| Computerized tomography — also called CT, CT scan and CAT scan — is an X-ray technique that produces more detailed images of your internal organs than do conventional X-ray exams. X-rays are a form of energy — radiation. Conventional X-ray exams produce two-dimensional images. But CT uses an X-ray-sensing unit, which rotates around your body, and a large 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 outlines of your liver, stomach, intestines, kidney and spleen. A CT scan, however, reveals these bones and organs as well as your pancreas, adrenal glands, ureters and blood vessels — all with a higher degree of precision.
CT is a noninvasive way to view your internal organs and tissues. CT is used to help: - Diagnose muscle and bone disorders, such as osteoporosis
- Pinpoint the location of a tumor, infection or blood clot
- Guide procedures such as surgery, biopsy and radiation therapy
- Detect and monitor diseases such as cancer or heart disease, and monitor the progression of a disease
- Detect internal injuries and internal bleeding
Unlike MRI, CT exams can be done even if you have a pacemaker or 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 choose an alternative exam that doesn't involve radiation, such as ultrasound or MRI.
How you prepare for a CT exam depends on which part of your body is being scanned. You may be asked to remove your clothing and dress in a hospital gown. You also need to remove any metal objects, such as jewelry, that might interfere with image results. If your infant or toddler is having the test, the doctor may give your child an oral sedative to keep him or her calm and still. Movement could cause fuzzy and incorrect results. Ask your doctor how to prepare your child. Some CT scans require you to ingest a contrast medium before the scan. A contrast medium blocks X-rays and appears white on images, which can help emphasize blood vessels or other structures. You take contrast medium by mouth, enema or through an injection into a vein (intravenously). If your test involves contrast medium, your doctor may ask you to fast a few hours before the test, because contrast medium can cause nausea.
| | | Brain CT |  | Left: Arrows indicate an epidural hematoma, a collection of blood ... | | << ENLARGE | | 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 further. Although rare, there's a slight risk of allergic reaction to contrast medium. Most reactions result in hives or itchiness. For individuals with asthma who become allergic to contrast medium, the reaction can be an asthma attack. In very rare instances, an allergic reaction might cause swelling in the throat or other areas of the body. If you experience hives, itchiness or swelling in the throat during or in the hours after your CT exam, immediately tell your technologist or doctor.
CT scans use X-rays to create images of the inside of your body. X-rays are a form of energy — radiation. X-rays exist in nature, emanating from outer space, rocks and even the soil. You can't see or feel them, and at low doses they're not harmful. Because they're of higher energy than visible light, X-rays can penetrate objects — including your body. 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. Detectors inside the gantry measure the radiation leaving your body and convert the radiation 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.
| | | Abdominal CT |  | Left: Conventional X-ray film shows bones and vague outlines of organs. ... | | << ENLARGE | |
| What can you expect during the procedure? | | | You can have a CT scan in a hospital or an outpatient clinic. Expect the exam to last about an hour or less, depending on the preparation needed and whether it includes the use of contrast medium. The scan itself may only last up to several minutes. If your infant or small child is having a CT scan, you may be allowed to stay with your child during the test. If so, you may be asked to wear a lead apron to shield you from exposure to X-rays.
| | | CT slices |  | CT allows doctors to see multidimensional images (slices) of your body. ... | | << ENLARGE | | If you're having the CT scan, you lie on a narrow table that slides in or out of the opening of a large device called the gantry. The table can be raised, lowered or tilted. Straps and pillows may help you stay in position. During a head CT, the table may be fitted with a special cradle that holds your head. As the X-ray tube rotates around your body, the table slowly moves through the gantry. While the table is moving, you may need to hold your breath to avoid blurring the images. You may hear clicking and whirring noises. Each rotation yields several images of thin slices of your body.
 | During the CT scan, a technologist is in a separate room supervising your exam and monitoring the images as they appear on the computer screen. The technologist can see and hear you, and you can communicate via intercom. A CT scan is painless. If your exam involves use of an intravenous contrast medium, you may feel a brief sensation of heat or experience a metallic taste in your mouth. If you receive contrast medium through an enema — to help highlight your lower gastrointestinal region — you may feel a sense of fullness, cramping or an urge to defecate. After the exam you can dress and return to your normal routine. If you used a contrast medium, your doctor or the radiography staff gives you any special instructions. These likely include drinking lots of fluids to help pass the medium through your body.
CT images usually are printed from the computer onto film. A radiologist interprets these images and sends a report to your doctor.
Risks of CT are similar to those of conventional X-ray. During the CT scan, you're briefly exposed to ionizing radiation. But doctors and other scientists believe that CT scans provide enough valuable information to outweigh the associated risks of radiation. Still, inform your doctor if: - You're pregnant.
If you're pregnant, your doctor may recommend another type of exam to reduce the possible risk of exposing your fetus to radiation. - You have asthma or allergies.
If you have asthma or allergies and your CT scan requires contrast medium, there's a slight risk of allergic reaction to the contrast medium. - You have certain medical conditions.
If you have diabetes, asthma, heart disease, kidney problems or thyroid conditions, these also may increase your risk of an allergic reaction to contrast medium.
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