Ans. a. Gadolinium (Ref: Grainger 5/e p32-51, 1249-1253; Sutton 7/e p419-421; Wolfgang 7/e p1147)Gadolinium is a non-iodine containing contrast."Gadolinium is a lanthanide. Gd-DTPA is classified as an acyclic, ionic gadolinium contrast medium Its paramagnetic property reduces the T1 relaxation time. Gadolinium based agents may cause a toxic reaction known as nephrogenic systemic fibrosis (NSF) in patients with severe kidney problems."Contrast used inCT scanMRIlodinatedQParamagnetic (non-iodinated): GadoliniumQ Contrast AgentsIonic Contrast AgentsNonionic Contrast Agents* Ionic contrast agents ionize in water in 2 ions per molecule* All monomers contain 1 benzene ring with 3 iodine atoms and dimers contain 2 benzene rings with 6 iodine atoms per molecule.* Therefore ionic monomers have 3 iodine atoms per 2 ionic/ osmotic particles 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 particles in solution with a contrast agent ratio of 6:2 or 3:1.* Nonionic agents do not ionize so they have only 1 osmotic particle 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 non ionic monomers is 3:1 and nonionic dimers are 6:1. High Osmolar Contrast Agents (HOCM)Low Osmolar Contrast Agents (LOCM)* HOCM are all ionic monomersQ* HOCM have osmolality in range of 1500 mosmols/kg water at concentrations of 300 mg lodine/ml.* Osmolality is proportional to the ratio of iodine atoms to the number of particles in solution.* The contrast agents with lower ratio (3:2) are HOCM and they have more particles in solution per iodine atom.* LOCM may be ionic dimers, and nonionic monomers or dimers (i.e. both ionic and nonionic)Q.* LOCM osmolality is less than half of the osmolality of HOCM.* As compared to physiological osmolality of 300 mosmols/kg water, non-ionic dimers are physiologically isotonic in solution at 300 mg lodine/ml.* 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/kg H2O: 320 and 300 are iodine concentrations).* Agents with higher ratio (3:1 or 6:2 and 6:1) are LOCM.* lohexol (omnipaque) is a nonionic monomer (LOCM with 3:1 ratio)Qlodinated contrast agentslodinated contrast agents have low lipid solubility, low toxicity, low binding affinities for protein, receptor or membranes, low molecular weight and are very hydrophilic.On IV injection due to 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 do not cross the blood brain barrier unless the barrier is disruptedQ. These agents accumulate in tissues with abnormal vascularity (inflammation and malignancy) and in regions where BBB is disrupted.Contrast enhanced CT scans use intravenous injection of iodinated contrast mediumQ contrast enhanced MRI use non iodinated contrast medium containing paramagnetic metal ions gadolinium (Gd3+), Copper (Cu2+) or manganese (Mn2+)Q.Gd is the most powerful (with 7 unpaired electrons) but unfortunately most toxic of these ions and therefore it is necessary to encapsulate it by a chelate such as DTPA (diethylene triamine pentaacetic acid salt) forming Gd-DTPA.Paramagnetic agents are mainly positive enhancers that reduce the T1 and T2 relaxation times, increase tissue signal intensity on T1 weighted images and almost have no effect on T2 weighted MR images.