CT scan

Reviewed by Dr Sarah Burnett, consultant radiologist and Professor Alastair J Munro, professor of radiation oncology and Dr John Pillinger, GP

What is a CT scanner?
A CT (computerised tomography) scanner is a special kind of X-ray machine. Instead of sending out a single X-ray through your body as with ordinary X-rays, several beams are sent simultaneously from different angles.

How does a CT scanner work?
The X-rays from the beams are detected after they have passed through the body and their strength is measured.

Beams that have passed through less dense tissue such as the lungs will be stronger, whereas beams that have passed through denser tissue such as bone will be weaker.

A computer can use this information to work out the relative density of the tissues examined. Each set of measurements made by the scanner is, in effect, a cross-section through the body.

The computer processes the results, displaying them as a two-dimensional picture shown on a monitor. The technique of CT scanning was developed by the British inventor Sir Godfrey Hounsfield, who was awarded the Nobel Prize for his work.

What are CT scans used for?
CT scans are far more detailed than ordinary X-rays. The information from the two-dimensional computer images can be reconstructed to produce three-dimensional images by some modern CT scanners. They can be used to produce virtual images that show what a surgeon would see during an operation.

CT scans have already allowed doctors to inspect the inside of the body without having to operate or perform unpleasant examinations. CT scanning has also proven invaluable in pinpointing tumours and planning treatment with radiotherapy.

What is the CT scanner used for?
The CT scanner was originally designed to take pictures of the brain. Now it is much more advanced and is used for taking pictures of virtually any part of the body.

The scanner is particularly good at testing for bleeding in the brain, for aneurysms (when the wall of an artery swells up), brain tumours and brain damage. It can also find tumours and abscesses throughout the body and is used to assess types of lung disease.

In addition, the CT scanner is used to look at internal injuries such as a torn kidney, spleen or liver; or bony injury, particularly in the spine. CT scanning can also be used to guide biopsies and therapeutic pain procedures.

How is a CT scan prepared for?
If the patient is receiving an abdomen scan, for example, they will be asked not to eat for six hours before the test. They will be given a drink containing gastrografin, an aniseed flavoured X-ray dye, 45 minutes before the procedure. This makes the intestines easier to see on the pictures.

Sometimes a liquid X-ray dye is injected into the veins during the test. This also makes it easier to see the organs, blood vessels or, for example, a tumour.

The injection might be a little uncomfortable, and some people also experience a feeling of warmth in their arm.

How is a CT scan carried out?
The scanner looks like a large doughnut. During the scan the patient lies on a bed, with the body part under examination placed in the round tunnel or opening of the scanner. The bed then moves slowly backwards and forwards to allow the scanner to take pictures of the body, although it does not touch the patient. The length of the test depends on the number of pictures and the different angles taken.

Does a CT scan hurt?
The examination does not hurt but some people find it uncomfortable to lie in the tunnel. As there is little room inside the tunnel, people who suffer from severe claustrophobia sometimes have problems with CT scans.

Let the doctors and radiographers know if this might be a problem. Other people get slightly nervous because of the whirring noise the machine makes while working.

Is a CT scan dangerous?
Far more X-rays are involved in a CT scan than in ordinary X-rays, so doctors do not recommend CT scans without a good medical reason. Some patients may experience side effects due to allergic reactions to the liquid dye injected into the veins.

In very rare cases, this dye has been known to damage already weakened kidneys. It is important to let the X-ray doctors or technicians know if you have any allergies, asthma or kidney trouble, prior to having the X-ray dye injected.

How is a CT scan read?
A CT scan can give the doctor a much clearer picture of the inside of the body than an ordinary X-ray. For example, different types of tissue such as bone, muscle and fatty tissue are easy to see on a CT scan. When looking at the abdomen, the scan shows various organs such as the pancreas, spleen and liver.

When it is necessary to look at the brain, the areas containing liquid - the ventricles - are also clearly defined. Very small shadows on the lungs can also be detected using CT and there are now studies looking into using it as a screening test for lung cancer.

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