Iodinated contrast agents (typically iodine-substituted benzene derivatives) are bound either in nonionic or ionic compounds. Ionic contrast agents consist of the negatively charged anion and the positively charged cation. Used components of the anion are for example diatrizoate, iodamide, iothalamate or metrizoate and of the cation the sodium or meglumine ion. The osmotic pressure depends on the number of particles in solution. Ionic contrast agents have a greater osmolarity; double that of nonionic contrast agents due to delivering more iodine atoms per molecule.
Ionic contrast agents were developed first and are still in use depending on the examination. Iodine based contrast media are water soluble and as harmless as possible to the body. However, ionic agents have more side effects compared to nonionic contrast agents due to their high osmolarity.

See also Ionic Dimer.

The first-generation contrast agents were all ionic monomers, consisting of a tri-iodinated benzene ring with 2 organic side chains and a carboxyl group. Diatrizoate or iothalamate are common iodinated anions, conjugated with a cation, sodium or meglumine. The ionization at the carboxyl-cation bond makes the agent water soluble.
Ionic monomers have the highest osmolality (high-osmolar contrast media (HOCM) possess an osmolality seven to eight times higher than plasma) of the different groups of contrast agents (CM ratio=1.5) and the lowest viscosity. The osmolality in solutions of ionic monomers ranges from 600 to 2100 mOsm/kg (human plasma = 290 mOsm/kg). These high osmolality is related to some of the adverse reactions. HOCM's have been widely replaced by newer contrast agents with improved tolerability and safety profiles.
Examples of HOCM's are Renografin®-60, Hypaque 76, Hypaque Meglumine, Hypaque Sodium and Conray®.

See also Ionic Contrast Agents.

Ionic dimers are used as x-ray contrast agents. They are formed by joining 2 ionic monomers and eliminating 1 carboxyl group and contain 6 iodine atoms for every 2 particles in solution (CM ratio=3).
Ionic dimers have an intermediate osmolarity, intermediate viscosity and an intermediate hydrophilicity between that of ionic and nonionic monomers. The only commercially available ionic dimer is ioxaglate (Hexabrix). Ioxaglate is used primarily for peripheral arteriography.