Magnetic field dependence of solvent proton relaxation in aqueous solutions of Fe3+ complexes

It might appear that the Fe3+ ion would be particularly useful as an agent for enhancing contrast in NMR images since it has a relatively large magnetic moment and occurs in vivo in a variety of forms. Moreover, the concentration of Fe3+ changes locally in certain disease states (e.g., beta-thalassemia) and in trauma (formation of methemoglobin), and can be altered in the gastrointestinal tract by the ingestion of readily available dietary supplements. However, the Fe3+ ion is insoluble above pH approximately 4, and soluble chelate and protein complexes of Fe3+ tend to sequester the ions from solvent; hence, the efficacy of Fe3+ ions for relaxing water protons ought to be low under typical physiological conditions. We report the magnetic field dependence of the relaxation rate of solvent protons (NMRD profiles) for solutions of a variety of Fe3+ complexes to demonstrate the phenomenology relevant to NMR imaging. From these data we make some estimates to show that, despite the low relaxation rates of solvent protons in solutions of Fe3+ complexes, certain observed changes in image contrast are consistent, quantitatively, with inferences that can be drawn from solution data.