Nonionic contrast agents are a group of contrast media that do not dissociate into charged particles. Many of the side effects of contrast agents are due to the injection of a solution with high osmolality compared to blood. Nonionic contrast agents have a low osmolality and tend to have less toxicity. However, as with all iodinated contrast agents, serious or fatal reactions have been associated with their use.
Due to their lower neurotoxicity, nonionic contrast agents can also be used for myelography. The only FDA approved agent, iodixanol, is iso-osmolar to blood, which makes it preferable for examinations of the central nervous system and cardiovascular studies. Because of its higher costs relative to other nonionic agents, the use is sometimes reserved for patients with high risk of nephrotoxicity.

See also Nonionic Dimer, Nonionic Monomer and CM Ratio.

Osmolality is the number of moles of osmotically active particles (molecules) present in solution per kilogram of water.
High osmolality of contrast agents is related to some of the adverse reactions.

Osmolality of iodinated contrast media:

Visipaque is an iso-osmolar contrast agent. The osmolality equal to that of blood and the sodium and calcium concentration in a ratio equivalent to blood provides high patient safety and comfort.

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.

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.