Unlike conventional flat plate X-rays, the CT scanner uses a rotating X-ray tube. The patient is moved through this moving beam which collects a series of images called tomograms.
The computerised images are more detailed than normal X-rays and help show clear pictures of organs inside the body, blood vessels, bones and tumours.
What to expect during a CT scan
A CT scan is usually carried out as a day patient procedure. After the scan, you can go back to your normal daily activities.
Before the scan, you will be asked to remove anything containing metal. This includes watches, jewellery, piercings, dentures, hearing aids and some wigs.
You will be asked about any existing health conditions you have, medication you take and about any allergies before the scan. For brain and abdomen scans, a special contrast needs to be used to give more detailed images. This may be a drink, enema or injection. This contrast can cause allergic reactions in some people.
The radiographer will need to know if there's any chance a woman is pregnant, as X-rays could harm an unborn child
You will probably have to wear a hospital gown.
For some CT scans, an injection of contrast dye is needed first to help show details more clearly.
A radiographer will ask a person to lie flat and still on a bed that then goes inside the scanner. You may go in head or feet first.
The radiographer will operate the scanner from a separate control room and will talk to you through an intercom. The scan will not cause any pain and typically takes around 10 to 30 minutes.
The duration depends on which part of a person is being scanned.
You are unlikely to get the results of the scan straight away, as the images will need to be assessed by a consultant.
Uses of CT scans
CT scans are often requested by doctors to look at:
As well as diagnosing new conditions, CT scans may be used by doctors to check inside the body before another planned procedure. This might include radiotherapy treatment or finding the right place to take a tissue sample or biopsy from.
Who shouldn't have a CT scan
Although the X-ray dose from diagnostic X-rays and CT scans are small, doctors are wary about putting people through unnecessary scans as radiation can cause cancer.
As well as being unsuitable for pregnant women, CT scans are not usually recommended for children who are at a greater risk of the radiation dose building-up.
Because the CT scanners use a doughnut shaped machine, there's less of a risk than with MRI scans for people who are claustrophobic - anxiety over being in enclosed spaces.
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