Types of Radiocontrast Media

Both high-osmolar contrast media (ionic) and low-osmolar contrast media (nonionic or organic) agents contain iodine and are administered intravenously. Most intravascular contrast media are derivatives of tri-iodobenzoic acid. The iodine molecule is an effective x-ray absorber in the energy range where most clinical systems operate. Iodinated contrast media are the most efficient products to enhance the visibility of vascular structures and organs during radiographic procedures. The ionic type creates more charged particles and causes a high osmolality in blood, which may cause a potentially life-threatening contrast media reaction in some individuals with medical conditions. The nonionic agents generate less dissociation and particles and decrease this risk, but are much more expensive. The nonionic contrast media are much more widely used today. The iodine concentration of contrast media is determined by the number of iodine molecules in milligrams present in a milliliter of a solution (mg/mL).2

Concentration of any contrast media agent determines how radiopaque the agent will be. The higher the iodine concentration, the better the chance that more x-ray photons will be absorbed. Therefore, that particular contrast agent may be more radiopaque than a comparable low-iodine concentrated agent.

The osmolality of a solution is the measurement of the number of molecules and particles in a solution per kilogram of water. In other words, osmolality can be described as a measurement of the number of molecules that can crowd out or displace water molecules in a kilogram of water. The radiographic significance of the osmolality value of contrast media is that it is higher than the osmolality value of blood plasma. Any solution that has an osmolality value greater than blood plasma is said to be a hyperosmolar solution. Therefore, ionic and nonionic contrast media are hyperosmolar solutions when compared to blood plasma.

Since certain radiographic procedures, such as myelography, cannot use ionic contrast media, the discovery of nonionic contrast media in 1974 (e.g., metrizamide) revolutionized these procedures.2