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Computed tomography (CT) scanning — formerly known as CAT (computerized axial tomography) scanning — is a diagnostic imaging technique in which multiple projections of X-rays (passed through the body from several angles) are analyzed by a computer to produce cross-sectional images of structures. The digital images obtained through CT scanning are more detailed than the images obtained through standard X-rays. CT scanning combines radiography with computer analysis of tissue density. The procedure is a painless, noninvasive method of visualizing detailed views of internal organs and structures; in many cases, it can eliminate the need for more invasive procedures.
CT scanning reveals bone, and also shows soft tissues such as organs, muscles and tumors. Light and dark tones of the images can be adjusted to highlight tissues of similar density. By using graphics software, the computer data from multiple cross-sections can be assembled into three-dimensional images.
Why It Is Performed
CT scanning was originally designed to image the brain, but it is now also used to diagnose disorders of the body cavity and bones. It may be used to detect organ damage, tumors, bleeding and other abnormalities that cannot be visualized on standard X-rays. CT scanning can aid in surgery and in the diagnosis of many different conditions. It may be used in association with treatments such as radiation therapy, in which knowing the precise density, size and location of a tumor is essential to providing the correct dosage of radiation. CT scans of the blood vessels can be performed to locate and detail precisely the nature of conditions such as an aneurysm (abnormal ballooning of a weakened area in the wall of an artery).
How It Works
As the scanner moves, a computer calculates the amount of X-ray penetration through the specific planes of the body parts being examined. Each amount is given a numerical value, called the density coefficient. This numerical information is supplied to a computer, which translates the values into different shades of gray. The images produced by the computer are displayed on a monitor and photographed, creating a series of images that depict a cross-section of the parts being examined.
How It Is Performed
CT scanning is performed in a hospital radiology unit or outpatient diagnostic clinic. The equipment includes a tube-shaped, X-ray scanner, a computer and a monitor. The person being scanned lies on a narrow table that slides into a tube-shaped scanner. The scanner then takes multiple X-ray images. In helical, or spiral CT scanning, there is continuous scanning as the examination table slides through the unit. Spiral scanning reduces the amount of time needed for scanning the entire body, enhances the contrast if a contrast medium is used, and improves the reconstruction and manipulation of the images.
What to Expect
Beforehand, the person performing the procedure usually explains what is involved in the scanning process. For some CT scans, an intravenous contrast medium containing iodine may be administered to make the blood vessels show up more clearly, or a flavored barium drink may be given to provide contrast to internal structures.
The person being scanned lies on a narrow table that slides into the scanner. The table moves continuously in spiral scanning, or may stop and start several times, depending on the part of the body being scanned, to obtain scans at different angles. A small area, several sections or the whole length of the body may be scanned.
After the scan, there will be special instructions if a contrast medium was used. For example, the person may be instructed to drink large amounts of water to help flush the contrast medium through the system. A radiologist will then interpret the results of the scan and provide a report to the referring physician.
Excerpted from: Complete Medical Encyclopedia, American Medical Association, 2003.