Contrast Dye Used for X-Rays and CAT Scans

Contrast agents are used for some imaging studies. Fuse / Getty Images

Contrast dye is a solution that is used to accentuate specific structures when looking at a body image.  Radiocontrast agents are substances that are used in studies such as x-rays, fluoroscopy, and computed tomography (CT) scans.  MRIs use other agents that help to accentuate the magnetic properties of a part of the body.

Radiographic Agents

In x-ray and CT studies, the radiocontrast agents are substances that absorb the x-ray photons, and does not allow them to be detected by the x-ray film or CT scanner.

An example commonly used is the element barium, that is delivered in the compound barium sulfate.  Contrast dye can be injected into your blood vessels (to show the vessels), it can be ingested orally (to show the upper gut), or inserted into the rectum (to show the lower gut). In some CT scans, all three types of contrast (so-called "triple contrast") are used.

In orthopedics, the most common use of radiographic agents is to inject the agent into a joint, or space within the body.  Often the space is identified with the radiographic agent to confirm a needle is in proper position before a medication is injected.

MRI Contrast Agents

MRI contrast works differently, but also accentuates the differences between tissues. The MRI contrast alters the magnetic properties of tissue. The altered properties will differentiate tissue types on the MRI image.

The most commonly used MRI contrast agent is the element gadolinium.

  As with radiographic agents, gadolinium can be injected into the blood vessels or injected into a joint.  Gadolinium enhanced MRI scans can be very helpful at showing subtle findings such as labral tears in the shoulder and hip cartilage damage.  Without the contrast agent added, these more subtle findings may not be seen.

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