Nuclear magnetic resonance spectroscopy is governed by longitudinal (T1) relaxation. For protein and nucleic acid experiments in solutions, it is well established that apparent T1 values can be enhanced by selective excitation of targeted resonances. The present study explores such longitudinal relaxation enhancement (LRE) effects for molecules residing in biological tissues. The longitudinal relaxation recovery of tissue resonances positioned both down- and upfield of the water peak were measured by spectrally selective excitation/refocusing pulses, and compared with conventional water-suppressed, broadband-excited counterparts at 9.4 T. Marked LRE effects with up to threefold reductions in apparent T1 values were observed as expected for resonances in the 6-9 ppm region; remarkably, statistically significant LRE effects were also found for several non-exchanging metabolite resonances in the 1-4 ppm region, encompassing 30-50 % decreases in apparent T1 values. These LRE effects suggest a novel means of increasing the sensitivity of tissue-oriented experiments, and open new vistas to investigate the nature of interactions among metabolites, water and macromolecules at a molecular level.
Keywords: NMR spectroscopy; longitudinal relaxation enhancement; magnetic resonance spectroscopy; metabolites; neurochemistry.
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