Computer-assisted imaging using a succession of X-ray beams to produce cross-sectional images through the body
Computerized tomography (CT) scanning uses X-rays in conjunction with a computer. A series of X-rays is passed through the body at slightly different angles to produce highly detailed cross-sectional images (“slices”) of the body, called tomograms. CT scanning obtains detailed information about organs painlessly and can replace exploratory surgery in many cases.
How does it work?
The CT scanner consists of an X-ray source and an X-ray detector, both of which rotate during the procedure so that they remain opposite each other. CT scanning uses X-rays in a different way from an ordinary X-ray machine to give a higher-quality image. Ordinary X-rays show only a few levels of density, while the detector within a CT scanner can see hundreds of different levels of density, including fibrous tissue in solid organs such as the liver. Instead of sending one beam of radiation through your body, the X-ray source inside a CT scanner emits a succession of narrow beams as it moves through an arc. The X-ray detector then picks up the radiation after it has passed through various body tissues (see Having a CT scan). After each arc is completed, the bed is moved forwards a small distance.
The information from the detector is then sent to the computer, which builds up cross-sectional images of the body and displays them on a monitor. These images can be stored as computer files or printed on conventional X-ray film. More sophisticated computers can produce three-dimensional images from standard CT data.
Current CT scanners use the spiral (or helical) technique, in which the scanner rotates around you as the bed moves continuously through the scanner so that the X-ray beams follow a spiral course. This type of CT scanning produces three-dimensional images and reduces the time taken to complete the scans.
What is it used for?
The CT scans performed most often are those of the head and trunk. CT scanning of the head is commonly used to investigate the brain following a stroke or head injury, or if a brain tumour is suspected. Chest CT scans are used to detect lung disease and secondary tumours as well as abnormalities of the blood vessels supplying the heart and lungs. Abdominal CT scans are used to detect tumours, to investigate internal bleeding due to trauma, and to diagnose disorders in which organs are enlarged or inflamed, such as polycystic kidney disease. CT scans can also be used to guide biopsy procedures.
CT scanning is able to produce much clearer images of bone than MRI. Blood vessels can also be imaged. These images may be enhanced by using a contrast medium (a substance that makes a fluid-filled or hollow structure visible on the image).
What are the risks?
Imaging techniques that use radiation may damage body cells, which may increase the risk of cancer in the long term. The dose for a CT procedure depends on the number of cross sections imaged. Scanning time is reduced for spiral CT scans, but the dose is the same as in a normal CT scan. Radiation exposure during CT scans is generally quite low but nevertheless may be significant.
From the 2010 revision of the Complete Home Medical Guide © Dorling Kindersley Limited.
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