Computed Tomography (CT scan)

Computed Tomography (CT scan)

What is it?

A computed tomography (CT) scan uses X-rays and a computer to create detailed images of the inside of the body.

How does it work?

A CT scanner emits a series of narrow beams through the human body as it moves through an arc, unlike an X-ray machine which sends just one stationary radiation beam. The final picture is far more detailed than an X-ray one.

Inside the CT scanner there is an X-ray detector which can see hundreds of different levels of density. It can see tissues inside a solid organ. This data is transmitted to a computer, which builds up cross-sectional images of the part of the body and displays it on the screen.

Sometimes a contrast dye called contrast media (CM) is used because it helps to demonstrate certain structures much more clearly on the screen. Depending on the clinical indication for an abdominal scan the patient may have to take oral CM. The CM appears white on the scan as it travels through the digestive system. If images of the lower abdomen/pelvis are required, such as the rectum, the patient may be given a rectal CM. If the exam aims to visualize the blood vessels the contrast media will be injected.

Preparation needed before a CT scan?

 It depends on which part of your body is to be scanned.  As a general rule, you will need to remove any metal objects from your body, such as jewellery, hair clips, etc. It is best not to wear clothes with metal zips, studs, etc. You may be asked not to eat or drink for a few hours before your scan - depending on the part of your body to be scanned. If you need an injection of contrast, it may also be necessary to stop certain medicines before the procedure.

 In some situations, depending on what the clinical indication for your exam is, one of the following may be needed. This helps to give better contrast between different organs and tissues on the scan pictures.

•For abdominal and pelvic scans you may be asked to take CM before the scan. This helps to show up the stomach and bowel more clearly.

•For some pelvic scans, you may have rectal CM

•For pelvic scans, women may be asked to insert a tampon into the vagina.

•Sometimes CM is injected into the bloodstream via a vein in your arm. The dye may give you a flushing feeling and an odd taste in your mouth, which soon goes.

The CT scan itself is painless. You cannot see or feel X-rays. You will be asked to stay as still as possible, as otherwise the scan pictures may be blurred. The examination may take between 5-30 minutes, depending on which part (or parts) of the body is being scanned.


CT scanning involves the use of X-rays. Women who are or might be pregnant must inform a member of staff in advance. The amount of radiation used is more than an ordinary X-ray of the chest or body and is equal to the natural radiation that we receive from the atmosphere over a period of approximately three years.

Many CT examinations involve you having a contrast medium injected into a vein to increase the quality of information obtained from the scan. The injection usually causes nothing more than a warm feeling passing around your body. However there is a very small risk of other possible reactions. These reactions are rare and can be treated immediately. In order to prevent any possible reactions you will be asked in detail for your medical history including history of diabetes or allergies, renal and cardiac problems and what medication you are on.


Even though there could be slight risks associated to having a CT scan by requesting this examination, your doctor would deem that the benefits outweigh the risks. Bear in mind there are greater risks from missing a serious disorder by not having your scan.