St. Aloisius Medical Center
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CT Scan

Find out what a CT scan is, how it's done and how to prepare for one.

A CT scan — also called CT, computerized tomography or CAT scan — is an X-ray technique that produces images of your internal organs that are more detailed than those 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 two-dimensional images. But CT scans use an X-ray generating device that rotates around your body and a very 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 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, kidneys and blood vessels — all with a higher degree of precision.

When is a CT scan recommended?

Your doctor may recommend a CT scan to help:

  • 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
  • Detect and monitor diseases such as cancer or heart disease
  • Detect internal injuries and internal bleeding

Unlike MRI, 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?

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.

If your infant or toddler is having the test, the doctor may give your child a sedative to keep him or her calm and still. Movement blurs the images and may lead to 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 can take the contrast medium by mouth, enema or an injection into a vein (intravenously). 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 result in hives or itchiness. For individuals with asthma who become allergic to the contrast medium, the reaction can be an asthma attack. In very rare instances, an allergic reaction might cause 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.

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. 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.

What can you expect during a CT scan?

You can have a CT scan in a hospital or an outpatient facility. Expect the exam to last no more than an hour, depending on the preparation needed and whether it includes the use of contrast medium. The scan itself may take just a few minutes.

During the CT scan you lie on a narrow table that slides through 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 CT scan of the head, the table may be fitted with a special cradle that holds your head still.

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 this time, a technologist in a shielded room supervises the CT scan and monitors the images as they appear on the computer screen. The technologist can see and hear you, and you can communicate via intercom.

If an infant or small child is having the 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 X-ray exposure.

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 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 or the radiography staff may give you special instructions. These likely include drinking lots of fluids to help remove the medium from your body.


CT images are sent to an electronic data file and then reviewed on a computer. A radiologist interprets these images and sends a report to your doctor.


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:

  • 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 a contrast medium, there's a slight risk of an allergic reaction to the contrast medium.

  • You have certain medical conditions. Diabetes, asthma, heart disease, kidney problems or thyroid conditions also increase your risk of a reaction to contrast medium.

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