Computed Tomography (CT)

Computed Tomography (CT) is a medical imaging procedure that uses X-rays and computers to create detailed pictures of the inside of your body.

Overview

Computed Tomography (CT) is a special kind of X-ray imaging procedure that takes many pictures of your body from different angles. Think of it like a loaf of sliced bread — a CT scan can show each “slice” of your body one at a time, or it can put all the slices together to create a full 3-dimensional (3D) picture.

Here is how it works: A motorized platform rotates an X-ray source and detector around a circular, donut-shaped structure called a gantry. The patient lies on a bed that slowly moves through the center of the gantry. The X-ray tube rotates around the body, sending focused beams of X-rays through it. Special digital detectors on the opposite side of the X-ray source pick up the X-rays that pass through the body and send the information to a computer.

Each time the X-ray source completes one full rotation, the computer uses math to build a 2-dimensional (2D) image “slice” of the body. The thickness of each slice usually ranges from 1 to 10 millimeters. The computer can then stack all the slices together to create a 3D image showing the skeleton, organs, tissues, and any abnormalities. Learn more by watching this video: How does a CT scan work?

Modern CT machines take pictures in a helical (or spiral) fashion. This is faster and produces better quality 3D pictures than older non-helical CT machines, and can improve the detection of small abnormalities.

CT is a noninvasive procedure and does not cause any pain. The length of a CT procedure depends on the size of the area being scanned, but it usually lasts only a few seconds to a few minutes. Sometimes CT studies needing more time are also indicated. For most people, CT is performed on an outpatient basis at a hospital or radiology center, without an overnight hospital stay.

Related Terms:

  • Computed tomography is also known by several other names:
  • Helical CT (also called spiral CT)
  • CT colonography (also known as virtual colonoscopy)

Related Health Effects and Risks

Radiation Exposure and Cancer Risk

CT scans use X-rays, which are a form of ionizing radiation. Exposure to ionizing radiation is known to increase the risk of cancer. However, the risk of developing cancer from a single CT scan is generally small, and the benefits of a CT scan, especially when used to diagnose a serious condition like cancer, greatly outweigh any potential health risks. Talk with your doctor about the expected benefits and potential risks of an X-ray or CT procedure for you.

Everyone is exposed to some background level of naturally occurring ionizing radiation every day. Examples include radon from the ground and radiation from outer space. The average American receives an estimated effective dose of between 3 to 10 millisieverts (mSv) per year from natural sources, depending on where they live. By comparison:

  • Low-dose CT scan of the chest (1.5 mSv) is comparable to 2-6 months of natural background radiation.
  • Typical CT scan of the chest (6.1 mSv) is comparable to 7 months to 2 years of natural background radiation.
  • CT colonography (6 mSv) procedure delivers an effective dose of comparable to 7 months to 2 years of natural background radiation.
  • Whole-body CT delivers an effective dose of about 10 to 20 mSv, comparable to 1 to 7 times the unavoidable annual background dose.

Risks for Children

Children are more sensitive to radiation than adults because of their growing bodies and the rapid pace at which the cells in their bodies divide. Because children have potentially many more years of life ahead of them, there is a greater opportunity for them to develop radiation-related cancers.

Because children are more sensitive to ionizing radiation and have a longer life expectancy, parents may want to ask the technologist or doctor if the machine settings have been adjusted for children.

Risks During Pregnancy

For CT examinations that do not include the abdomen or pelvis, radiation exposure to the fetus is primarily from scattered radiation and is typically very low. When a CT scan is medically necessary, your healthcare team will consider the expected benefits and any potential risks to you and your baby. If imaging of these areas is needed, doctors prefer to use exams that do not use radiation, such as magnetic resonance imaging (MRI) or ultrasound. However, if neither of those can provide the answers needed, or there is an emergency, CT may be an acceptable alternative.

Diagnostic Uses

CT scans are used to identify disease or injury in many parts of the body. Here are some of the conditions CT can help find:

Head and Brain, and Spinal Cord

  • Injuries, tumors, and blood clots that can lead to stroke
  • Hemorrhage (bleeding in the brain)
  • Abnormal brain function or deposits in patients with cognitive impairment
  • Spinal conditions

Heart and Blood Vessels

  • Heart disease and abnormalities
  • Coronary artery disease (atherosclerosis), blood vessel aneurysms, and blood clots
  • Possible blockages in blood vessels

Lungs

  • Tumors
  • Pulmonary embolisms (blood clots in the lungs)
  • Excess fluid, emphysema, or pneumonia

Abdomen and Digestive System

Bones and Joints

  • Complex bone fractures
  • Bone density
  • Severely eroded joints
  • Bone tumors — CT usually produces more detail than a conventional X-ray for these conditions

Cancer

  • Screening for cancer
  • Helping diagnose the presence of a tumor
  • Providing information about the stage of a cancer
  • Determining exactly where to perform a biopsy
  • Guiding certain local treatments
  • Helping plan external-beam radiation therapy or surgery
  • Determining whether cancer is responding to treatment
  • Detecting recurrence of a tumor

Special Types of CT

Contrast Agents

Dense structures like bone are easily imaged by X-rays but soft tissues can be harder to see. Contrast agents have been developed to make certain areas of the body more visible on a CT scan.

  • Intravenous (IV) contrast agents are injected into the bloodstream. They help illuminate blood vessels and look for possible blockages, including those in the kidneys and heart.
  • Oral contrast agents, such as barium-based compounds, are used for imaging the digestive system. This includes the esophagus, stomach, and gastrointestinal (GI) tract.

In some patients, contrast agents may cause allergic reactions, or in rare cases, temporary kidney failure. Some people experience mild itching or hives. Symptoms of a more serious allergic reaction include shortness of breath and swelling of the throat or other parts of the body. People should tell the technologist immediately if they experience any of these symptoms so they can be treated promptly.

In patients with severely reduced kidney function or acute kidney injury, IV contrast may increase the risk of further kidney injury. Your healthcare team will weigh this potential risk against the benefits of the examination and may take steps to reduce the risk when appropriate. Kidney function can be checked with a simple blood test before injecting the contrast.

Cancer Screening with CT

Colorectal Cancer Screening

A procedure called CT colonography (also known as virtual colonoscopy) can screen for large colorectal polyps and tumors. Virtual colonoscopy uses CT to produce a series of pictures of the colon and rectum from outside the body. However, if polyps or other abnormal growths are found during a virtual colonoscopy, a standard colonoscopy is needed to remove them.

Lung Cancer Screening

Low-dose CT of the lungs is used to screen for lung cancer in adults who are at increased risk because of their smoking history. Annual lung cancer screening with low-dose CT is recommended for people ages 50 to 80 years who have a 20 pack-year or more smoking history and either currently smoke or quit smoking within the last 15 years.

Combined Positron Emission Tomography (PET)/CT

Combined PET/CT uses two imaging methods, CT and PET in one procedure. CT creates anatomic pictures of the organs and structures in the body. Then PET creates pictures that provide information about the chemical reactions taking place in tissues or cells. Cancer cells often use different metabolic pathways than normal cells.

Combining CT and PET may provide a more complete picture of a tumor’s location and growth or spread than either test alone. The combined procedure may make it easier to diagnose cancer, determine how far a tumor has spread, and plan and track response to treatment. Combined PET/CT may also reduce the number of extra imaging tests and other procedures a patient may need.

Preparing for a CT Scan

During a CT procedure, the person lies very still on a table, and the table passes slowly through the center of a large donut-shaped X-ray machine. The person might hear whirring sounds during the procedure. At times, the person may be asked to hold their breath and remain still to prevent blurring of the image.

CT is a noninvasive procedure and does not cause any pain. However, lying in one position during the procedure may be slightly uncomfortable. The length of a CT procedure depends on the size of the area being scanned, but it usually lasts only a few seconds to a few minutes, unless a longer procedure is necessary.

Some people are concerned about experiencing claustrophobia during a CT procedure. However, most CT scanners surround only portions of the body, not the whole body. So, people are not fully enclosed in a machine and are unlikely to feel claustrophobic.

Questions for Parents

When a child needs a CT scan, parents can ask their healthcare provider three key questions:

  1. Why is the test needed?
  2. Will the results change the treatment decisions?
  3. Is there an alternative test that doesn’t involve radiation?

If the test is clinically justified, parents can be reassured that the benefits will outweigh the small long-term risks.

Find a Clinical Trial

Clinical trials uncover better ways to prevent, diagnose, treat, and understand diseases and conditions. To ensure results apply to everyone, volunteers of all ages, sexes, and backgrounds, including both healthy individuals and those with specific medical conditions, are needed. Find clinical trials on computed tomography.

Community Support

See the X-ray topic page for information about reducing the risk of X ray technologies.

Note: This resource list is for informational purposes only. It is not comprehensive, and an organization’s inclusion does not constitute an endorsement by NIH.  

Research Information

The National Institutes of Health (NIH) is the largest public funder of biomedical research in the world. NIH invests most of its budget in medical research seeking to enhance life and to reduce illness and disability. NIH-funded research has led to breakthroughs and new treatments helping people live longer, healthier lives, and building the research foundation that drives discovery.  

NIH supports research to develop new CT technologies and reduce their risk to health.

Find NIH-funded research projects using NIH RePORTER, a searchable database of current and past research projects supported by NIH and other federal agencies.