The Leeds Teaching Hospitals NHS Trust

Computerised Tomography (CT)

x ray scanner

A computerised tomography (CT) scan uses X-rays and a computer to create detailed images of the inside of the body.

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 ten and twenty minutes depending on the part of your body being scanned.

How it is performed

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.

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 20 minutes. You should be able to go home soon after the scan has been completed.

Contrast medium

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, or can be injected into your bloodstream.

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 radiographer 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.


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.

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.


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.