What Is a CT Scan?

A computed tomography (CT or CAT) scan allows doctors to see inside your body. It uses a combination of X-rays and a computer to create pictures of your organs, bones, and other tissues. It shows more detail than a regular X-ray.

You can get a CT scan on any part of your body. The procedure doesn't take very long, and it's painless.

How Do CT Scans Work?

They use a narrow X-ray beam that circles around one part of your body. This provides a series of images from many different angles. A computer uses this information to create a cross-sectional picture. Like one piece in a loaf of bread, this two-dimensional (2D) scan shows a “slice” of the inside of your body.

This process is repeated to produce a number of slices. The computer stacks these scans one on top of the other to create a three-dimensional (3D) image. This can give your doctor a better view of your organs, bones, or blood vessels. For example, a surgeon may use this type of scan to look at all sides of a tumor to prepare for an operation.

How Are CT Scans Done?

You'd probably get a scan at a hospital or radiology clinic. Your doctor might tell you not to eat or drink for a few hours before the procedure. You may also need to wear a hospital gown and remove any metal objects, such as jewelry.

A radiology technologist will perform the CT scan. During the test, you’ll lie on a table inside a large, doughnut-shaped CT machine. As the table slowly moves through the scanner, the X-rays rotate around your body. It’s normal to hear a whirring or buzzing noise. Movement can blur the image, so you’ll be asked to stay very still. You may need to hold your breath at times.

How long the scan takes will depend on what parts of your body are being scanned. It can take anywhere from a few minutes to a half-hour. In most cases, you’ll go home the same day.

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What Is It Used For?

Doctors order CT scans for a long list of reasons:

  • CT scans can detect bone and joint problems, like complex bone fractures and tumors.
  • If you have a condition like cancer, heart disease, emphysema, or liver masses, CT scans can spot it or help doctors see any changes.
  • They show internal injuries and bleeding, such as those caused by a car accident.
  • They can locate a tumor, blood clot, excess fluid, or infection.
  • Doctors use them to guide treatment plans and procedures, such as biopsies, surgeries, and radiation therapy.
  • Doctors can compare CT scans to find out if certain treatments are working. For example, scans of a tumor over time can show whether it’s responding to chemotherapy or radiation.

What Is a CT Scan with Contrast?

In a CT scan, dense substances like bones are easy to see. But soft tissues don’t show up as well. They may look faint in the image. To help them appear clearly, you may need a special dye called a contrast material. They block the X-rays and appear white on the scan, highlighting blood vessels, organs, or other structures.

Contrast materials are usually made of iodine or barium sulfate. You might receive these drugs in one or more of three ways:

  • Injection: The drugs are injected directly into a vein. This is done to help your blood vessels, urinary tract, liver, or gallbladder stand out in the image.
  • Orally: Drinking a liquid with the contrast material can enhance scans of your digestive tract, the pathway of food through your body.
  • Enema: If your intestines are being scanned, the contrast material can be inserted in your rectum.

After the CT scan, you’ll need to drink plenty of fluids to help your kidneys remove the contrast material from your body.

Are There Any Risks?

CT scans use X-rays, which produce ionizing radiation. Research shows that this kind of radiation may damage your DNA and lead to cancer. But the risk is still very small -- your chances of developing a fatal cancer because of a CT scan are about 1 in 2,000.

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But radiation’s effect adds up over your lifetime. So your risk increases with every CT scan you get. Talk to your doctor about the procedure’s potential dangers and benefits, and ask why the CT scan is necessary.

Ionizing radiation may be more harmful in children. That’s because they’re still growing. They also have more years to get exposed to radiation. Before the procedure, you may want to ask the doctor or technician if the CT machine’s settings have been adjusted for a child.

Tell your physician if you’re pregnant. If you need imaging for your stomach area, your doctor may recommend an exam that doesn’t use radiation, such as an ultrasound.

What Are the Side Effects?

Some people are allergic to the contrast materials. Most of the time, the reaction is mild. It can lead to itchiness or a rash. In very few cases, the dye may trigger a life-threatening reaction. For this reason, your health care provider may want to monitor you for a short period after your CT scan. Tell your doctor about any allergies you have to medications, seafood, or iodine.

Your doctor should know, too, if you have diabetes and are taking the drug Metformin. He’ll let you know if you should stop taking your medication before or after your procedure.

Although it’s rare, contrast materials can lead to kidney problems. Let your doctor know if you have any kidney issues before the CT scan.

WebMD Medical Reference Reviewed by William Blahd, MD on December 23, 2016

Sources

SOURCES:

National Institute of Biomedical Imaging and Bioengineering: “Computed Tomography.”

Johns Hopkins Medicine: “Computed Tomography (CT or CAT) Scan of the Abdomen.”

National Cancer Institute: “Computed Tomography (CT) and Cancer.”

Food and Drug Administration: “Computed Tomography (CT).”

Mayo Clinic: “CT Scan.”

PLOS One: “Consistent Surgeon Evaluations of Three-Dimensional Rendering of PET/CT Scans of the Abdomen of a Patient with a Ductal Pancreatic Mass.”

American Cancer Society: “CT Scan for Cancer.”

Nature Communications: “Mutational Signatures of Ionizing Radiation in Second Malignancies.”

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