Magnetic field dependence of solvent proton relaxation induced by Gd³⁺ and Mn²⁺ complexes

The effect of solute paramagnetic ions on the longitudinal magnetic relaxation rate 1/T¹ of solvent water protons depends on magnetic field strength and on the chemical environment of the ions. The variation of 1/T¹ with field has been measured for solutions of Gd³⁺ and Mn²⁺ ions in three grossly different environments near physiological pH: the hydrated aquoion; chelated by EDTA (ethylenediaminetetraacetic acid) and DTPA (diethylenetriaminepentaacetic acid); and bound to the protein concanavalin A. It is demonstrated that over the field range at which NMR imaging is currently being done, the chemical environment can alter 1/T¹ of solvent protons by more than an order of magnitude. The relevance of these results to the potential utility of these ions as agents for enhancement of contrast in NMR images is discussed.