What is a CT scan (CAT scan)?

A CT scan is a test that uses x-rays and a computer to create detailed pictures of the inside of your body. It takes pictures from different angles. The computer puts them together to make a 3 dimensional (3D) image.  

CT (or CAT) stands for computed (axial) tomography.

You usually have a CT scan in the x-ray or radiology department as an outpatient. A ​radiographer​  operates the scanner and looks after you while you have your scan. The whole appointment can take about an hour and a half, depending on which part of your body they are scanning. 

You might have a CT scan combined with another test such as a PET scan (PET-CT scan). Or you might have a different type of CT scan depending on what information your doctor wants to know. For example, a CT urogram is a different type of CT scan. This type looks at the ​urinary system​ .

Read more about these scans on our A-Z list of tests and scans

Preparation for a CT scan

Some CT scans need special preparation beforehand. You should follow the information given by your healthcare team or in your appointment letter, as some hospitals may have slightly different instructions.

For most scans, you have a drink or an injection of ​contrast medium​ or both. This is a dye that shows up body tissues more clearly on the scan.

You may be asked to stop eating and drinking for 2 to 4 hours before your scan if you have a contrast medium. Your scan team will tell you exactly how long you need to do this for. Let them know if not eating is a problem for you.

You have the injection through a small thin tube called a cannula into your arm. The cannula stays in place until after your scan. This is in case you have any problems, such as an allergic reaction after having the injection.

You should take your regular medication unless you are told otherwise.

It is important that you call the scanning staff before your scan if you are pregnant or think you might be pregnant. 

You might have blood tests before your CT scan. This is to make sure your kidneys are working well. There is a small risk that the contrast medium can affect your kidneys.

CT scans of the tummy (abdomen)

If you are having a CT scan of your abdomen, you might need to:

• drink a liquid contrast medium some time before the scan

• drink more of the liquid contrast or water in the x-ray department

• stop eating or drinking a few hours before your scan

You usually have the contrast medium by injection and also as a drink. This helps to show up the ​digestive system​  more clearly in the scan.

CT scans of the chest

You may need to stop eating and drinking beforehand. You might have an injection of the contrast medium during the scan. This is to help show up the tissues close to the area containing cancer. For example, if your doctor wants to know if the cancer is affecting your blood vessels. It may help to show whether cancer can be removed with surgery or not.

Pelvic CT scans

The pelvis is the lower cavity of the body in the area between the hip bones. It contains the pelvic organs, including the bladder, lower bowel and reproductive organs. If you are having a CT scan of the pelvis, you might need to:

• stop eating or drinking for some time before the scan

• have an injection of contrast medium

Occasionally, for a ​rectal​ scan, you might have an enema of contrast medium. This helps the outline of the bowel show up more on the scan. Your radiographer will give you more information about this. 

CT scans of the head

For some brain scans, you might have an injection of the contrast medium dye. This is to make the scan clearer.

CT colonography

You might have a very detailed scan of the bowel called a CT colonography or virtual colonoscopy. 

For this test you need an empty bowel. You'll have strong laxatives and follow a special diet about 2 days before the test. You might also have a special liquid to drink called gastrorgrafin. It's a type of dye but also acts like a laxative. Your doctor or nurse will tell you more about this. 

Find out about having a CT colonography

Getting your results

You should get your CT scan results within 1 or 2 weeks.

Ask your doctor, radiographer or nurse how long it will take to get them. Contact the doctor who arranged the test if you haven’t heard anything after a couple of weeks.

Waiting for test results can be a worrying time. You might have contact details for a specialist nurse. You can get in touch with them for information and support if you need to. It may help to talk to a close friend or relative about how you feel.