CT enema scan - Overview

A CT enema scan looks at your large bowel (colon). It is also called a virtual colonoscopy. The scan can help your doctor to find the cause of your symptoms.

It is done in the X-ray department by a radiographer (a person who takes X-rays), or by a radiologist (a doctor who uses scans to diagnose and treat illnesses).

Having a CT enema scan

During a CT enema scan, a small tube is put into your bottom. Carbon dioxide gas is put through the tube to inflate the large bowel (colon).

  • A special contrast dye is used to help the large bowel show up better on the scan.
  • Pictures are then taken (by X-rays) to create a detailed image of the inside of your body.
  • The pictures show cross-sections, or slices, of your body. They are put together by a computer and can be viewed on a screen.
  • The radiologist will review and create a report on these images. 

A CT enema scan needs special preparation. It is important that you follow the right preparation instructions. We will tell you which preparation you need to follow.

Risks of a CT enema scan

During a CT scan, you are exposed to X-rays. They are a type of radiation called ionising radiation. This may cause cancer many years or decades after you are exposed to it.

The level of radiation used during a CT scan and the health risks are low. Also, the benefits of the scan are thought to be greater than any risks. Everyone is at risk of getting cancer during their lifetime.

  • The contrast dye that is used contains iodine, which some people are allergic to. We will check if you have any allergies.
  • If you have an injection of contrast dye, it might give you a taste of metal in your mouth, and a warm feeling (a hot flush). Some people might feel as if they are wetting the bed. These feelings are normal, and stop after a couple of minutes.
  • You will have an iodine injection, which can affect your kidneys. You will not be given this injection if you have very poor kidney function.

The carbon dioxide gas given to inflate the bowel is not suitable for all patients. We will review your medical history to check and may decide to use air instead.

There is a very small risk that inflating the large bowel with carbon dioxide gas or air may cause a small tear (perforation) in the bowel. The risk of this is about 1 in 3,000 cases and is much lower than that of a colonoscopy. The CT scan is reviewed by the radiographer to make sure there is no evidence of this before you leave the department.

If you have had an allergic reaction to contrast dye or iodine in the past, you must tell your doctor or contact the CT department.

If you are or might be pregnant

Radiation can be harmful for an unborn baby. We recommend that you have the CT scan within 10 days from the first day of your period. If your appointment is not within this time or you think that you might be pregnant, please contact the CT department.

Other treatment options

The alternatives to the CT enema scan are a colonoscopy or sigmoidoscopy. Please speak to your doctor if you have any questions or concerns about this scan, or if you would like more information on the other options.

Asking for your permission (consent)

We want to involve you in decisions made about your care and treatment. If you decide to have a CT enema scan, you will be asked to sign a consent form to say that you understand what it involves and agree to have the treatment. 

You can read more about our consent process.

Before a CT enema scan

If you need a hoist (piece of medical equipment to help lift or move you safely), transport or a translator, please contact the radiology department.

If you have diabetes

Make sure you have a morning appointment. If you take insulin, you should be given a 9am appointment. Please call the CT department if this has not been done.

If you are taking medicines for diabetes these might need to be changed around the time of your procedure. Call the CT nurse coordinator for advice.

If you are able to become pregnant and have regular periods

You must make sure the scan is within 10 days of the first day of your last period.

If you are taking the contraceptive pill

It may not be effective once you have taken the bowel preparation, so you must use other precautions. Speak to your GP, pharmacist or family planning clinic for further advice.

If you have any allergies to medicines

Contact the CT nurse coordinator to let them know.

The day before your scan

You will need to make some changes to your diet the day before your scan.

You will also need to take some preparation medicine as your bowel needs to be empty and clear of poo. A clear bowel gives a better CT scan result.

You will have been advised on which preparation you need to do before your scan. You can read the diet and preparation instruction by clicking the relevant link. If you are not sure, or have any questions, contact the CT nurse coordinator.


The morning of your scan

Drink clear fluids only.

You can take your usual morning medicines by 8am with a glass of water. If you take medicines that help prevent blood clots and heart attacks (such as anticoagulant or antiplatelet), these can be taken as usual. 

During a CT enema scan

The scan will take 20 to 30 minutes, but you will be in the X-ray department for about 1 hour.

We will try to keep to your appointment time, but sometimes we have to scan people urgently at short notice. This means that your appointment could be delayed. We will let you know if this is the case.

  • When you arrive in the X-ray department we will ask you to complete a short pre-scan checklist to confirm your medical history.
  • You will be asked to change into a hospital gown. You might find it more comfortable to bring a dressing gown and slippers with you, but you do not have to.
  • You will have an injection of contrast dye into your arm. You will not have this if you have an iodine allergy or poor kidney function.
  • You will meet the radiographer and the radiologist, who will explain what happens in the procedure.
  • The scanner is shaped like a huge ring doughnut standing on its side. You will need to lie on a bed which moves slowly through the machine as the scan is taken. The CT scanner is not enclosed.
  • We will help position you on the bed. You will then be asked to roll onto your left side so we can put a small flexible tube into your bottom.
  • Carbon dioxide (or air) will be put into your bowel through the tube to inflate your bowel. The carbon dioxide will not cause you any harm.
  • Once your bowel is inflated, we will ask you to turn onto your back for the first scan. You will then be asked to lie on your tummy for the final scan, while the tube is in place.
  • You will need to lie very still while each picture is taken.
  • We will ask you to hold your breath for about 10 seconds at a time. This helps to stop the images being blurry.

The tube might feel a little strange in your bottom, but you should not feel any pain.

You may have some discomfort from the carbon dioxide (or air) being inserted into your bottom, which can feel like stomach cramps. 

The most difficult part is keeping still. Try to relax as much as possible.

If you find it uncomfortable to lie still or if you feel any pain, please tell the radiographer or radiologist.

After your scan

In most cases, as soon as the scan is finished you can go home, or go back to your ward if you are staying in hospital. 

You can eat and drink as usual, and do your usual activities straight after the scan.

If you had a contrast dye injection, we might ask you to wait for 10 minutes before you leave the department. This is to make sure there are no side effects from the dye.

If you feel well, you can return to work on the same afternoon after your scan. Or, you might want to take the rest of the day off, and go to work the next day.

Contact your GP or the CT department if:

  • you have a rash
  • you have itchiness or redness
  • you feel unwell

Allergic reactions are rare. In very rare cases a reaction can cause swelling of the:

  • face
  • lips
  • tongue
  • throat

If this happens, go straight to your nearest emergency department (A&E).

Results of your scan

The pictures taken during the scan are read by the radiologist, who will make a detailed report for the doctor who referred you for the scan.

Your doctor will talk to you about the results and any treatment you might need.

  • If you were referred by a doctor at Guy’s and St Thomas’ hospitals, they will usually get the results in 4 working days.
  • If you were referred by a doctor at another hospital, or by your GP, it can take longer. Please wait 2 weeks after your scan before making a follow-up appointment to talk about your results.

Resource number 3048/VER5
Publish date: November 2023
Review date: November 2026

Contact us

If you have any questions or concerns about having a CT scan, please contact:

CT department at Guy’s, phone 020 7188 5574
CT department at St Thomas’, phone 020 7188 5464
CT nurse coordinator, phone 020 7188 9033 or 07784 362210

You can also contact the radiology department, phone 020 7188 5542

Monday to Friday, 9am to 5pm.


Do you have any comments or concerns about your care?

Contact our Patient Advice and Liaison Service (PALS)

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