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A computed tomography (CT or CAT) scan uses an X-ray sensing unit that circles around the body and creates image slices, which a computer pieces together to produce three-dimensional images of the inside of the body. A CT scan shows more of your body with more precision than conventional X-ray scans.
A CT scan is useful for:
- Identifying bone and muscle disorders, including osteoporosis
- Revealing the location of a tumor or infection
- Guiding surgery, biopsies or radiation therapy
- Monitoring the progress of a disease that grows worse over time
- Finding internal injuries or bleeding
During a CT scan, you lie on a table inside a doughnut-shaped machine called a gantry. An X-ray tube inside the machine rotates around your body and beams small doses of radiation through it at various angles.
As X-rays pass through your body, different tissues absorb different amounts. Detectors inside the gantry measure the radiation leaving your body and convert the radiation into electrical signals. A computer gathers these signals and gives them a color ranging from black to white depending on signal intensity. The computer then puts the images together and displays them on a computer monitor. A technician in a separate room supervises your exam and watches the images on the computer. He or she can see and talk with you through an intercom.
A CT scan is painless. It can be performed on an infant or toddler. Unlike magnetic resonance imaging (MRI), a CT scan can be done even if you have a pacemaker or cardioverter defibrillator.
Sometimes a CT scan will be done after you have swallowed a contrast medium. The contrast medium blocks the X-rays and shows up white in the scan. This can help parts of your body show up better in the scan. For example, when a CT scan is combined with a myelogram, your doctor is able to see excellent nerve detail. During a myelogram, a dye is injected into the spinal canal. The dye, in turn, lights up the nerve roots making them easy to see during the CT scan. This type of diagnostic test is very sensitive. A myelogram is an excellent way for your doctor to see how the bone is affecting the nerve roots.
Usually having a CT scan done takes less than an hour. Most of that time is spent in preparation for the actual scan. As with an X-ray, a radiologist who is specially trained to read the images will look at the scan and send a report to your doctor or surgeon.
A CT scan is about as safe as an ordinary X-ray. There is some brief exposure to radiation. However, the information that a CT scan provides outweighs the risks of the radiation exposure.
If any of the following apply to you, please tell your doctor:
- You are or think you might be pregnant. In this case, your doctor may want to recommend another type of diagnostic test.
- You have asthma or allergies. If it is necessary for you to have a contrast medium while the CT scan is done, there is a small possibility that you would have an allergic reaction to the medium.
- You have medical conditions such as diabetes, asthma, heart disease, kidney problems or a thyroid condition. These also may increase your risk of an allergic reaction to any contrast medium that may be needed during your CT scan.