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 detailed image 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.
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 help 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.
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.
Radiation Dose from CT
Estimates of the effective dose from a diagnostic CT procedure can vary by a factor of 10 or more depending on the type of CT procedure, patient size and the CT system and its operating technique. A list of representative diagnostic procedures and associated doses are given in Table 1.
Radiation Dose Comparisons
|Diagnostic Procedure||Typical Effective Dose (mSv)1|
|Chest x-ray (PA film)||0.02|
|Upper G.I. exam||6|
|Coronary artery calcification CT
|Coronary CT angiogram||16|
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.
How much is too much?
The more scans you have, the higher your lifetime exposure and therefore the higher your risk. The American College of Radiology recommends limiting lifetime diagnostic radiation exposure to 100 mSv. That is equal to 10,000 chest x-rays, or up to 25 chest CTs.