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Computerised tomography scanning

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Alison Coutts, MSc, RGN.

Lecturer in applied biological sciences, St Bartholomew school of nursing and Midwifery, City University, London.

Computerised tomography

Alison Coutts, MSc, RGN, is lecturer in applied biological sciences, St Bartholomew school of nursing and Midwifery, City University, London.

What is CT?

Computerised tomography (CT) sometimes used to be called computerised axial tomography (CAT scan). It uses X-rays to generate an image (CT scan) of parts of the body. X-rays are, of course, also used in traditional plain films, but CTscans are more precise, and have a number of other advantages. They have been used since the 1970s.

X-rays are forms of electro-magnetic radiation of extremely short wave-length. Visible light is also electromagnetic radiation, but of longer wavelength, and radiation used for radiotherapy is of even shorter wavelength. X-rays are produced using an electric current to accelerate electrons across a vacuum onto a tungsten anode. When these rays encounter tissue, they can be absorbed, scattered (attenuated) or pass straight through. The more dense the tissue, the more likely the rays are to be attenuated. In plain films, those X-rays that pass through the material land on photographic plate, and cause it to become darker. CT scanners use electronic sensors (photomultiplier tubes) to measure the amount of X-rays hitting them, and convert this mathematical value into a digitised shade of grey. CT scanners are also able to produce detailed images by rotating the X-ray tube and the photomultiplier tubes around the patient, and a computer processes the information about the attenuation in each element into an image, resulting in a two-dimensional picture. Older CT scanners provide an image a slice at a time, each measuring from 2mm to 10mm. In helical CT the image is acquired continuously, with no gaps. The most modern scanners are multislice scanners that allow very thin slices to be imaged. These can be taken very rapidly, so that the whole body can be scanned in a single breath-hold. The image can also be manipulated so that tissue to be viewed in three dimensions, from any angle, not just the axial image.

As the image is digital and can be stored on a computer, CTs can be transmitted so that, for instance, several health professionals can view the image at the same time but in different places.

Uses of CT

CT is useful following a suspected head injury or on presentation of neurological signs. Plain film X-rays show skull fractures, but the brain parenchyma can be injured even in the absence of a fractured skull. CT will also be useful in imaging complex bony structures, such as the spine. CT scans can be obtained quickly - a modern scanner can scan the entire brain in less than one minute. This has obvious advantages for patient comfort.

CT can also be used with contrast media - that is when a compound is introduced that will show up on the scan. These media can be used to suggest changes in the function of the tissue, rather than just structure. For example, a water-soluble iodine compound can be given intravenously, and it accumulates in areas where the blood-brain barrier has broken down. Other-agents can be used to highlight areas of inflammation, or of increased vascularity.

Nursing care

There is no specific preparation required for patients unergoing a CT scan, unless contrast media is to be used. The procedure is relatively quick and the patient usually remains lying down - sometimes some effort is required to position the patient to give the axis required, and sometimes the table itself may tip during the procedure.

Safety

The greater detail of CT scans compared with plain –rays comes at a cost - the radiation the patient receives is much higher. The scanner delivers a dose of about 8mSv, while a plain film of the chest involves doses of about 0.17mSV. For comparison, the Health and Safety Executive recommends a dose of no more than 20mSv a year for workers in the nuclear industry, and 1mSv a year for the general population. However, most experts believe that there is no safe dose, and that even tiny amounts of radiation can damage the DNA, leading to an increased risk of cancer, particularly in the very young. In view of the very large doses delivered by the CT scan, the patient must be left during the scan itself, and the health professionals retire behind a lead shield.

Reference

Banerjee, A.K. (2006) Radiology Made Easy. Cambridge: Cambridge University Press.

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