What Is a Contrast Agent?

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  • Written By: Mary McMahon
  • Edited By: Bronwyn Harris
  • Last Modified Date: 27 September 2019
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A contrast agent is a substance which is used in a medical imaging study to make it easier to see the internal structures of the body. Numerous different types of contrasts are used; they are sometimes called “dyes” because they appear to dye the tissue which is being studied. If you are scheduled for medical imaging, a radiologist will discuss the use of contrasts with you if one is required. Most agents are very safe, and they will be expressed from the body very quickly after the studies; in rare cases, someone has an allergic reaction.

One of the most common types of contrast agent is an ingested agent like barium sulfate, which is taken by mouth for studies of the gastrointestinal tract. This substance is radioopaque, meaning that x-rays cannot pass through it. As a result, the x-ray film is not exposed, and the doctor has a bright white picture of the patient's intestines. For MRIs, a substance which affects the magnetism of the tissue of interest is used.

A contrast agent can also be injected. A wide variety of agents are used for injection with various types of imaging machines. Injected agents can sometimes be uncomfortable for patients; they may cause a burning sensation or a strange feeling, but they are usually safe. Such agents can illuminate the inner workings of the brain and central nervous system with incredible detail.


Contrast agents may also be inserted through the rectum or inhaled for very rare imaging studies. Generally, a radiologist decides on what type of contrast should be used, although the patient record will be consulted to check for any potential allergic risks. The contrast may be given to the patient right before the study, or he or she may be given some additional time to ensure the agent reaches the desired destination; once the contrast agent has suffused the area of interest, the radiologist takes images of the site for later analysis.

The use of a contrast agent is generally very safe for the patient, as most radiologists want to avoid harmful reactions. However, allergic reactions do happen. If an imaging study with contrast is ordered, ask the radiologist about common side effects, and alert him or her if you begin to feel unwell. If the radiologist feels that you may be having an adverse reaction, he or she will take the appropriate steps to treat you.


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