.

CT Colonography or Virtual Colonoscopy

National Health Service: Your CT scan: 
Available from CT scan - NHS [Accessed on July 2022]

United States Food and Drug Administration: Computed Tomography (CT).
Available from Computed Tomography (CT) | FDA [Accessed July 2022]

Computed Tomography (CT or CAT scan) Procedures

Radiation Dose from X-Ray and CT Exams