Before having a computerised tomography (CT) scan, you'll be asked about any existing health conditions, whether you are taking any medication, and if you have any allergies.

This is to make sure that there's no risk of an adverse reaction during the scan.

Women of childbearing age will also be asked if they're pregnant. CT scans aren't recommended for pregnant women unless there's an urgent medical reason, as there's a small chance that the X-rays could harm the unborn child.

Tell the radiographer if you feel anxious or claustrophobic about having a CT scan. A radiographer is a healthcare professional trained to carry out X-rays and other types of scans. They'll be able to give you advice to help you feel calm and, if necessary, arrange for you to have a sedative (medication to help you relax).

The scan

Before the scan, you may be asked to remove your clothing and put on a gown. You'll also be asked to remove any jewellery, as metal interferes with the scanning equipment. If you're having a head scan, you may also be asked to remove dentures, hair clips and hearing aids.

The CT scanner is a large circular machine. You'll be asked to lie on your back on a motorised bed that can be moved in and out of the scanner. The radiographer will position the bed so that the part of your body being investigated is lined up with the scanner.

The radiographer will operate the scanner from an adjoining room. While the scan is taking place, you'll be able to hear and speak to them through an intercom.

While each scan is being taken, you'll need to lie very still and breathe normally. This ensures that the scan images aren't blurred. You may be asked to breathe in, breathe out, or hold your breath at certain points.

The X-ray unit inside the ring will rotate around you. Each time it goes round it creates a new X-ray scan. The bed will move forward slightly after each scan is completed.

Depending on the area of your body being investigated, a CT scan may last up to 10 minutes. You should be able to go home soon after the scan has been completed.

Contrast medium

For some CT scans, such as those investigating the brain or abdomen, you may be given contrast medium beforehand. This is a liquid that contains a dye that shows up clearly on the images of certain tissues or blood vessels. It helps distinguish blood vessels from other structures in your body.

Contrast medium can be given in different ways, depending on the part of your body being scanned. It can be swallowed in the form of a drink, given as an enema in your back passage, or can be injected into your bloodstream (intravenously).

If your kidney function is poor, contrast medium isn't usually given intravenously as it can depress kidney function further.

In rare cases, contrast medium can cause an allergic reaction. Tell the radiologist if you have had an allergic reaction to iodine or contrast medium in the past, or if you have any other allergies.

Contrast medium is harmless and will pass out of your body in your urine.

Your results

The results of your CT scan won't be available immediately. A computer will be used to process all the information, which will then be analysed by a radiologist (a doctor who specialises in interpreting images of the body).

The radiologist will write a report and send it to your doctor. Before leaving hospital, check when you should expect your results.