Paramagnetic species relaxation agents

The observed proton relaxation rate, l/Ti ohs is the sum of a diamagnetic contribution, ITi d, and the paramagnetic relaxation rate enhancement, 1 Tip, which is linearly proportional to the concentration of the paramagnetic species, [Gd]. This relation is valid in dilute solutions and in the absence of intermolecular interactions between the contrast agent molecules. Given the... 

The contrast media for MRI, according-to their effect on image enhancement, are categorized as (l)relaxivity-based Tl agents and (2) susceptibility-based T2 agents (96, 97). The term where i = 1, 2, represents the rate of relaxation of solvent nuclei. Relaxivity (Ri) is obtained as the slope of relaxation rate per 1 mM of paramagnetic species, in the unit of mM s It represents the efficiency with which the contrast agent enhances the proton relaxation rate of water and is defined as follows (94) ... 

Rates of nuclear relaxations can be altered by the deliberate addition to the solutions used for NMR spectroscopy of species containing paramagnetic nuclei. The usual relaxation agent employed with substrates dissolved in organic solvents is the chelate chromium tris(acetylacetonate) (Cr(acac)3), [Me C6 CH C(6)Me]3Cr, i.e. chromium(III)-2,4-pentane-dionate. It acts to reduce values of via dipole-dipole interactions, leaving chemical shift values unaltered. Cr(acac)3, when used at concentrations of about 0.05 M, substantially reduces values of T, particularly those for quaternary nuclei simultaneously it removes the nuclear Overhauser enhancement from proton-decoupled and other resonances. Use of this reagent could therefore considerably shorten the times required for quantitative NMR measurements. 

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