Effects of the operating magnetic field on potential NMR contrast agents

Paramagnetic metal ions have shown promise as contrast agents for nuclear magnetic resonance (NMR) imaging. Their ability depends upon modification of the relaxation times (T1 and T2) through dipolar interactions. These interactions cause the effectiveness of the agents to be sensitive to the operating magnetic field. Studies are presented of the operating field dependence (frequency dispersion) of two metal-chelate complexes, Gd+3-ethylenediaminetetraacetate (EDTA) and Mn+2-EDTA, in a physiologically balanced electrolyte solution. Inversion recovery experiments were performed on two concentrations of each metal-chelate complex at five resonant frequencies. The frequency dispersion curves were similar in appearance for those of the corresponding aqueous solutions. The Mn+2 complex showed no unusual concentration effects. The Gd+3 complex showed an unexpected concentration dependence in the dispersion behavior. This is attributed to a difference in the dipolar correlation time between the two solutions. With its unique correlation time in electrolyte solutions, predictions of relaxation rate changes in studies in vivo may be easier for the Mn+2-EDTA complex.