What Affects the Test

The following may stop you from having the test or may change the test results:

• Pregnancy. CT scans are not usually done during pregnancy.
• Barium and bismuth used for another test. These substances show up on a CT scan. If a CT scan of the belly is needed, it should be done before any tests that use barium, such as a barium enema.
• Metal objects in the body. These items, such as surgical clips or metal in joint replacements, may prevent a clear view of the body area.
• You are not able to lie still during the test.

What To Think About

• Sometimes your CT test results may be different than those from other types of X-ray tests, magnetic resonance imaging (MRI), or ultrasound scans because the CT scan provides a different view.
• An ultrasound test, which doesn't use dangerous radiation, may give results similar to a CT scan. If you are concerned about radiation exposure, ask your doctor if you can have an ultrasound instead of a CT scan.
• Children who need a CT scan may need special instructions for the test. The child will likely need to hold his or her breath during the scan. If the child is too young to hold still or is afraid, the doctor may give the child a medicine (sedative) to help him or her relax.
• If your child is scheduled for a CT scan, talk with your child's doctor about the need for the scan and the risk of radiation exposure to your child.
• Special CT scanners called spiral (helical) CT scanners and multi-slice (or multi-detector) CT scanners are sometimes used for this test. Many modern scanners are multi-slice scanners. These scanners can be used for many conditions, such as finding kidney stones, a pulmonary embolism, an enlarged prostate gland, or atherosclerosis. These special CT scanners can:
  • Take better pictures of blood vessels and organs so other imaging tests may not be needed.
  • Complete scans and provide pictures in less time.
• CT results are often compared to positron emission tomography (PET) results to help find cancer. Some new scanners do both scans at the same time.
• An electron beam CT scan is another type of CT scan that can find atherosclerosis and coronary artery disease. An electron beam CT scan is much faster than a standard CT scan and can take a good picture of a coronary artery while the heart is beating. Electron beam CT scans are not widely available. Another type of CT scanner, the multi-slice CT scan, is nearly as fast as electron beam CT scanners and is more widely available.
• A CT angiogram can show two- and three-dimensional pictures of blood vessels. For more information, see the topic Angiogram of the Head and Neck.
• Coronary calcium scans can help find out risk of heart disease. This test is not done very often, because a physical exam and other tests often give enough information about your heart. This test is not advised for routine screening.
• MRI may give different information than a CT scan about certain conditions. For more information, see the topic Magnetic Resonance Imaging (MRI).
• Experts disagree about the use of a CT method called full-body scanning to screen for coronary artery disease and cancers. Full-body scanning is expensive, can lead to unnecessary tests or surgery, and may increase the chance of cancer from the radiation exposure. Most doctors do not recommend these studies unless a person has a specific risk for a certain disease.