CT scan — Overview describes what happens during this imaging procedure and how to prepare.
A CT scan — also called CT or computerized tomography — is an X-ray technique that produces images of your body that visualize internal structures in cross section rather than the overlapping images typically produced by conventional X-ray exams.
Conventional X-ray exams use a stationary X-ray machine to focus beams of radiation on a particular area of your body to produce 2-D images on film or a digital detector, much like a photograph. But CT scans use an X-ray unit that rotates around your body and a powerful computer to create cross-sectional images, like slices, of the inside of your body.
A conventional X-ray of your abdomen, for example, shows your bones as well as subtle overlapping outlines of your liver, stomach, intestines, kidney and spleen. A CT scan, however, clearly reveals these bones and organs as well as their inner structure and detailed anatomy of your pancreas, adrenal glands, kidneys and blood vessels.
Who is a CT scan for?
Your doctor may recommend a CT scan to help:
CT scans can be done even if you have a pacemaker or an internal cardioverter defibrillator — devices implanted in your chest to help regulate your heartbeat. However, if you're pregnant or suspect you might be, tell your doctor. Your doctor may suggest postponing the procedure or choosing an alternative exam that doesn't involve radiation, such as an ultrasound or MRI.
How do you prepare for a CT scan?
How you prepare for a CT scan depends on which part of your body is being scanned. You may be asked to remove your clothing and wear a hospital gown. You'll need to remove any metal objects, such as jewelry, that might interfere with image results.
Preparation sometimes involves fasting
Depending on the part of your body being scanned, your doctor may ask you to take laxatives, enemas or suppositories, or temporarily modify your diet.
Reactions to contrast medium
In rare instances, an allergic reaction can be serious and potentially life-threatening — including swelling in your throat or other areas of your body. If you experience hives, itchiness or swelling in your throat during or after your CT exam, immediately tell your technologist or doctor.
If you've had a reaction to a contrast medium in the past, and you need a diagnostic test that may require a contrast medium again, talk to your doctor. Be sure to let your doctor know if you have kidney problems. Contrast material that's injected into a vein is removed from your body by your kidneys and could potentially cause further damage to your kidneys in some circumstances.
Preparing your small child for a scan
CT scan images of the brain
Left: Arrows indicate an epidural hematoma, a collection of blood between the skull and the outer covering of the brain, that's compressing the frontal lobe. Right: Contrast medium injected into a vein during this CT scan of the head highlights tumors in both sides of the brain.
How is a CT scan done?
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 sends small doses of radiation through it at various angles. As X-rays pass through your body, different tissues absorb different amounts of radiation.
Detectors inside the gantry measure the radiation that has passed through your body and converts it into electrical signals. A computer gathers these signals and assigns them a color ranging from black to white, depending on signal intensity. The computer then assembles the images and displays them on a computer monitor.
CT scan image of the abdomen
Left: Conventional X-ray film shows bones and vague outlines of organs. Right: CT with contrast medium clearly shows several organs and blood vessels. Short, thick arrows show a tumor in the pancreas that has spread (metastasized) to the liver.
What can you expect during a CT scan?
Length of the scan
How you are positioned
What the device does
The role of the technologist
What you may feel
Back to normal
During your small child's exam
CT scan slices
CT scans allow doctors to see multidimensional images (slices) of your body. This slice shows heart and lung tissue.
Results of a CT scan
CT images are stored as electronic data files and usually reviewed on a computer. A radiologist interprets these images and sends a report to your doctor.
Risks of a CT scan
CT scan risks are similar to those of conventional X-rays. During the CT scan, you're briefly exposed to radiation. But doctors and other scientists believe that CT scans provide enough valuable information to outweigh the associated risks. Be sure to inform your doctor if:
Last Updated: 01/12/2008
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