CT scans

How does a CT work?

A CT scanner is a type of X-ray machine. For a normal X-ray, for example if you break a bone, an x-ray tube will send x-rays in one direction, into the area doctors want to look at. Some of the x-rays pass right through your body. These are picked up on the other side and turned into a picture.

A CT scanner is shaped like a ring. It contains an x-ray tube and a detector. These are opposite each other. When the machine is working, the x-ray tube and the detector will spin around the ring. The x-ray tube will send x-rays into your body from all angles and those that pass through will be picked up by the detector. The detector then sends signals to a computer which makes a picture.

Each picture looks like a thin slice through your belly. The CT scanner will make lots of these pictures, up and down your belly. Then put them together to make a 3D picture of the whole area.

Dye (contrast medium)

A dye might also be used during a CT scan. This helps the scan to get clearer pictures of a particular part of your body. It is good for looking at blood vessels. You might hear this dye called a “contrast medium”.

You might be given the dye as a drink. Or as an injection into a tube (canula) in your hand or arm. After your scan the dye will come out in your pee.

The dye can sometimes have side effects. These are usually mild and do not last very long. They include:

• a rash
• a headache
• feeling dizzy
• feeling or being sick

The dye can sometimes cause an allergic reaction. This is very rare. But let your medical team know before your scan if you have a history of allergic reactions.

Back to top

Before the scan

Most people can have a CT scan. But in some cases you might need extra checks first.

If your doctor says you need a CT scan always tell them if:

On the day of the scan

This is general advice, but sometimes you might be asked to do something different. Always check your appointment letter or message for any special instructions. If you are not sure, contact the scanning department. The contact information should be on your appointment letter.

Scan appointments do not always run on time. It is a good idea to take a bottle of water with you and a book, phone, or something else to pass the time. You could also take a snack for after the scan.

You can read our advice about coping with the stress of appointments here. It includes some simple techniques to help you cope with anxiety in the waiting room.

Back to top

Having a CT scan

Preparation

CT scans are usually done by a scanning specialist called a radiographer.

If you are going to have a dye injected a small tube called a canula will be put into one of your veins. This is usually in the back of your hand.

The radiographer will explain the test to you and make sure you are as comfortable as possible on the bed.

You cannot have someone else in the room with you because of the X-ray radiation. So the person doing the scan will go to a separate room to operate the scanner. But they will be able to see you all the time through a window or on a screen. They will also be able to hear you, talk to you, and give you instructions.

If you have any questions or worries, you can talk to the person doing the scan at any time.

During the scan

CT scans are usually very quick. Each scan takes about 10 – 20 seconds.

If you are having a dye injected, you might have more than one scan. For example, a scan before the dye and then more scans to get pictures of the dye moving through your blood vessels. This should still only take a few minutes.

You might feel the dye going in, this can be cold or warm. Some people get a warm feeling in their bladder. This can make it feel like you have wet yourself, but it is just the dye moving through your body.

The CT scanner is not very loud so you will not need to wear headphones. But you might hear a whirring sound while the pictures are being taken.

After the scan

When the scan has finished the radiographer will come back into the room and you will be able to get off the bed.

If you had dye injected, you will have the canula removed. You will be asked to wait in the scanning department for 15 to 20 minutes so that the medical team can be sure you will not have a reaction. Allergic reactions to the dye are very rare so this is just to be on the safe side.

After your scan you can go back to all your normal activities and eat and drink as normal.

Back to top

Getting your results

If you are having other tests as well, your medical team will usually wait until they have all the results so that they can look at them all together.

If you do not have a follow up appointment booked, contact your liver medical team to find out how and when you will get your results.

Your results might give your medical team enough information to make a diagnosis. If you have been diagnosed with a liver condition you can find out more information about it here. 

Or you might need to have more tests such as an MRI scan or a liver biopsy.

Find out more about other tests for liver disease  

Back to top

Special thanks

We would like to thank the patients and doctors who helped up to create this information. Including Dr Christopher Clarke, Consultant Abdominal Radiologist, Nottingham University Hospitals NHS Trust.

Published: May 2025