Contrast Agent Toxicity

Excerpt

Contrast agents, also called contrast media, are substances used to enhance the radiodensity of a targeted tissue by altering the way that electromagnetic radiation or ultrasound waves pass through the body. These substances can be administered to the patient orally, rectally, or intravenously. The type of contrast agent used depends on the modality and purpose of the imaging the patient will receive.

For patients undergoing radiographic imaging like x-rays or computed tomography (CT) scans, contrast agents are either iodine or barium-based. Iodinated contrast agents are classified based on their osmolality, ranging from approximately 300 to 1200 osmol/kg H2O. Because iodine is the radiopaque substance in all iodinated contrast agents, the radiopacity produced by the administration of these contrast agents depends on their concentration of iodine. Iodinated contrast agents are most frequently administered via intravascular injection, but the substance quickly redistributes to the extravascular space due to the capillary permeability of the contrast molecules.

Sometimes, iodinated agents are also employed as oral or rectal contrasts to enhance images of the stomach and bowels. However, barium sulfate is the predominant contrast agent in use for gastrointestinal imaging. Barium-based contrast agents are typically given as a solution of finely pulverized barium powder mixed into a slurry with approximately 236.5 to 473.2 mm of liquid. The patient orally ingests this solution or administers an enema into the rectum.

In the setting of magnetic resonance imaging (MRI), most contrast agents are based on chelated gadolinium. Unlike iodinated or barium contrast agents that attenuate x-rays to improve imaging, gadolinium contrast agents enhance the signal intensity of biologic tissues by decreasing the time it takes water protons to align with the magnetic field created by the imaging machine. The chelating agents in gadolinium contrast agents also allow the substance to remain in circulatory vessels longer than radiographic contrast agents before extravasating.

For ultrasound (US) imaging, the use of contrast agents is significantly less common compared to other imaging modalities. However, microbubble contrast agents consisting mainly of a gas core and a stabilized biological shell are sometimes used to enhance US images. These bubbles range between 1 to 10 uM in size, approximately the dimensions of a red blood cell, and are administered to the patient intravenously.

The use of contrast agents has become ubiquitous in recent clinical practice. With the proliferation of contrast usage, concerns about the toxicities of the different agents have increased rapidly as well.