Relaxation of solvent protons by paramagnetic ions and its dependence on magnetic field and chemical environment: implications for NMR imaging.

Paramagnetic ions have been used to alter the magnetic relaxation rates 1/T1 and 1/T2 of solvent water protons since the first observations of a proton resonance signal almost four decades ago. The earliest theories of relaxation indicated that the influence of solute paramagnetic ions on relaxation rates of solvent protons should depend both on the chemical environment of the solute ions and on the magnetic field strength. Much knowledge, both experimental and theoretical, has since been amassed regarding relaxation effects in solutions of hydrated ions (aquoions) and of complexes of these ions with macromolecules, mainly proteins. The phenomena are well understood at this point, though the understanding is more retrospective than predictive for ion-protein complexes. Nonetheless, from what is now known about homogeneous solutions, and from current reports on the introduction of paramagnetic ions into tissue to alter contrast in NMR images by affecting relaxation rates, it is clear that the solution results are particularly germane, and transferable, to tissue investigations. The main features of relaxation of solvent protons in the presence of solute paramagnetic ions, as hydrated aquoions and complexed with protein, are presented here, with attention to those ions most relevant to in vivo studies, and with emphasis on the influence of the magnetic field and the chemical environment of these ions on solvent proton relaxation rates.