Computed Tomography (CT)

Computed tomography (CT) is a method of body imaging in which a thin X-ray beam rotates around the patient. Small detectors measure the amount of X-rays that make it through the patient or particular area of interest. A computer analyzes the data to construct a cross-sectional image. These images can be stored, viewed on a monitor, or printed on film.  


What is a CT scan?

A CT (computed tomography) scan, also known as a “CAT” scan, uses X-rays to form pictures of the inside of the body.

How is CT different from a regular X-ray?

A regular X-ray sends images from a stationary machine through the body to make a “shadow” picture. In CT, the X-ray machine rotates around the body and makes pictures that are like “slices” through the body, providing much more information than a regular X-ray.

What are the benefits of CT?

Your doctor will use the results to better diagnose and manage your condition. CT may be used to check for many diseases and causes of pain, such as blood clots in the lungs or appendicitis. Failure to diagnose and treat these conditions could seriously affect your health.

What are the risks of CT?

There is a very small risk from radiation, because CT uses X-rays and radiation can cause cancer. To put this in perspective, all of us are exposed to radiation every day, mainly from the sun and soil. The amount of radiation during a typical body CT scan is about the same as the radiation we each get in two years from these background sources, and the risk of getting a fatal cancer from this amount of radiation is about 1 in 2000. This is about the same excess risk of death in a motor vehicle accident from travelling 40,000 miles in a car.

How can the risk of cancer be reduced?

At UCSF, our scanners feature the latest technology, allowing us to use the lowest possible amount of radiation needed for each CT scan.

What is X-ray contrast, and why is it used?

X-ray contrast or “dye” is a special liquid that is injected through a small tube placed in a vein in your forearm during a CT scan. This liquid shows up on the pictures and provides much more information about the blood vessels and organs inside the body. Most CT scans are performed with contrast.

What are the risks of X-ray contrast?

The risks of X-ray contrast include nausea, vomiting, arm pain, allergy, kidney damage and leakage. All of these are rare and most are temporary, requiring no treatment or simple “first aid” measures. However, you should be aware that:

  • Kidney damage from contrast sometimes requires treatment by a nephrologist (a doctor who specializes in kidney disease) and is occasionally irreversible. This occurs primarily in diabetic patients and patients with known kidney disease.
  • Surgery or surgical consultation for severe leakage of contrast into the tissues of the arm is required for one in every 6,000 injections.
  • Allergic reactions can occur and very rarely are severe enough to be life-threatening (about one in every 20,000 to 100,000 injections). Allergic reactions may include headache, sneezing, vomiting, confusion, weakness, warmth, sweating, nervousness, trouble breathing (through mouth or nose), sickness in stomach, metallic taste, watery eyes, itchy eyes, itchy bumps on skin, sections of fluid-filled bumps on skin, red-colored skin, pale skin or uncontrollable shaking. If you experience any of these symptoms during your CT scan, immediately tell your doctor or the department staff.

How can the risks of X-ray contrast be reduced?

Before contrast is injected, you will be screened for any risk factors that may place you at a higher risk of a contrast reaction. If you are at higher risk, your case will be reviewed by the doctors in the Radiology Department. Alternatives to contrast or changes in the type and amount of contrast used may be considered or discussed with you and your referring doctor. It is therefore important for your own safety that you complete this screening form as accurately as possible. Your information will be kept private. Sometimes, patients who have had a previous allergic reaction to contrast will be given special medicines to prevent this from happening again (this is called “premedication”).

Are there alternatives to CT?

In a serious or emergency situation, the benefits of CT clearly outweigh the risks and CT is usually the best option. However, in other situations you may wish to discuss with your referring doctor not having a CT scan or having another type of test (for example, ultrasound or MRI).

If my CT scan result is normal, does that mean it should not have been done?

A normal CT provides valuable information and does not mean the test was a “mistake.” It is  important for your doctor to know what you do not have, as well as what you do have.

What if I might be pregnant?

It is very important that you tell the Radiology staff if there is even the slightest possibility that you may be pregnant, since X-rays can be harmful to the developing baby.

What if I am on a diabetic medication containing metformin?

If you are on a diabetic medication containing metformin, please inform the staff. The radiologist will be notified. Metformin should be discontinued for 48 hours after the procedure. Contact your primary physician after the scan. In those instances when metformin is discontinued, it should be reinstituted only when renal function has stabilized.

What are oral and rectal contrast?

Sometimes, because of known or suspected disease in the stomach or bowel, you may be asked to drink contrast mixture or water right before your CT scan. Sometimes contrast may be given in the rectum. This is only done if it is considered essential.

How do I prepare for my CT scan exam?

Read about how to prepare for your exam.

What if I still have questions or concerns?

You should discuss any remaining questions or concerns with either your referring doctor or with the staff of the Radiology Department, or read about our radiation safety.

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