Definition:
A computed tomography (CT) scan is an imaging method that uses x-rays to create cross-sectional pictures of the body. See also:
Alternative Names:
CAT scan; Computed axial tomography scan; Computed tomography scan
How the test is performed:
You will be asked to lie on a narrow table that slides into the center of the CT scanner. Depending on the study being done, you may need to lie on your stomach, back, or side. Once you are inside the scanner, the machine's x-ray beam rotates around you. (Modern "spiral" scanners can perform the exam in one continuous motion.) Small detectors inside the scanner measure the amount of x-rays that make it through the part of the body being studied. A computer takes this information and uses it to create several individual images, called slices. These images can be stored, viewed on a monitor, or printed on film. Three-dimensional models of organs can be created by stacking the individual slices together. You must be still during the exam, because movement causes blurred images. You may be told to hold your breath for short periods of time. Generally, complete scans take only a few minutes. The newest multidetector scanners can image your entire body, head to toe, in less than 30 seconds.
How to prepare for the test:
Certain exams require a special dye, called contrast, to be delivered into the body before the test starts. Contrast can highlight specific areas inside the body, which creates a clearer image. Some people have allergies to IV contrast and may need to take medications before their test in order to safely receive this substance. Contrast can be given several ways, and depends on the type of CT being performed. - It may be delivered through a vein (IV) in your hand or forearm.
- It may be given through the rectum using an enema.
- You might drink the contrast before your scan. When you actually drink the contrast depends on the type of exam being done. The contrast liquid may taste chalky, although some are flavored to make it taste a little better. The contrast eventually passes out of your body through your stools.
If contrast is used, you may also be asked not to eat or drink anything for 4-6 hours before the test. If you weigh more than 300 pounds, have your doctor contact the scanner operator before the exam. CT scanners have a weight limit. Too much weight can cause damage to the scanner's working parts. Since x-rays have difficulty passing through metal, you will be asked to remove jewelry and wear a hospital gown during the study.
How the test will feel:
Some people may have discomfort from lying on the hard table. Contrast given through an IV may cause a slight burning sensation, a metallic taste in the mouth, and a warm flushing of the body. These sensations are normal and usually go away within a few seconds.
Why the test is performed:
CT rapidly creates detailed pictures of the body, including the brain, chest, spine, and abdomen. The test may be used to:
References:
Shaw AS, Dixon AK. Multidetector computed tomography. In: Grainger RC, Allison D, Adam, Dixon AK, eds.Diagnostic Radiology: A Textbook of Medical Imaging. 4th ed. New York, NY: Churchill Livingstone; 2001:chap 4.
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