Having a Computed Tomography (CT) Colonography Examination

Patient Information

Radiology Department

• Author ID:      LW
• Leaflet Ref:    Rad 014
• Version:         12
• Leaflet title:    Having a CT Colonography Examination
• Last review:    September 2025
• Expiry Date:   September 2027

Introduction

Please read the entire leaflet thoroughly before starting your preparation.

The following information explains how the test is done, what to expect and the risks involved.

For health and safety reasons, please contact the CT Department if your weight is equal to or more than 222 Kg (35 stone), as the CT scanner has a weight limit.

What is a CT Colonography?

Computed Tomography (CT) Colonography is a test used to look inside your bowel and abdomen. A CT Colonography involves using a CT scanner to produce two and three dimensional images of the whole of the large bowel (colon and rectum).

You may have been referred for this test to help your doctor find out what is causing symptoms such as abdominal (stomach) pain, weight loss, a change in your usual bowel habit and blood in your faeces (stools).

Before the test you will be asked to follow a special diet and you will be given some Gastrografin (x-ray dye) to drink at home. The special diet will help to empty your bowel before the test and the Gastrografin is used to coat any remaining faeces that remain in the bowel. This is called “Faecal Tagging”; please refer to the specific instruction leaflet: 

Rad 022 Preparation for your CT Colonography Examination.

During CT Colonography, gas will be used to inflate your bowel via a thin tube placed into your rectum (“back passage”). Then CT scans will be performed with you lying on your back and your front or side.

You may also be given an injection of a “muscle relaxant” which will help to relax the muscles within the bowel only. This injection will help you to tolerate the gas used to inflate your bowel.

You may be given an injection of an iodine based contrast agent or “x-ray dye” which is used to enhance the images, making them easier to interpret and can highlight pathology or diseases on the scan.

After the scans, Doctors will look at your images for signs of pathology or diseases that affect the bowels. Occasionally you may be offered further tests to provide additional information.

Are there any alternatives to CT Colonography?

There is one other way of looking at the large bowel: Colonoscopy.

Colonoscopy involves a flexible tube with a camera on the end to directly visualize the large bowel. This continues to be the main test for looking at the bowel. It is more invasive than CT Colonography and usually requires sedation. However, a biopsy or sample of tissue can be taken at the same time, if required.

Although gold standard, Colonoscopy tests will only give us information about your large bowel. CT Colonography also provides information about the other structures in your abdomen.

Are there any risks?

CT Colonography is regarded as a very safe test. Problems that may occur are similar to any other test for imaging the bowel.

Carbon dioxide gas is introduced into the abdominal cavity to expand it so that the colon is clearly seen. There is a small risk that inflating the colon may cause a tear (perforation). The risk of perforation is about 1 in 3000 and is lower than that of a colonoscopy.

Other possible side effects / adverse effects

• Abdominal pain / bloating
• “Faint like” reactions
• Reactions to the injected contrast agent (”X-ray dye”)
• Dehydration or an electrolyte imbalance caused by the Gastrografin

It is important to drink plenty of fluids.  If you feel ill after taking Gastrografin please contact us on 01942 822368 or contact your   own General Practitioner (GP).

What are the risks of radiation?

There is a link between developing cancer and excessive exposure to radiation. We keep the radiation dose to an absolute minimum and the dose you are exposed to during a CT Colonography is approximately the equivalent to three years natural background radiation (i.e. the amount of natural radiation you would receive from living in the United Kingdom for 3 years). However, your doctor feels that the benefits of an accurate diagnosis far outweigh any potential health risks.

Pregnancy

If there is any chance of you being pregnant, please contact the CT Scan Department for advice before starting the preparation.

If you take oral contraceptives, diarrhoea may reduce their effectiveness.

Do I need sedation for this test?

Sedation or pain relief is not needed for this test. Some people find that it can be a little uncomfortable, but as soon as the test is finished the discomfort will ease.

Do I need to do anything before the test?

The most important thing to do is follow the preparation instructions. The bowel lining needs to be clear for us to get good pictures so you will need to follow a low fibre / low residue diet for two days before the test.

Please refer to the information leaflet: Rad 022 Preparation for your CT Colonography Examination.

Can I take my medication as usual?

If you take Metformin (Glucophage) tablets for any condition, please telephone to inform us at least 48 hours before the CT appointment as you may be required to stop this medication prior to the test.

If you are taking iron tablets stop taking them 7 days before your CT Colonography appointment.

You should continue with all other medication as normal. 

On the day of the CT Appointment

Please pay attention to your appointment letter. Your scan may be performed at either Wigan Royal Albert Edward Infirmary or Leigh Infirmary. If you attend the wrong site you will likely be turned away without a scan

On arrival to the CT department, you will be greeted by a Radiology Assistant or Radiographer who will direct you to the changing rooms where you will be required to change into a gown. You may wish to bring your own dressing gown to wear over your gown as dressing gowns are not supplied by the Department.

What happens during my CT Colonography?

The Radiographer will explain the test and answer any questions before asking for your consent. Please let them know if you had any problems with your bowel preparation.

The procedure usually takes about 30 minutes.

You may have a small tube, called a cannula, inserted into one of the veins in your arm.

• You will be asked to lie down on the scanner onto your left side
• The Radiographer or Radiologist will pass a small flexible tube into your rectum (back passage)
• A muscle relaxant is often injected to avoid bowel spasm (you must wait 1 hour before driving after this injection)
• You may be given an injection of an iodine based intravenous contrast medium via the cannula
• Gas (carbon dioxide) will be gently introduced into your bowel through the tube in your back passage. This is done at a controlled rate by a machine specifically designed for this purpose
• Despite the muscle relaxant you may still feel some bloating and discomfort in your abdomen
• Once the Radiographer is satisfied with the amount of gas in your bowel, CT scans will be taken with you lying in 2 positions: first on your front and then on your back or side
• Each scan will take about 10-20 seconds (one breath hold)
• Sometimes the Radiographer may need to take extra scans to ensure we can fully see your entire bowel. Occasionally we will perform a scan of your chest at the same time for additional information

Relatives and escorts

You can bring a relative or friend with you to the Radiology Department, however they will not normally be allowed in the examination room.

Children / vulnerable adults

Please be aware that the department does not have the facilities to supervise children / vulnerable adults. If you need to bring a child or vulnerable adult with you, you must bring an appropriate adult to supervise them whilst you are having the examination.

Interpreters

If you require an interpreter, please contact the department before you begin your preparation so that this can be arranged. (Please Note: Hospital policy states that relatives cannot act as interpreters).

What happens after CT Colonography?

A Specialist Radiologist will review the images from your CT Colonography and send a report to your doctor. 

Any further questions?

We will do our best to make your visit as comfortable and stress free as possible.

If you have any questions about the time and date of your appointment, please phone the Appointments Clerks on 01942 822368, 8:30am until 5pm Monday to Friday.

There is an answer phone available outside of these hours where you can leave a message and we will call you back.

If your call cannot be answered please leave your name and contact details and we will return your call as soon as possible.