<prompt> Which of the following statements about contrast in radiography is true:Ionic monomers have three iodine atoms per two paicles in solutionOsmolar contrast agents may be ionic or non ionicGadolinum may cross the blood brain barrierlohexol is a high osmolar contrast media  <response>: 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 ionicosmotic 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 nonionicQ. HOCM have osmolality in range of 1500 mosmolskg water at concentrations of 300 mg I2ml. 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 mosmolskg water, nonionic dimers are physiologically isotonic in solution at 300 mg iodineml. Normal plasma osmollity is 300 mosmolskg water at iodine concentration of 300mgml. So LOCM means that osmolality is lower than the HOCM not physiological. Lowest osmolalityosmolarity is seen in non ionic dimer agents which becomes almost physiologically isotonic or iso-osmolarQ visipaque 320 is 290 mosmolkg and isovist 300 is 320 mosmolkgH2O; 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 ratioQ 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 bo high capillary permeability they all are distributed rapidly into extravascular, extracellular interstitial space except in CNSQ 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. 