A computerised tomography (CT) scan uses X-rays and a computer to create detailed images of the inside of the body.
CT scans are also sometimes known as CAT scans, which stands for computerised axial tomography.
During a CT scan, you'll usually lie on your back on a flat bed. The CT scanner consists of an X-ray tube that rotates around your body. You'll usually be moved continuously through this rotating beam.
The X-rays will be received by a detector on the opposite side of your body and an image of the scan will be produced by a computer.
Unlike an MRI scan, where you're placed inside a tunnel, you shouldn't feel claustrophobic.
The images produced by a CT scan are called tomograms and are more detailed than standard X-rays. A CT scan can produce images of structures inside the body, including the internal organs, blood vessels, bones and tumours.
The scan is painless and will usually take between five and 10 minutes depending on the part of your body being scanned.
Read more about how a CT scan is performed.
When CT scans are used
CT scans can be used to diagnose and monitor a variety of different health conditions, including brain tumours, certain bone conditions, and injuries to internal organs such as the kidneys, liver or spleen. They're also now being used to look at the heart.
They're also often used to look inside the body before another procedure takes place, such as radiotherapy treatment or a biopsy (where a small sample of tissue is taken so that it can be examined under a microscope).
Read more about when you might need a CT scan.
CT scans are usually carried out on an outpatient basis, which means you'll be able to go home on the same day as the procedure.
Your scan results won't be available immediately. A computer will need to process the information from your scan, which will then be analysed by a radiologist (a specialist in interpreting images of the body).
After analysing the images, the radiologist will write a report and send it to your doctor. This usually takes a few weeks.
CT scans are only used when the doctor responsible for your care decides there's a clear medical benefit.
Although CT scans are generally safe, they do expose you to slightly more radiation than other types of imaging tests. The amount of radiation you're exposed to can vary depending on the type of scan you have.
In most cases, the benefits outweigh any potential risks because a CT scan can provide your doctor with much clearer images than those produced by a normal X-ray.
However, CT scans aren't routinely recommended for pregnant women because there's a risk that the X-rays could harm the unborn baby.
Children are also more at risk of developing a build-up of radiation than adults. A CT scan will therefore only be recommended if a child has a serious condition that puts them at greater risk.
Read more about the risks of CT scans.