Influence of paramagnetic ions and pH on proton NMR relaxation of biologic fluids.

The extent to which various concentrations of the paramagnetic metal ions [gadolinium (III), manganese (II), chromium (III), iron (III), nickel (II), copper (II), and cobalt (II)] affect proton magnetic relaxation times of distilled water, 4% human serum albumin (HSA), and dog plasma was studied in vitro. The pH of water and HSA varied from 4 to 8. Nuclear magnetic resonance relaxation parameters, T1 and T2, were measured at 10.7 MHz using inversion recovery and spin-echo radiofrequency sequences, respectively. The presence of Mn(II), Gd(III) and Cr(III) in water significantly reduced T1, while Fe(III), Ni(II), Cu(II) and Co(II) had only a minimal effect. In 4% HSA and dog plasma Mn(II) and Cu(II) had the greatest effect on T1. At neutral pH, Gd(III) and Cr(III) had little effect on T1, while Mn(II) induced a large shortening of T1. All of the metal ions changed T2 less than T1. These differences in proton relaxation enhancement caused by the various ions in the three solutions studied are due to variations in the effective magnetic moment, the degree of binding of the ions to protein, and the chemical form of the ion associated with changes in pH. Thus, it is impossible to predict the effect of metal ions on proton relaxation in vivo based solely on in vitro studies, because of the complexity of various biologic fluids in vivo.