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:

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.

(LOCM) Low-osmolar contrast media have a wide range of indications due to their lower side effects. The type of contrast media is an important risk factor for an adverse reaction.
LOCM have not completely replaced contrast media with higher osmolality due to their higher cost. Guidelines of professional organizations give recommendations for the selective use of low-osmolar contrast media for certain high-risk patients.
There are ionic and nonionic iodinated contrast materials with low osmolality available:
An adverse reaction occurs in low-risk patients who receive conventional ionic contrast agents more often than in high-risk patients who receive nonionic LOCM.

See also Contrast Enhancement, Biliary Contrast Agents, Safety of Contrast Agents and Contrast-Induced Nephropathy.

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.

Ultravist® is an iodinated contrast agent with low osmolality and low viscosity, enabling ease of administration and rapid iodine delivery rate. Ultravist® is available in 4 concentrations. Introduced in Europe 1985 and in the US in 1995, Ultravist® is being used today in over 100 countries worldwide.
See also Low-Osmolar Contrast Media.