This page is about CT colonography, which is a test that looks at the inside of your large bowel. You can find the following information
CT colonography is a test that uses CT scans to check the large bowel (colon). It is also called virtual colonoscopy or CT colonoscopy. You have it in the CT scanning department at the hospital. It takes about 30 minutes.
Before your appointment you will receive details of how to prepare for the scan. Your bowel needs to be empty. So you will need to take strong laxatives the day before. Or you may need to drink a special liquid (contrast medium) over 1 or 2 days. In some hospitals, you may have both. You will also need to follow a special diet for 2 or 3 days before the test.
Having a CT colonography
At the hospital, you may have an injection of a medicine that helps to relax the muscles of your bowel wall. You may also have an injection of a dye (contrast medium). This helps to show up your bowel more clearly on the scans.
Just before the test the radiographer puts a small tube a few centimetres into your back passage to pump a gas inside. This helps to open up the bowel and give clearer pictures on the scan.
A nurse or the radiographer helps you to lie on your back, and the scanning table moves into the CT scanner. The radiographer and other staff then leave the room. But they can still see you, and you can talk to each other through an intercom system. You often have a second scan lying on your front.
Once the scan is finished, the radiographer removes the tube from your back passage and shows you to the toilet. You may need to stay in the department for around 30 minutes. You can then get dressed and go home and eat and drink normally.
CT colonography is a safe test for most people but has some possible risks. Your doctor or radiographer will talk to you about these beforehand.
Getting your results
You should get your results within a couple of weeks. Contact your doctor’s secretary or GP if you haven't heard anything after this time.
CT (computed tomography) colonography uses CT scans to check the large bowel (colon). It is also called virtual colonoscopy or CT colonoscopy.
You have this test as an outpatient in the CT scanning (or radiology) department at the hospital. The test is done by a senior radiographer or specialist doctor (radiologist). It usually takes around 30 minutes but you should expect to be in the department for about an hour or so.
You usually have this test to help find the cause of your symptoms.
View a transcript of the video showing what happens when you have a CT colonography (Opens in a new window)
Before your appointment you will receive details of what you need to do to prepare for the scan. Your bowel needs to be empty so the doctor can have a clear view of the bowel lining.
Clearing the bowel
You will need to take strong laxatives the day before the test. Or you may need to drink a special liquid (contrast medium) called gastrografin over 1 or 2 days. In some hospitals, you may have both the laxatives and the contrast medium.
Gastrografin is a type of dye that contains iodine, and helps make the scan pictures clearer. It also has a laxative effect and causes diarrhoea. You may need to collect the laxatives or gastrografin from the hospital at least a few days before your test.
Laxatives can affect people differently. You will need to go to the toilet often and for some people this is urgent. You may also have stomach cramps. It is a good idea to stay at home close to a toilet for at least several hours after taking the laxatives or gastrografin.
You need to follow a special diet (such as a low fibre diet) for 2 or 3 days before the CT colonography. You usually can’t eat solid food from the day before the test until after the scan. When you start taking the laxatives, it is important to drink plenty of clear fluids, such as water and tea or coffee without milk.
If you take iron tablets, you usually need to stop taking them about a week before the test. If you normally take medicines that cause constipation, you should also stop taking these about 4 days beforehand. Your appointment letter will give you information about this.
If you are pregnant or diabetic, contact the radiology department for advice as soon as you can before the test.
At the hospital, a radiographer or an assistant will meet you and ask you to change into a gown. The radiographer will talk to you about the test and answer any questions you may have. They will ask about your medical history and if you are allergic to anything. When you are ready, you lie on the scanning couch, on your left side.
The CT scanning machine is large and shaped rather like a ring doughnut. The couch goes into the scanning machine. Below is a photograph of a CT scanning room.
You may have an injection of a medicine that helps to relax the muscles of your bowel wall. You may also have an injection of a dye (contrast medium) at the same time. This helps to show up your bowel more clearly on the scans. You have the injections through a fine tube (cannula) in your arm.
Just before the test the radiographer puts a small tube a few centimetres into your back passage to pump carbon dioxide or air inside. This helps to open up the bowel and give clearer pictures on the scan. You may feel some discomfort from the carbon dioxide or air. Rarely, people have pain.
A nurse or the radiographer helps you to lie on your back and the scanning table moves into the CT scanner. The middle part of your body is in the centre of the ring. The radiographer and other staff leave the room while you have the scan. But they can still see you and talk to you through an intercom.
You often have a second scan lying on your front. At certain times the radiographer may ask you to hold your breath for a few seconds. It is important that you lie very still during the scan.
A computer matches up the two scans and makes a virtual scan of the inside of your bowel.
Once the scan is finished, the radiographer removes the tube from your back passage and shows you to the toilet. They may ask you to stay in the department for around 30 minutes to make sure you are feeling alright. They will remove the small tube from your arm.
You can then get dressed and go home. You can usually eat and drink normally.
CT colonography is a safe test for most people but has some possible risks. Your doctor and radiographer make sure the benefits of having the test outweigh these risks.
Effects of clearing the bowel
Clearing the bowel can cause dehydration for some people. So it is important to drink plenty before and after your scan. Gastrografin may make some people feel sick or be sick. It can also cause a skin rash or very rarely, an allergic reaction. If you think you are having a reaction, contact your GP straight away or go to the nearest accident and emergency department.
Effects of the contrast medium
The injection of contrast medium may make you feel hot and flushed for a minute or two. You may also have a metallic taste in your mouth. You may feel that you are passing urine, but you are not. This feeling is common and goes away quickly.
Very rarely, people have an allergic reaction to the contrast medium. If you feel ill or have problems breathing during the test, tell the radiographer straight away.
A sore tummy
You may have bloating and some discomfort in your abdomen after the test. This is due to the carbon dioxide or air put into your bowel. The discomfort should pass quite quickly.
A tear in the bowel
There is a very small chance that putting the gas into your bowel may damage or tear it (perforation). This is very rare. If your bowel is damaged, you may need antibiotics and fluids through a drip. And you may need surgery to repair the tear. Contact your GP or hospital straight away if you have severe pain in your abdomen or bleeding from the back passage after the test.
The medicine used to relax your bowel may cause temporary blurred vision. If this happens, you shouldn’t drive for an hour or so, until your eyesight is back to normal. Tell your radiographer before the test if you have glaucoma (increased pressure within the eye) or heart problems.
CT scanners use X-rays to produce images, so you will be exposed to some radiation. The radiation is kept to a minimum. The risk of the radiation causing any problems in the future is very small. Pregnant women should not have CT colonography because the radiation may harm the unborn baby.
It can take time for test results to come through. How long will depend on why you are having the test but it may take up to a week or two. The hospital will send the results to your specialist, who gives you the results.
Understandably, waiting for results can make you anxious. Try to remember to ask your doctor or the radiographer when you should get your results. If you haven’t heard anything a couple of weeks after your test, ring your doctor’s secretary to check if the results are back.
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