Contrast media are a group of chemical agents developed to aid in the characterization of pathology by improving the contrast resolution of an imaging modality. Specific contrast media have been developed for every structural imaging modality, and every conceivable route of administration.
Barium sulfate contrast media have been used for many decades, and are well-established as oral agents for the examination of the GI tract. Their use is generally restricted to radiographic and fluoroscopic examinations. Occasionally they are also used for CT examination of the GI tract (e.g. CT colonography in iodinated contrast media-allergic patients). They are cheap and well-tolerated by most patients, complications from their use are rare.
Iodinated contrast media are the mainstay contrast agents for use in radiographic, fluoroscopic, angiographic and CT imaging. They are a versatile group of agents used for intravenous, oral and other routes of administration, such as urethral and intra-articular.
MRI contrast media are most commonly gadolinium-based contrast agents (GBCAs), which are the agents used for the vast majority of contrast-enhanced MRI scans. Historically, they were occasionally used for vascular and CT scans but due to nephrotoxicity this use has been (largely) abandoned.
Ultrasound contrast media have been gaining traction in recent years for generally niche applications, e.g. characterization of liver lesions.