A i.e. Ionic monomers have three iodine atoms per two paicles in solution Ionic contrast agents dissociate (ionize) in water in 2 ioins (or osmotic paicles) per molecule whereas nonionic agents do not ionize or dissociate. All monomers contain I benzene ring with 3 iodine atoms and dimers contains 2 benzene rings with 6 iodine atoms per molecule. Therefore ionic monomers have 3 iodine atoms per 2 ionic/ osmotic paicles in solutionQ and a contrast agent ratio of 3:2 (i.e. 1.5). And ionic dimers have 6 iodine atoms per 2 ionic/osmotic paicles in solution with a contrast agent ratio of 6:2 or 3:1. Nonionic agents donot ionize so they have only 1 osmotic paicle per molecule. And similarly monomers contain 3 iodine atom and dimers contain 6 iodine atoms per molecule of contrast agent. Therefore contrast agent ratio of nonionic monomers is 3:1 and nonionic dimers is 6:1. High osmolar contrast agents (HOCM) are all ionic monomersQ; whereas low osmolar contrast agents (LOCM) may be ionic dimers, and nonionic monomers or dimers (i.e. both ionic & nonionic)Q. HOCM have osmolality in range of 1500 mosmols/kg water at concentrations of 300 mg I2/ml. Whereas LOCM have osmolality which is less than half of the osmolality of HOCM (i.e. 600-700 for nonionic monomer, 560 for ionic dimer and 300 for nonionic dimer). So compred to physiological osmolality of 300 mosmols/kg water, nonionic dimers are physiologically isotonic in solution at 300 mg iodine/ml. Normal plasma osmollity is 300 mosmols/kg water at iodine concentration of 300mg/ml. So LOCM means that osmolality is lower than the HOCM (not physiological). Lowest osmolality/osmolarity is seen in non ionic dimer agents which becomes almost physiologically isotonic or iso-osmolarQ (visipaque 320 is 290 mosmol/kg and isovist 300 is 320 mosmol/kgH2O; 320 & 300 are iodine concentrations). Osmolality is propoional to the ratio of iodine atoms to the number of paicles in solution. The contrast agent with lower ratio (3:2) are HOCM and they have more paicles in solution per iodine atom (or in other words less iodine atoms per paicle). And agents with higher ratio (3:1 or 6:2 and 6:1) are LOCM. Iohexol (omnipaque) is a nonionic monomer (LOCM with 3:1 ratio)Q Iodinated contrast agents have low lipid solubility, low toxicity, low binding affinities for protein, receptor or membranes, low moleculer wt & are very hydrophilic. On iv injection b/o high capillary permeability they all are distributed rapidly into extravascular, extracellular interstitial space (except in CNS)Q but do not enter blood or tissue cells. Pharmacokinetics of all extracellular MRI contrast agents (all gadolinium except Gd-BOPTA) are similar to iodinated water soluble contrast media. They donot cross the blood brain barrier unless the barrier is disruptedQ. These agents accumulate in tissues with abnormal vascularity (inflammation & malignancy) and in regions where BBB is disrupted.