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CT colonography (CT pneumocolon or virtual colonoscopy)

Patient information A-Z

  • Part 1- The examination
  • Part 2- Preparation instructions

Part 1- CT Colonography examination

Please read all of the leaflet thoroughly before starting your preparation

This leaflet is for patients who are referred for a CT Colonography examination with faecal tagging. This procedure is also sometimes called a CT pneumocolon or a CT virtual colonoscopy. The leaflet will explain what the procedure involves and what the risks are.

Direct line to CT Scanning reception: 01223 217427

Please contact the CT department between 08:00 – 17:00 (Monday to Friday) as soon as possible if:

  • you cannot attend for your appointment for any reason
  • you are or there is any chance you may be pregnant
  • you weigh more than 180kgs or 28 stone
  • you are claustrophobic
  • you have limited mobility
  • In the lead up to your appointment you or someone you live with are suspected or confirmed of having COVID-19. This would be suggested by:
    • A positive COVID-19 test
    • Fever of > 37.5 oC
    • New persistent cough
    • Myalgia – muscles aches
    • Difficulty in breathing
    • Loss of taste or smell
    • Nausea / vomiting
    • New diarrhoea (although please note that you do not need to call if diarrhoea occurs after you start taking Gastrografin, as this is an expected side effect)

Every year thousands of people fail to keep their appointments. If you do not turn up, we cannot give your slot to someone else. If you are unable to attend, no longer need the scan or would like to re-arrange your appointment, please call 01223 217427.

Please note that we have both male and female radiographers performing these examinations. They will also be accompanied by another radiographer acting as a chaperone during the examination.

If you would prefer a radiographer of the same sex as yourself performing your examination, please contact CT reception prior to your appointment so we can make arrangements to accommodate this.

What is CT Colonography?

‘CT’ stands for Computed Tomography, which means using x-rays to produce ‘sliced’ pictures through a part of your body, which can be reviewed by the doctor in two and three dimensions. CT Colonography is a test used to look at your large bowel (colon and rectum). The scanner is circular with a large hole in the middle and is open at both ends. During the scan you will be asked to lie on the scan table. The table will move in and out the scanner as it takes images.

The purpose of the test is to try and find out what may be causing your symptoms (for example, a change in bowel habit, weight loss or anaemia).

How long will the examination take?

The whole examination should take about half an hour. Unless there are delays to the list, your total time in the department should be about one hour.

What happens during the examination?

Firstly, the radiographer will explain the procedure, and answer any questions you have. You will then be asked to change into a hospital gown to prevent any metal objects showing up on the pictures. The radiographer will put a cannula into one of your arms.

You will be asked to lie down on the scanner table on your right side and a very small and flexible tube will be inserted into your back passage. This will allow the radiographer to slowly introduce some carbon dioxide (CO2) and inflate the bowel. This will enable the doctors to see the walls of your bowel clearly. This may make you feel bloated.

Most patients are given two injections through the cannula. One is called Buscopan and it relaxes the bowel muscles and makes it more comfortable for you. The other is an iodine-based injection to show your blood vessels. Scans will then be taken with you lying in different positions depending on which bit of the bowel has or hasn’t inflated. It is usually on your right hand side and on your back. However, we may do additional scans and you may be asked to lie on your left hand side or on your front.

CT colonography (CT pneumocolon or virtual colonoscopy) scanner table

What should I expect after the examination?

The examination will last for about half an hour. You will be asked to stay in the department for about 15 minutes. You will be offered a hot drink and something to eat (feel free to bring something else with you).

You may wish to rest at home for the rest of the day, as:

  • You may experience abdominal pains over the next few hours. The CO2 will gradually be absorbed over the hour or so after the scan.
  • You may have some residual diarrhoea.
  • You may have blurred vision following the injection. Your eyesight will return to normal within an hour. Do not drive or operate heavy machinery until your eyesight has returned to normal.

Are there any risks or side effects?

CT Colonography is generally regarded as a safe test. Problems are rare and are similar to those which could happen with other methods of examining the bowel.

These include the following:

  • There is a risk of damage to the lining of your bowel wall. This is very rare, fewer than 1 in 3000 tests.
  • There is a risk you may react to either the x-ray dye or the Buscopan. The radiographer will ask you questions to reduce this risk before giving you the injections.
  • Very rarely, the injection can cause painful blurred vision in people who are already at risk of glaucoma. If this occurs you should call your GP as an emergency.
  • The bowel preparation, Gastrografin, may cause dehydration or an electrolyte imbalance. It is important that you drink plenty of fluids. In some people it can also cause nausea and vomiting, skin rashes and very rarely an allergic type reaction.
  • If you are concerned that you may be experiencing any of the above, please do not take any more and contact the CT department on 01223 217427 or your GP.
  • Taking the bowel preparation might prevent the absorption of the oral contraceptive pill. Additional contraceptive precautions should be taken until your next period begins.

As with all x-ray examinations, there is a risk from the use of radiation. The dose is kept to a minimum and it is equivalent to a barium enema or a few years of natural background exposure.

Are there alternative tests?

The alternative way of imaging the bowel is colonoscopy.

Colonoscopy involves passing a narrow flexible instrument through the rectum into the colon to examine the colon lining. This remains the standard test for the large bowel. It is more invasive than CT Colonography and usually requires sedation. It does, however, allow tissue samples to be taken if needed.

Colonoscopy only gives information on the large bowel, whereas CT Colonography also provides extra information on the other structures within your abdomen.

When and how will I get the results of the examination?

The pictures taken of your bowel will be reported by the radiologist (radiology doctor) who will send a report to whoever referred you for the procedure.

If you have an appointment booked, the results will be ready by then. If you do not have another outpatient appointment and you do not hear anything within three weeks, please contact the department that referred you for the scan.

Part 2- How to prepare for your scan

What do I have to do before my CT Colonography?

To enable the doctor to have a clear view of your bowel lining, your bowel will need to be prepared before the test. To do this we will ask you to drink a liquid called Gastrografin and to follow a limited diet the day before the test. It may also have strong laxative effect, but will vary from person to person. However, its purpose is not to clear your bowels unlike the colonoscopy preparation but to act like a dye that coats the bowel wall.

If you have concerns about your suitability for the procedure due to a lack of mobility when taking the Gastrografin, please speak to the team that has referred you for the scan.

During this time you will need to:

  • Stay near a toilet at all times.
  • Drink plenty of fluids as instructed to prevent dehydration.
  • Continue to take your regular medication.
  • Use extra precautions if you take the oral contraceptive.

Can I take other medication along with Gastrografin?

If you are taking iron tablets please stop them for 7 days prior to your scan.

You should continue with your all other regular medication as prescribed by your doctor unless directed otherwise.

If you are a diabetic, please read the chart in this leaflet on page 8. The change in your diet may upset your diabetes. Monitor your blood sugar regularly. If you have any questions about controlling your diabetes whilst taking your preparation, please contact the diabetes nurse specialist.

If you are taking beta-blockers, hypersensitivity or allergic-type reactions may be aggravated. Please contact the CT department on 01223 217427 if this happens.

Is there a reason why I should not take the Gastrografin?

If you are hypersensitive to iodine-containing contrast media or have hyperthyroidism then you should not take the Gastrografin. Please contact the CT department for further advice.

If you are or think you might be pregnant, please contact the CT department on 01223 217427 before taking the medication.

How do I take my Gastrografin?

The preparation for the scan is over a 24 hour period. Please start the diet and preparation the day before your appointment.

(Do NOT stop taking your normal medication except those detailed in the instructions below)

DAY BEFORE the examination

Before breakfast - 07:00

Mix 75mls (three quarters of the bottle) Gastrografin with an equal amount of Water. Cordial may be added. Drink the liquid mixture.

Breakfast – before 8am

This meal is limited to boiled or poached egg and/or white bread with a scraping of butter or margarine.

Lunch – 12:30-13:30

A small portion of steamed, poached or grilled white fish or chicken.

A small portion of boiled potatoes (2 eggs sized) OR two slices of white bread.

Jelly for dessert (this may contain sugar)

After 16:00 –

NO SOLID FOOD to be eaten until after your examination.

Clear soup/broth, for example Bovril, may be consumed followed by jelly (this may contain sugar).

Evening - 19:00 –

Mix the remaining 25mls of Gastrografin with an equal amount of water.

Cordial may be added. Drink the mixture.

Please note –

Drink as much liquid as possible.

Tea and coffee WITHOUT milk are permitted.

Alcohol is NOT permitted.

Because food intake is limited you may require the occasional sugary drink.


DAY OF the examination

No solid food until after the examination.

If diabetic, do not take your diabetes medication until after the examination.

Please note –

Drink as much clear liquid as possible.

Alcohol is NOT permitted.

Because food intake is limited you may require the occasional sugary drink.

Advice for diabetic patients undergoing CT Colonography

Please follow these instructions if your procedure is in the morning (before 13:00). For any diabetes related questions during bowel preparation, please contact your GP or the diabetes specialist nurses on 01223 348790 or bleep 152078.

Non-urgent advice: Instructions

Food and drink
  • You must follow the low residue diet the day before the CT Colonography. On the day of the procedure you must not eat anything. However, you should drink as much clear fluid as possible, at least a glass every hour, to prevent dehydration.
  • Make sure you have some sugary drinks available, for example Lucozade (100mls), or apple or orange juice (200mls). Drink these if your blood sugar falls below four.
  • Ensure that your bedtime blood sugar is at least 10.
  • Test your blood sugar regularly throughout the day.
Insulin – taken once daily

No change to insulin dose necessary.

Insulin – taken twice daily

Day before the procedure

  • Normal doses.

Day of procedure

  • Do not take morning insulin, but bring it with you to the department and take half dose with food after procedure.
  • Take normal evening insulin.
Insulin – taken four times a day

Day before the procedure

  • Normal doses.

Day of procedure

  • Do not take morning insulin.
  • After procedure take normal short acting insulin with food.
  • Take your normal evening and bedtime insulin doses.
Tablets for diabetes

Day before the procedure

  • Normal dose.

Day of procedure

  • Do not take your morning tablets. Restart at next dose after your procedure unless directed otherwise by your radiographer.

For further information

How to find us

The PET-CT department is between the outpatients and oncology departments on the ground floor. Parking at CUH is limited. Please use public transport if possible. If you do need to bring your car then we have two patient and visitor car parks.

You can find information - including information on parking concessions - on our website at Visiting our hospitals

CT colonography (CT pneumocolon or virtual colonoscopy) map directions

We are smoke-free

Smoking is not allowed anywhere on the hospital campus. For advice and support in quitting, contact your GP or the free NHS stop smoking helpline on 0800 169 0 169.

Other formats

Help accessing this information in other formats is available. To find out more about the services we provide, please visit our patient information help page (see link below) or telephone 01223 256998.

Contact us

Cambridge University Hospitals
NHS Foundation Trust
Hills Road, Cambridge

Telephone +44 (0)1223 245151