CT Scan

What It Is

In a CT scanner, an x-ray tube rotates around the body while detectors on the opposite side measure how much of the x-ray beam passes through. A computer uses these measurements to create cross-sectional images, which can also be combined into three-dimensional views.

Different tissues absorb x-rays differently. Bone absorbs a lot and looks white, air looks black, and soft tissues appear in shades of gray. Contrast material can make blood vessels and some organs stand out more clearly.

CT is fast and widely available, which makes it useful when quick information is needed, such as after an injury. Because it uses radiation, clinicians weigh the expected benefit of the information against the small potential risk, especially for children and for people who need repeated scans.

Common Terminology

Computed tomography (CT)

An imaging method that uses x-rays and computer processing to make cross-sectional images.

Contrast (iodinated)

An iodine-based material given by vein, or a different contrast taken by mouth, to highlight certain structures.

CT angiography (CTA)

A CT exam focused on blood vessels, usually using IV contrast.

Radiation dose

The amount of radiation energy absorbed during the scan, kept as low as reasonably achievable.

Density

How much a tissue blocks x-rays, described in reports as hypodense, hyperdense, or isodense.

Low-dose CT

A CT technique using less radiation, used for certain purposes such as lung cancer screening.

Why the Test Is Done

Clinicians order CT scans for many reasons. Common general uses include:

• Evaluating injuries, including head trauma, internal bleeding, and broken bones.
• Investigating causes of abdominal pain, such as appendicitis or kidney stones.
• Examining the lungs and chest, including for infections, blood clots, or nodules.
• Looking at blood vessels with CT angiography.
• Detecting, staging, or monitoring tumors and guiding some biopsies or procedures.
• Screening in specific situations, such as low-dose CT for lung cancer in certain higher-risk adults.

What the Results Generally Describe

A CT report from a radiologist may describe, in general terms:

• The type of exam, the body area, and whether contrast was used.
• The appearance, size, and density of organs and structures.
• Any areas that look different from expected, with measurements.
• Comparison with prior imaging, when available.
• An "Impression" summarizing key findings; results are interpreted by the radiologist and ordering clinician, and there is no single normal range.

How the Test Is Done

A CT scan generally involves these steps:

• The person may change into a gown and remove metal items in the area being scanned, since metal can affect the images.
• If IV contrast is planned, an IV line is placed; oral contrast, if used, is taken beforehand.
• The person lies on a table that moves through a ring-shaped scanner called a gantry.
• The technologist may give breathing instructions, such as holding the breath briefly.
• IV contrast can cause a brief warm feeling or a metallic taste.
• The scan itself often takes only seconds to a few minutes.

Preparation and What to Expect

General preparation and safety points include:

• Some exams with contrast involve not eating for a few hours beforehand; facilities give specific instructions.
• Screening questions cover pregnancy, prior contrast reactions, allergies, kidney function, and diabetes medications.
• Clinicians generally avoid unnecessary repeat scans and use dose-reduction techniques, especially for children.
• Most people can return to normal activities right after the scan.
• Allergic-type reactions to iodinated contrast are uncommon; severe reactions are rare and imaging staff are prepared to respond.

When Results Are Discussed With a Clinician

CT results are generally reviewed with the ordering clinician. Topics often discussed include:

• What the main findings mean in the context of symptoms and history.
• Whether any findings are incidental and whether follow-up is suggested.
• Whether additional tests, such as MRI or ultrasound, may add useful information.
• Radiation considerations if more imaging is planned.
• Any unfamiliar terms in the report.

Questions to Ask a Healthcare Professional

General questions people often find useful to bring to an appointment:

• Why is a CT scan being recommended for this situation?
• Will contrast be used, and is any preparation needed?
• How is the radiation dose kept as low as possible?
• Are there alternatives, such as MRI or ultrasound?
• How long will the scan take?
• When will the results be available?

Sources

This guide was written in plain language from the following authoritative references.

1. CT Scans — MedlinePlus (U.S. National Library of Medicine)
2. Computed Tomography (CT) - Body — RadiologyInfo.org (RSNA and ACR)
3. Computed Tomography (CT) — U.S. Food and Drug Administration (FDA)

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