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Many x-ray techniques use the natural contrasts and varying densities that exist in body tissues representing air, water (in soft tissue), fat, and bone. The lungs and gastrointestinal (GI) tract normally contain air or gases. Other body structures are encased in a fatty envelope. Bone contains naturally occurring mineral salts. However, diagnosis of certain pathologic conditions requires visualization of details that cannot be revealed through plain x-rays. In these cases, details can be highlighted by the presence of contrast media in the area. These contrast substances can be administered through oral, rectal, or injection routes.

The ideal contrast agent should be relatively harmless (low toxicity, nonantigenic, nonallergenic, and inert), should not interfere with any physiologic functions, and should allow high and repeated dosing at a moderate cost. A contrast medium may be classified as either radiopaque (not permitting the transmission of x-rays) or radiolucent (permitting partial transmission of x-rays). The adverse pharmacodynamics of contrast media causes death in an estimated 1 of every 20,000–40,000 administrations. The most commonly used contrast agents are water-soluble noniodine agents for GI examinations and intravascular procedures. Ultimately, one must always be alert to the possibility of an adverse reaction to contrast media. Consequently, emergency supplies and equipment should be readily available when using these agents, along with trained personnel.

The following contrast agents are used routinely for x-ray studies:

  1. Alimentary canal contrast agents

    1. Water-soluble agents (e.g., diatrizoate meglumine and diatrizoate sodium)

    2. Water-insoluble agents (e.g., barium sulfate [BaSO4])

    3. Gases (CO2 gas, gas-producing calcium citrate, and magnesium citrate)

  2. Injectable contrast agents

    1. Nonionic iodinated contrast (low osmolar agents; e.g., iohexol)

    2. Ionic iodinated contrast (high osmolar agents; e.g., diatrizoate sodium)

  3. Specific-use agents

    1. Bile agents (e.g., iothalamate meglumine)

    2. Iodized oil contrast agents