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.
Some CT scans require you to drink a contrast liquid before the scan or have contrast injected into a vein in your arm during the scan. A contrast medium blocks X-rays and appears white on images, which can help emphasize blood vessels, bowel or other structures. If your test involves a contrast medium, your doctor may ask you to fast for a few hours before the test.
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.
Although rare, the contrast medium involved in a CT scan poses a slight risk of allergic reaction. Most reactions are mild and result in hives or itchiness. For people with asthma who become allergic to the contrast medium, the reaction can be an asthma attack.
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.
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.
You can have a CT scan in a hospital or an outpatient facility. Expect the exam to last no longer than an hour, depending on the preparation needed and whether it includes the use of a contrast medium. The scan itself may take less than a minute on the newest machines. Most scans take just a few minutes to complete.
CT scans are 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 the contrast medium through an enema — to help highlight your lower gastrointestinal region — you may feel a sense of fullness or cramping.
After the exam you can return to your normal routine. If you were given a contrast medium, your doctor, a nurse or the CT technologist performing the scan may give you special instructions. You may be asked to wait for a short time in the radiology department to ensure that you feel well after the exam. After the scan, you’ll likely be told to drink lots of fluids to help your kidneys remove the medium from your body.
CT scan recommend for:
- Diagnose muscle and bone disorders, such as bone tumors and fractures
- 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
- Detect internal injuries and internal bleeding
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.