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CT scan

A CT (computed tomography) scanner is a large, ring-shaped machine with a hole in its centre which is used to diagnose and monitor a number of different health conditions. These include bleeding in the brain, heart problems, bone fractures or certain types of cancer. A CT scan is sometimes referred to as a CAT scan.

It’s shaped like a doughnut. An X-ray tube inside the ring creates a beam of X-rays. As you lie flat on the CT scanner table, the X-ray tube rotates around your body. The X-rays pass through your body and are picked up by detectors on the opposite side. Signals picked up by the detectors are then collected by a computer and built up into two-dimensional images or ‘slices’ of your body, or part of it. These two-dimensional images may then be stacked to create a three-dimensional image.

A diagram showing a man having a CT scan

The images from a CT scan appear in different shades of grey. Different tissues in your body (eg bones and muscles) absorb different amounts of radiation. Sometimes though, the grey shades are very similar. Your radiographer may use a dye called contrast medium to make the image clearer.

What is a CT scan used for?

A CT scan can be used to diagnose and monitor a number of different health conditions including certain types of cancer, bleeding in the brain, heart problems and bone fractures.

A CT scan is widely used in the diagnosis of acute abdominal (tummy) pain and stroke. It can also help surgeons plan surgery before an operation.

How do you prepare for a CT scan?

You may be in the hospital for up to a couple of hours, as you’ll need to prepare for your scan in advance. How you prepare for your CT scan will depend on which part of your body needs to be scanned and your current health condition. You may be asked to drink a dye or have this injected, not to eat in the hours leading up to your scan, remove accessories or change into a hospital gown. These are explained in more detail below.

CT scan usually takes about 10–20 minutes (depending on the type of scanner, this may be shorter). So you’ll usually have the scan and go home on the same day.

Your radiographer (a health professional trained to perform imaging procedures) will discuss with you what will happen before, during and after a CT scan. If you’re unsure about anything, don’t be afraid to ask. No question is too small. It’s important that you feel fully informed so you’re happy to go ahead with the scan.

Before having a CT scan, your radiographer will check if you have any of the following medical conditions:

Long-term health conditions, such as kidney problems, diabetes or high blood pressure can affect how well your kidneys work. Your radiographer may ask you to have an injection of a special dye called a contrast medium. The dye will make your tissues show up more clearly on the images. If your kidneys don’t work well, having an injection of the dye may damage them even more – so it’s important that you tell your radiographer. If you’re at a high risk of kidney damage, you should drink plenty of water before the CT scan. You may be given an IV drip before the scan to keep you hydrated.

The injection may make you feel warm for a minute or two. You may have a metallic taste in your mouth. You may also feel like you need to pass urine but this doesn’t last long.

If you have a CT scan of your abdomen (tummy), you may be asked to drink the contrast medium. Occasionally the dye is given as an enema, which is inserted into your rectum (back passage). This can make you feel constipated. You may be asked not to eat or drink anything for a specific number of hours before your scan.

Before your CT scan, you may be asked to take off your clothes and put on a hospital gown. This will depend on which part of your body needs to be scanned; there’ll usually be a private area where you can do this. You may also be asked to remove any jewellery, glasses, contact lenses, dentures, hair clips and hearing aids. Metal, including dental fillings, can affect the images created by the scanner.

What are the alternatives to a CT scan?

Depending on why you need a CT scan, you may be able to have an ultrasound or magnetic resonance imaging (MRI) scan instead. This will depend on the part of your body that’s being looked at, as well as your individual circumstances. A CT scan tends to give more complete pictures of your organs than an ultrasound and X-ray and is quicker than an MRI. Your doctor will discuss with you which type of scan is most suitable.

What happens during a CT scan?

A CT scan usually takes about 10–20 minutes, but may be shorter depending on the type of CT scanner.

You’ll be asked to lie on the CT scanner table, which slides into, or out of, the CT scanner ring. Your radiographer will control the positioning of the table. You may be asked to hold your breath or not to swallow at certain points during the scan. For the rest of the time, it's important to lie very still. The CT scanner will usually make some whirring noises when you’re inside it.

Your radiographer will operate the scanner from a control room behind a window. They’ll be able to see, hear and speak to you at all times. If you’re feeling claustrophobic during a CT scan, it’s important to tell them. Closing your eyes during the scan may help to make you more comfortable and less nervous.

What to expect afterwards

Once the scan is finished, the CT scanner table will move back out of the scanner ring. The radiographer will come back into the room and lower the table so you can get off the table. You’ll usually be able to go home when you feel ready.

Before you go home, ask your radiographer when you can expect to get your results. It can take several days, or even a week or two, for your results to come through.

Your results will be reviewed by a radiologist (a doctor who specialises in using imaging methods to diagnose medical conditions). Usually, the radiologist will send a report to your GP or the doctor who referred you for the CT scan.

What are the risks?

As with every test, CT scans have some risks. But the benefits of having a CT scan usually outweigh these risks. Your radiographer will explain the risks and benefits to you.

A CT scan exposes you to some radiation. But you’ll only have a CT scan if it’s absolutely necessary. You're exposed to natural background radiation in the atmosphere all the time. Different radiology tests and scans expose you to different doses of radiation. A CT scan gives out more radiation than other types of X-ray imaging, such as a plain X-ray. But doctors and radiographers are trained to keep your exposure to a minimum.

With some CT scans, your radiographer will use a dye (contrast medium). Allergic reactions to the dye are rare. But if you have any itching or difficulty breathing, tell your radiographer straightaway. Very occasionally, the dye can affect your kidneys. See our preparation section for more information.

If you're pregnant, you’ll usually be advised not to have a CT scan. This is because there’s a risk the radiation could harm your unborn baby. You may be advised to have a CT scan if it’s an emergency or considered necessary by your doctor. If you're pregnant or think you may be, let your doctor or radiographer know.


  • Other helpful websites Other helpful websites

    Further information


    • Computed tomography. The MSD Manuals., last full review/revision January 2015
    • CT Scan. Cancer Research UK., updated April 2015
    • Radiology. Oxford Handbook of Clinical Medicine (online). 9th ed. Oxford Medicine Online., published online January 2014
    • Computerised tomography (CT) scans. PatientPlus., last checked July 2015
    • Risks of medical radiation. The MSD Manuals., last full review/revision January 2015
    • Radiographic contrast agents and contrast reactions. The MSD Manuals., last full review/revision January 2015
    • Diabetic Nephropathy. PatientPlus., last checked May 2016
    • Medical radiation: uses, doses, measurements and safety advice. Public Health England., last updated January 2016
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    Reviewed by Laura Blanks, Specialist Health Editor, Bupa Health Content Team, August 2016

    Expert reviewer, Dr Daniel Boxer, Consultant Radiologist

    Next review due August 2019

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