If you have a computerised tomography (CT) scan, you'll be exposed to radiation in the form of X-rays. The amount of radiation used can vary.

Radiation is measured in units called millisieverts (mSv). Different types of CT scan use different amounts of radiation:

• CT scan of the head – 1.4 mSv, which corresponds to seven-and-a-half months' worth of background radiation
• CT scan of the chest – 6.6 mSv, or three years' worth of background radiation 
• CT scan of the whole body – 10 mSv, which corresponds to four-and-a-half years' worth of background radiation

Benefits versus risks

The benefits of having a CT scan to help diagnose a medical condition, or to check the symptoms of an existing condition, will usually greatly outweigh any potential risk. CT scans are quick and accurate, and often eliminate the need for invasive surgery.

However, if you don't have any symptoms, the benefits of having a CT scan may not outweigh the risks, particularly if it leads to further unnecessary testing and added anxiety.

The benefits and risks should therefore always be weighed up before deciding to have a CT scan. It's recommended that you only have one following a medical referral.

Read more about the risks of radiation exposure.

Pregnant women and children

CT scans aren't usually recommended for pregnant women because there's a small risk that the X-rays may harm the unborn child. Before having a scan, tell your doctor if there's a chance you may be pregnant.

Children are at greater risk from a build-up of radiation than adults. They should only have a CT scan if it's justified by a serious condition that risks their health.