CT stands for computerised tomography. A CT scan, also known as a CAT scan, is a specialised X-ray test. It can give quite clear pictures of the inside of your body. In particular, it can give good pictures of soft tissues of the body which do not show on ordinary X-ray pictures. Note: the information below is a general guide only. The arrangements, and the way tests are performed, may vary between different hospitals. Always follow the instructions given by your doctor or local hospital.
How is a CT scan done?
CT stands for computerised tomography, and CAT for computerised axial tomography (sometimes the word computed is used instead of computerised). The CT scanner looks like a giant thick ring. Within the wall of the scanner there is an X-ray source. Opposite the X-ray source, on the other side of the ring, are X-ray detectors. You lie on a couch which slides into the centre of the ring until the part of the body to be scanned is within the ring. The X-ray machine within the ring rotates around your body. As it rotates around, the X-ray machine emits thin beams of X-rays through your body, which are detected by the X-ray detectors.
By NithinRao (Own work) [Public domain], via Wikimedia Commons.
The detectors detect the strength of the X-ray beam that has passed through your body. The more dense the tissue, the less X-rays pass through. The X-ray detectors feed this information into a computer. Different types of tissue with different densities show up as a picture on the computer monitor, in different colours or shades of grey. So, in effect, a picture is created by the computer of a slice (cross-section) of a thin section of your body.
As the couch moves slowly through the ring the X-ray beam passes through the next section of your body. So, several cross-sectional pictures of the part of your body being investigated are made by the computer. Newer scanners can even produce 3-dimensional pictures from the data received from the various slices of the part of the body being scanned.
What is a CT scan used for?
A CT scan can be done on any section of the head or body. It can give clear pictures of bones. It also gives clear pictures of soft tissues which an ordinary X-ray test cannot show, such as muscles, organs, large blood vessels, the brain and nerves. The most commonly performed CT scan is of the brain - to determine the cause of a stroke, or to assess serious head injuries. Other uses of a CT scan include:
- To detect abnormalities in the body, such as tumours, abscesses, abnormal blood vessels, etc, when they are suspected by symptoms or other tests.
- To give a surgeon a clear picture of an area of your body before certain types of surgery.
- To pinpoint the exact site of tumours prior to radiotherapy.
- To help doctors find the right place to take tissue samples (biopsies).
What preparation do I need to do before a CT scan?
Usually very little. It depends on which part of your body is to be scanned. You will be given instructions by the CT department appropriate for the scan to be done. As a general rule, you will need to remove any metal objects from your body, such as jewellery, hair clips, etc. It is best not to wear clothes with metal zips, studs, etc. You may be asked not to eat or drink for a few hours before your scan - depending on the part of your body to be scanned. If you need an injection of contrast, as described below, it may be necessary to stop certain medicines before the procedure. This may apply to people taking metformin, a medicine used to treat diabetes. If you are taking this medication your doctor should give you instructions about what to do.
In some situations, depending on what part of the body is being scanned, one of the following may be needed. These aim to block a certain amount of X-ray going through various tissues. This helps to give better contrast between different organs and tissues on the scan pictures.
- For abdominal and pelvic scans you may be asked to have a special drink before the scan. This helps to show up the stomach and bowel more clearly.
- For pelvic scans, some fluid may be put into your back passage (rectum).
- For pelvic scans, women may be asked to insert a tampon into the vagina.
- Sometimes a dye (contrast agent) is injected into the bloodstream via a vein in your arm. The dye may give you a flushing feeling and an odd taste in your mouth, which soon goes.
The CT scan itself is painless. You cannot see or feel X-rays. You will be asked to stay as still as possible, as otherwise the scan pictures may be blurred. Conventional CT scans can take between 5-30 minutes, depending on which part (or parts) of the body is being scanned. More modern CT scans (helical CT scans) take less than a minute and also use less radiation.
Can anybody be with me during the scan?
Because the scan uses X-rays, other people should not be in the same room. The operator controls the movement of the couch and scanner from behind a screen or in a separate control room. (This protects them from repeated exposure to X-rays.) However, you can talk to them, usually via an intercom, and you will be observed at all times on a monitor.
Some people feel a little anxious or claustrophobic in the scanner room when they are on their own. A mild sedative may be offered if you are particularly anxious.
What can I expect after the scan?
You can return to your normal activities as soon as the scan is over. However, if you had a sedative for the scan, you will need someone to accompany you home. You will not be able to drive until the effect of the sedative wears off.
The pictures from the scan are studied by an X-ray doctor (radiologist) who sends a report to the doctor who requested the scan.
Are there any possible complications?
Complications are rare. Rarely, some people have an allergic reaction to the dye (contrast agent) which is sometimes used. This can be treated immediately. Very rarely, the dye may cause some kidney damage, most commonly in people already known to have kidney problems.
If possible, pregnant women should not have a CT scan, as there is a small risk that X-rays may cause an abnormality to the unborn child.
Risks of X-ray radiation used in CT scans
CT scans use X-rays, which are a type of radiation. Exposure to large doses of radiation is linked to developing cancer or leukaemia - often many years later.
The dose of X-ray radiation needed for a CT scan is much more than for a single X-ray picture but is still generally quite a low dose. The risk of harm from the dose of radiation used in CT scanning is thought to be very small but it is not totally without risk. As a rule, the higher the dose of radiation, the greater the risk. So, for example, the larger the part of the body scanned, the greater the radiation dose. And, repeat CT scans over time cause an overall increase of dose. Also, the younger you are when you have a CT scan, the greater the lifetime risk of developing cancer or leukaemia.
Various studies have aimed to estimate the risk of developing cancer or leukaemia following a CT scan (see references below). In general, the risk is small. In many situations, the benefit of a CT scan greatly outweighs the risk. However, as the same study concludes ... "although clinical benefits should outweigh the small absolute risks, radiation doses from CT scans ought to be kept as low as possible and alternative procedures, which do not involve ionising radiation, should be considered if appropriate".
Because of the small risk, the Committee on Medical Aspects of Radiation in the Environment (COMARE) has recommended that routine whole-body CT scans should not be offered to people without symptoms as part of 'health checks'. They also offer various other recommendations on the use of CT scans (see references below).
Dr Tim Kenny
Dr Colin Tidy
Dr Adrian Bonsall