Computed Tomography (CT): How they work, when they’re used, benefits, and limitations

Computed Tomography (CT) Machine

A CT scan, also known as a computed tomography scan, is a diagnostic medical imaging test that uses X-rays and computer processing to produce detailed cross-sectional images of the body. Unlike a traditional X-ray, which only shows a single image, a CT scan can produce multiple images of the same area from different angles, providing a more comprehensive view of the body. 

How do CT scans work?

CT scans use X-rays and computer processing to produce detailed cross-sectional images of the inside of the body. The patient lies on a table that moves through the center of the CT scanner, and an X-ray tube rotates around the patient, emitting a series of narrow beams that pass through the body and are picked up by detectors on the other side. 

The computer processing combines the information from multiple X-ray beams to produce detailed images that can be stacked on top of each other to create a 3-dimensional representation of the body.

Which conditions are diagnosed using CT scan?

Computed Tomography (CT) scans are used to diagnose a wide range of conditions, and are particularly useful for visualizing internal organs, bones, soft tissue, and blood vessels. Some of the conditions that are best diagnosed using CT scans include:

  • Trauma: CT scans are often used in emergency situations to diagnose internal injuries and bleeding following an accident or trauma.
  • Abdominal conditions: CT scans are often used to diagnose conditions of the liver, pancreas, and other abdominal organs.
  • Bone injuries: CT scans can detect fractures, dislocations, and other injuries to bones, especially those that may not be visible on traditional X-rays.
  • Brain and nervous system conditions: CT scans can help diagnose issues such as strokes, blood clots, and brain tumors.
  • Cancer: CT scans can be used to detect tumors, monitor the growth or spread of cancer, and help plan radiation therapy or surgery.
  • Chest conditions: CT scans are useful for diagnosing conditions such as lung cancer, lung infections, and blood clots in the lungs.
  • Heart and blood vessel conditions: CT scans can be used to visualize the coronary arteries and detect blockages that may cause heart attacks.

What can I expect during a CT scan?

Here is what you can typically expect during a CT (Computed Tomography) scan:

  1. Preparation: You may need to remove any clothing, jewelry, or other items that may interfere with the scan. Depending on the type of scan, you may also need to drink a contrast material beforehand to help highlight specific areas of the body.
  2. Moving through the machine: The table will slowly move through the machine, and the X-ray tube will rotate around you. You may hear whirring or buzzing sounds, but the scan is painless.
  3. Holding still: During the scan, you will need to lie still and hold your breath for short periods of time, as directed by the technologist. This helps ensure that the images are clear and accurate.
  4. Length of time: A CT scan typically takes only a few minutes, although the exact length of time will depend on the type of scan being performed.
  5. After the scan: After the scan, you can return to their normal activities. If you drank a contrast material, you should drink plenty of fluids to help flush it out of their system.

Overall, CT scans are relatively quick and non-invasive, and most people find them to be relatively easy to tolerate. The technologist will be on hand to answer any questions and provide support throughout the process.

Benefits of CT

Computed Tomography (CT) scans offer several benefits over other imaging modalities:

  • Speed: CT scans are relatively fast and can produce images in just a few minutes. This makes them a useful tool in emergency situations, where rapid diagnosis is critical.
  • Detail: CT scans produce highly detailed images of the inside of the body, including bones, organs, and soft tissue. This allows for the precise visualization of internal structures and can help diagnose a wide range of conditions.
  • Multiplanar imaging: CT scans can produce images in multiple planes, including axial, coronal, and sagittal views. This provides a comprehensive view of the body and can help diagnose conditions that might not be visible on other types of imaging.
  • Three-dimensional imaging: CT scans can generate 3-dimensional images of the body, which can provide additional information and help with the planning of treatments such as surgery or radiation therapy.

Limitations of CT

While CT (Computed Tomography) scans offer several benefits over other imaging modalities, there are also several limitations to consider:

  • Radiation exposure: Although CT scans involve relatively low levels of ionizing radiation, repeated exposure to radiation can increase the risk of cancer and other harmful effects. This is particularly a concern for children, who are more sensitive to radiation.
  • Difficulty with soft tissue: CT scans are best suited to visualize bones and hard tissues, and may not provide adequate detail for some soft tissues, such as muscles and nerves. Other imaging modalities, such as MRI, may be more suitable for these types of tissues.
  • Cost: CT scans can be expensive, especially for complex procedures or those that require the use of contrast material. This can limit access to CT scans for people who do not have adequate insurance coverage.

Who should avoid CT?

There are certain individuals who may need to avoid CT (Computed Tomography) scans or limit their exposure to ionizing radiation:

  1. Pregnant women: CT scans are generally not recommended during pregnancy, as the ionizing radiation can be harmful to the developing fetus. If a CT scan is necessary, the risks and benefits should be carefully considered and alternative imaging modalities, such as ultrasound, may be used instead.
  2. Children: Children are more sensitive to ionizing radiation than adults and are at a higher risk of developing cancer from radiation exposure. If a CT scan is necessary for a child, the lowest possible dose of radiation should be used, and alternative imaging modalities should be considered when appropriate.
  3. People with kidney problems: CT scans often require the use of contrast material, which can be harmful to people with kidney problems. If a CT scan is necessary for someone with kidney problems, alternative imaging modalities or special precautions may be necessary to minimize the risk of harm.
  4. People with a known allergy to contrast material: If a person has a known allergy to contrast material, they may need to avoid CT scans that require the use of contrast material.
  5. People who have received multiple or high-dose radiation treatments: People who have received multiple or high-dose radiation treatments, such as for cancer, may need to avoid or limit their exposure to additional ionizing radiation from CT scans.

Your doctor can help you make an informed decision about whether a CT scan is appropriate for you.

About the Author

Dave Harrison, MD

Dr. Harrison is a board certified Emergency Physician with a part time appointment at San Francisco General Medical Center and is an Assistant Clinical Professor-Volunteer at the UCSF School of Medicine. Dr. Harrison attended medical school at Tufts University and completed his Emergency Medicine residency at the University of Southern California. Dr. Harrison manages the editorial process for