Radiation in Medicine: CT Scans

CT, or computerized axial tomography (CAT) scans, are medical imaging tests that are used to take pictures of parts of the body at different angles to create detailed images of internal organs, blood vessels, and bones.

During doctor’s visits or annual physicals your doctor may ask questions about your medical history, perform an examination and possibly request a CT scan to detect, diagnose or plan treatment for a particular disease or injury.  CT scans involve exposure to ionizing radiation, which can present risks. If patients understand the benefits and risks they can make the best decisions about their health care.

Most people have had one or more medical imaging tests that use ionizing radiation. The type of imaging procedure that your doctor may suggest will depend on your health concern and the part of the body that is being examined.   Some other common examples of imaging tests include:

If your doctor suggests a CT scan or other medical imaging procedures you should consider the following:

  • Medical imaging procedures should be performed only when necessary.
  • The U.S. Food and Drug Administration (FDA) recommends discussing the benefits and risks of CT scans with your doctor.

CT Scans

Before the procedure

Your health care provider will explain the procedure and ask you if you have any questions.  Some CT procedures may use a “contrast dye” which allows doctors to see specific organ(s). The dye may be administered by swallowing, an intravenous (IV) line in your hand or arm, or an enema.

Make sure to let the doctor know if:

  • You have ever had a reaction to any contrast dye.
  • You are pregnant or suspect that you may be pregnant.

During the Procedure

  • You will be asked to remove any clothing and jewelry that may interfere with the procedure.
  • You must lie very still on a table while it moves slowly through an x-ray machine that gives off rotating beams of x-rays.
  • A special type of x-ray equipment is used to take pictures of your body at different angles.
  • A computer collects and organizes the x-ray information to show organs, bones and other tissues in great detail.

Medical imaging tests such as CT scans are painless procedures that allow doctors to diagnose diseases and injuries without being invasive. CT scans also help doctors to:

  • Obtain detailed pictures of the body including the brain, chest, spine and abdomen
  • Determine whether surgery is a good treatment option
  • Identify masses and tumors, including cancer
  • Diagnose certain blood system diseases

During a normal x-ray, diseases or injuries may not be easily detectable due to organs that may overlap.  A CT scan allows the doctor to see areas of the body that are not easily seen by an x-ray. 

As in many areas of medicine, there are risks associated with CT scans, which use ionizing radiation to create images of the body. Risks from exposure to ionizing radiation include:

  • A small increase in the likelihood that a person exposed to radiation will develop cancer later in life.
  • Health effects that could occur after a large acute exposure to ionizing radiation such as skin reddening and hair loss. 
  • Possible allergic reactions associated with a contrast dye injected into the veins to better see body structures being examined.

In the case of x-rays or other tests involving exposure to ionizing radiation, doctors and radiation experts can help reduce your exposure to and risk of harm from diagnostic ionizing radiation by:

  • Checking to see if you have had a similar test done recently that can provide them with the background information they need.
  • Checking to see if a test that does not use ionizing radiation can provide similar information.
  • Making certain the least possible amount of radiation needed to obtain a good quality image is used for your procedure.
  • Providing protective lead shielding to prevent exposing other areas of the body to radiation.

Radiation experts include the following:

  • Hospital radiation safety officer
  • Medical physicist
  • Radiation physicist
  • Diagnostic medical physicist

Talk to your physician about the potential risks and benefits from the medical procedures.  In many cases, the risk of an x-ray procedure to the mother and the unborn child is very small compared to the benefit of finding out about the medical condition of the mother or the child.

Many doctors use ultrasound to examine the abdomen, pelvic area, or heart. Ultrasound does not use ionizing radiation, so it does not expose women of childbearing age to radiation in the pelvic area. This is particularly important in pregnancy.  For more information, please see the Image Wiselyexternal icon website.

Risks that are considered small should not be taken if they’re unnecessary.  You can reduce risks from medical imaging procedures by telling your doctor if you are, or think you might be, pregnant whenever an abdominal x-ray is suggested by your doctor. Other options suggested by FDA that may be considered are as follows:

  • If you are pregnant, the doctor may decide that it would be best to cancel the medical imaging procedure, to postpone it, or to modify it to reduce the amount of radiation.
  • Depending on your medical needs, and realizing that the risk is very small, the doctor may feel that it is best to proceed with using a medical imaging procedure as planned.

In any case, you should feel free to discuss the decision with your doctor.  For more information on medical imaging and pregnancy, please see X-rays, Pregnancy and Youexternal icon.  Also, for more information on radiation safety in adult medical imaging, please visit the Image Wisely websiteexternal icon.

 Image Gently logo

It is important that x-rays and other imaging procedures performed on children use the lowest exposure setting needed to obtain a good clinical image.  The Image Gently Allianceexternal icon, part of the Alliance for Radiation in Pediatric Imaging, suggests the following for imaging of children:

  • Use imaging examinations when the medical benefit outweighs the risk.
  • Use the most appropriate imaging techniques, matched to the size of the child.
  • Use alternative imaging methods (such as ultrasound or Magnetic Resonance Imaging)when possible.

The FDAexternal icon also provides information for parents, patients, and healthcare providers to address concerns about the benefits and risks of medical imaging procedures for children.

There are medical imaging procedures such as Magnetic Resonance Imaging (MRI) or ultrasound that do not use ionizing radiation to diagnose illnesses or injuries.

What is an MRI?

MRI procedures, which can lasts from 30-60 minutes, use magnetic fields and radio waves to produce images of specific parts of the body.  MRI scans are often performed along with other medical imaging procedures to provide a more detailed view of the area of the body that is being examined.   For more information on MRI, please see FDA’s website on MRI (Magnetic Resonance Imaging).external icon

What is an ultrasound?

Ultrasound imaging uses high-frequency sound waves to see inside the body. There is no ionizing radiation used and in most ultrasound examinations, no contrast is given.

Page last reviewed: November 8, 2016