Detection and imaging of proton chemical exchange in biological tissues using nuclear magnetic resonance

Direct detection of metabolites using N.M.R. is a relatively insensitive technique. Using indirect detection, proton-proton chemical exchange between water and metabolites can be used to enhance the N.M.R. output signal. In the present studies the N.M.R. technique of saturation transfer is used to detect chemical exchange reactions in the slow to intermediate range, and to quantify the presence of metabolites in very low concentrations. The first step in this work was to demonstrate chemical exchange of metabolites in the presence of macromolecules for a model system consisting of ammonia as the chemically exchanging metabolite in low concentration and agarose as the macromolecule. Subsequently using saturation transfer, the ammonia protons were irradiated and a decrease in water's spin lattice relaxation time, T;The experimental results are: (1) chemical exchange can be used to determine the concentration of metabolites even in the presence of macromolecules in model systems, (2) saturation transfer amplifies the N.M.R. signal about 100 fold as compared to direct detection, (3) saturation transfer can be used to image an ex vivo kidney and (4) it was found that urea is a major component for proton-proton chemical exchange in the kidney. Suggestions are made to apply this to in vivo tissue and at different field strengths.