VOL: 98, ISSUE: 37, PAGE NO: 41
Alison Coutts, MSc, RGN, is lecturer in applied biological sciences, St. Bartholomew school of nursing and midwifery, City University, LondonComputerised tomography (CT) has been in use since the 1970s. It uses X-rays to generate an image of parts of the body. Unlike traditional X-rays, which use plain films to capture the image, CT scans provide more detailed information.
Computerised tomography (CT) has been in use since the 1970s. It uses X-rays to generate an image of parts of the body. Unlike traditional X-rays, which use plain films to capture the image, CT scans provide more detailed information.
X-rays are a type of electromagnetic radiation that can pass through objects that are opaque to visible light. When X-rays encounter tissue, they can be completely absorbed, scattered and attenuated, or pass straight through. The more dense the tissue, the more likely the X-rays are to be attenuated. In plain film, X-rays that pass through the tissue darken the photographic film leaving a paler silhouette where there is dense tissue such as bone.
CT scanners use electronic sensors (photomultiplier tubes) to measure the intensity of X-rays reaching them. They convert these measurements into digitised shades of grey, giving more detail than conventional X-ray films. CT scans are also able to produce detailed images by rotating the X-ray tube and the photomultiplier tubes around the patient. A computer then processes the information about the extent of attenuation in each segment into an image.
In helical CT the image is acquired continuously, allowing the tissue to be viewed in three dimensions. The image can also be manipulated so that tissues may be viewed from different perspectives. CTs can be viewed simultaneously by health professionals in different places because the image is digital and can be stored on computers and transmitted electronically.
Uses of CT
CT is particularly useful for:
- Internal organs within the abdomen and chest, such as the liver, kidneys, pancreas, intestines and lungs;
- Bone imaging, for orthopaedic examinations;
- Brain imaging;
- Vascular imaging, to examine blood flow.
CT is a useful investigation for patients with suspected head injury or who present with suspect neurological signs. Although X-ray films show skull fractures, unlike CT scans they cannot show damage to the brain parenchyma (its functioning tissues) which may be injured even in the absence of a fracture.
The procedure is also useful for producing images of 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, aiding patients' comfort.
CT can also be used in conjunction with contrast media - radiopaque materials that show up on the CT scan. Contrast media can be used to observe changes in the function of the tissue, rather than just its structure.
For instance, a water-soluble compound of iodine can be given intravenously, and will accumulate in areas where the blood-brain barrier has broken down. Other agents can be used to highlight areas of inflammation, or of increased vascularity. Other common uses of CT scans include:
- Diagnosis and staging of cancers;
- Assessing organ and bone damage caused by trauma;
- Planning radiotherapy treatment;
- Screening for and assessing cardiac disease;
- Bone mineral densitometry to assess osteoporosis;
- Guiding biopsy procedures for taking tissue samples (Impactscan, 2002).
There is no specific preparation required for patients undergoing a CT scan, unless contrast media are to be used. However, patients may be unnerved when they first see the scanner and should be told in advance what to expect.
They normally lie on a movable table which will raise, lower and move them in and out of the scanner as required. Sometimes, effort is required to position the patient to give the axis required, and sometimes the table itself may incline during the procedure. Patients also need to understand the importance of remaining still throughout the procedure, which can take up to an hour.
Unfortunately the greater detail provided by CT scans comes at a cost. The patient receives a far higher dose of radiation than they would with a plain film X-ray. A CT scan delivers a dose of 1-8milliSieverts (mSv). A plain film X-ray of the chest gives a dose of about 0.17mSv. The Health and Safety Executive recommends a maximum dose of 20mSv per calendar year for workers in the nuclear industry, and 1mSv a year for the general population.
However, most experts believe that there is no dose of radiation that is absolutely safe, and that even tiny amounts can damage DNA which may increase the risk of cancer.
In view of the size of the doses delivered by CT, patients must be alone in the scanning room. Health professionals must stand behind a lead shield which protects them by absorbing any stray X-rays.