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

CT scans are also sometimes known as CAT scans, which stands for computerised axial tomography.

During a CT scan, you will usually lie on your back on a flat bed. The CT scanner consists of an X-ray tube that rotates around your body. You will usually be moved continuously through this rotating beam. The rays will be analysed by a detector on the opposite side of your body.

Unlike an MRI scan, where you are placed inside a tunnel, you should not 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 10 to 30 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 conditons and injuries to internal organs such as the kidneys, liver or spleen.

They are also often used to look inside the body before another procedure takes place, such as radiotherapy treatment or a biopsy (where a small tissue sample is taken so that it can be examined under a microscope).

Read more about when you might need a CT scan.

Your results

CT scans are usually carried out on an outpatient basis, which means that you will be able to go home on the same day as the procedure.

The results of your scan will not 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 specialist or GP. This usually takes a few weeks.

Safety

CT scans are safe procedures and, in most cases, the benefits of having a scan outweigh any potential risks.

A CT scan does expose you to radiation. The amount is small and will not cause you any harm. However, CT scans are not recommended for pregnant women because there is a small risk that the X-rays that are used could harm the unborn baby.

Children are also more at risk than adults from a build-up of radiation. Therefore, a CT scan will only be recommended if a child has a serious condition that puts them at greater risk.

Read more about the risks of CT scans.

CT (computerised tomography) scans provide important information that doctors can use to help diagnose medical conditions.

The results of a CT scan can confirm or rule out a suspected diagnosis and they can also sometimes identify unsuspected conditions.

Unlike other imaging techniques, such as X-rays, CT scans can give detailed images of many types of tissue, including bone, lung tissue, soft tissue and blood vessels.

Types of CT scan that can be used to investigate particular areas of the body include:

• head scans - can be used to check for suspected brain tumours and bleeding or swelling of the arteries; head scans are also useful for investigating the brain following a stroke (when the blood supply to part of the brain is cut off)
• abdominal scans - can be used to detect tumours and diagnose conditions that cause internal organs, such as the liver, kidneys, pancreas, intestines or lungs, to become enlarged or inflamed
• vascular scans - can be used to assess conditions that affect the blood flow to different parts of the body
• bone scans - can be used to assess bone injuries and disease, particularly in the spine

CT scans are often used after serious accidents to look for internal injuries, such as tears in the spleen, kidneys or liver.

They are also sometimes used to prepare for further tests and treatments. For example, as CT scans can identify both normal and abnormal tissue, they can be useful when planning radiotherapy treatment.

CT scanning can also act as a guide during a needle biopsy (where a sample of tissue is taken so that it can be examined more closely).

CT scan screening

In recent years, there has been concern that, in some cases,  CT scans have been used unnecessarily. Some private medical organisations offer CT screening as a way of detecting conditions in patients who do not have symptoms or any significant risk factors for a disease. This can be both expensive and put you at unnecessary risk.

CT scans are not recommended as a way of giving you peace of mind if you do not have any symptoms. A scan may be recommended if you have symptoms caused by an injury or illness that need to be investigated. However, you should only have a CT scan following a medical referral.

The Department of Health asked a group of experts to investigate the risk of CT scans in healthy people, such as those who decide to pay for a CT scan as part of a private health assessment. In 2007, the Committee on Medical Aspects of Radiation in the Environment (COMARE) produced a report outlining a number of recommendations on the use of CT scans for screening.

The report recommeded that all radiation exposure and equipment used for exposure should be tightly controlled by a number of regulations (both in the NHS and commercially), and that all private sector organisations offering CT screening should provide comprehensive information about a person’s eligibility for screening and the dosage of radiation used.

For full details of all the recommendations, you can read the COMARE report (PDF, 549 kb).

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

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

Women of childbearing age will also be asked if they are pregnant. CT scans are not recommended for pregnant women (unless there is an urgent medical reason) because there is a small chance that the X-rays could harm the unborn child.

You should tell the radiographer if you feel anxious or claustrophobic about having a CT scan. A radiographer is a healthcare professional who is trained to carry out X-rays and other types of scans. They will be able to give you advice to help you to 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 to put on a gown. You will also be asked to remove items of jewellery because metal interferes with the scanning equipment. If you are having a head scan, you may also be asked to remove contact lenses, dentures, hair clips and hearing aids.

The CT scanner is a large, circular-shaped machine. You will be asked to lie on your back on a motorised bed that can be moved in and out of the CT 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 will be able to hear and speak to them through an intercom.

While each scan is being taken, you will need to lie very still and breathe normally. This ensures that the scan images are not blurred. You may be asked to breathe in, breathe out, or to 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. After each scan is completed, the bed will move forward a small distance.

Depending on the area of your body being investigated, a CT scan may last up to 30 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 your brain or abdomen, you may be given contrast medium beforehand. This is liquid that contains dye that shows up clearly on the images of certain tissues or blood vessels. Contrast medium helps distinguish blood vessels from other structures in your body.

There are a number of ways that contrast medium can be given, 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 it can be injected into your bloodstream (given intravenously).

If your kidney function is poor, contrast medium may not be given intravenously because it can depress kidney function further.

In rare cases, contrast medium can cause an allergic reaction. You should 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 it will pass out of your body in your urine.

Your results

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

The radiologist will write a report and send it to your specialist or GP. Before leaving hospital you should ask when to expect your results. It will usually take a couple of weeks.

If you have a computerised tomography (CT) scan, you will be exposed to radiation in the form of X-rays. The amount of radiation that is used is very small.

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

• CT scan of the head - 1.4 mSv
• CT scan of the chest - 6.6 mSv
• CT scan of the whole body - 10 mSv

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 outweigh any potential risk. In these cases, CT scans are quick and accurate and often eliminate the need for invasive surgery.

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

Therefore, the benefits and risks should always be weighed up before deciding to have a CT scan. It is recommended that you only have a CT scan following a medical referral.

Read more about the risks of radiation exposure.

Pregnant women and children

CT scans are not recommended for pregnant women because there is a small risk that the X-rays may harm the unborn child. Before having a scan, you should tell your doctor if there is a chance that you may be pregnant.

Children are at greater risk from a build-up of radiation than adults and should only have a CT scan if it is justified by a serious condition that puts them at an increased risk.