Abstract
Noninvasive measurement of absolute metabolite concentrations is essential for understanding the physiology of living systems. Surface-coil NMR spectroscopy can achieve this by double tuning the coil to a reference nucleus of known concentration (usually water 1H), as suggested by Thulborn and Ackerman in 1983. The spatial sensitivities of the two nuclei are matched by using identical nutation angles at the coil center for each nucleus; the ratio of signals from each nucleus is then proportional to the ratio of concentrations and independent of the particular sample geometry. Water 1H concentration is relatively constant from one tissue to another. Coil loading affects Q, sensitivity, and nutation angle. Circuit analysis of these effects is experimentally confirmed. The value of the variable matching capacitance gives Q, pulse length, and sensitivity for any particular sample. A practical scheme for measuring 31P concentrations, suitable for use by biologists, is presented. In vivo measurements of 31P metabolite concentrations in rat tissue are in good agreement with in vitro values measured by chemical analysis. Measurement of absolute concentrations without a reference nucleus is possible if the volume of interest can be made to fall entirely within the tissue, for example by using gradient localization.
Published Version
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