Alternative Names CAT scan, computed axial tomography, computed tomography imaging.
Definition Computed tomography (CT) is a computer-aided x-ray technique. X-rays consist of electromagnetic waves of energy. They penetrate the body to varying extents depending upon the density of the structures being viewed. The result is black and white images of interior portions of the body. A CT scan produces detailed cross-sectional views of the body, similar to slices of bread.
The technology behind CT scans has advanced rapidly in recent years. Older machinery used to take minutes to obtain enough information for a single "slice." Now, the same image can be produced in seconds. Newer scanners called spiral or helical scanners are so fast that they can scan the entire chest during one held breath. These devices can also produce three-dimensional scans.
Who is a candidate for the test? CT scans are performed to evaluate:
abnormalities that showed up on other types of x-rays.
injuries.
tumours related to cancer. CT scans can indicate the progress of some cancers if the cancers spread or metastasise and the effectiveness of treatment.
bony abnormalities.
brain abnormalities.
abdominal symptoms. Use of CT scans in these cases can often prevent exploratory surgery.
suspicious chest abnormalities.
CT scans are also used to guide needles when taking tissue samples. In addition, the technique is useful in gauging a person's recovery after an operation. CT scans can also be used to guide instruments for surgery deep in the brain.
How is the test performed? A person having a CT scan will need to undress and put on an examination gown. Next, the person will lie on a narrow table. The table will slide through a machine that looks like a doughnut. This is called the gantry. While in the gantry, an x-ray tube travels around the individual creating computer-generated x-ray images.
Some types of examinations require the individual to receive an intravenous injection of iodinated contrast, which is a dye that makes some tissues show up better. Scans of the intestines sometimes call for the person to drink diluted iodinated contrast solution prior to the examination. After the examination, the technologist will view the pictures. If they are adequate, the person is free to leave.
What is involved in preparation for the test? A person having the test will be asked to refrain from eating or drinking for 4 hours before the scan. All jewellery and metal objects that may interfere with the examination needs to be removed beforehand, as well. Women will be asked if they are pregnant. Individuals should check with their doctor or hospital x-ray department to see if any other preparation is needed.
What do the test results mean? A CT scan provides a direct image of soft tissue structures such as the, liver, lung, spleen, pancreas, lymph nodes and fatty tissues. CT is also good for identifying and tracking large abnormalities such as tumours. CT of the head can be used to evaluate strokes, tumours, bleeding and injuries. It can also be used to examine most brain structures. CT performs well in providing images of bony structures. These include the spine, facial bones, sinuses and skull. It also works well in viewing long bones for fractures, tumours or infection.
Author: Reviewer: HealthAnswers Australia Medical Review Panel Editor: Dr David Taylor, Chief Medical Officer HealthAnswers Australia Last Updated: 1/10/2001 Contributors Potential conflict of interest information for reviewers available on request
