Nuclear magnetic resonance (NMR) has been used as a spectroscopic method in physics and chemistry before it was developed to become a diagnostic imaging tool in medicine. When NMR Spectroscopy is applied to human tissue, metabolism can be studied in normal physiological and pathological states in vivo. Metabolite concentrations and rates can be monitored dynamically and with localisation of a defined region of interest. The "window" which is opened for observation, i.e. which quantities are measured, depends on the nucleus used for RF excitation.
Mechanisms of adenosine tri-phosphate (ATP) resynthesis, as a direct source of energy for muscle contraction, are phosphocreatine (PCr) splitting, glycolysis, beta-oxidation and, finally, oxidative phosphorylation. Whilst the dependency of these processes' fractional contribution to muscular energy supply on exercise type and duration is well known, quantitative models of the regulating mechanisms involved are still subject of current research. A large fraction of the established knowledge about metabolism is based on biochemical analysis of tissue acquired invasively (e.g. microdialysis and open-flow microperfusion) or representing averaged metabolic concentrations for the whole body (via serum metabolites or gas exchange analysis). Localised NMR spectroscopy, however, is capable of non-invasively acquiring time-resolved data from a defined volume of interest, in vivo.
In contrast to the vast majority of MRS studies investigating metabolism, where spectra of a single nucleus (commonly 1H, 31P or 13C) were acquired or several MR spectra with different nuclei were measured in separate experiments, this work opens an additional "window" on muscle metabolism by interleaved localised acquisition of 1H and 31P NMR spectra from human calf muscle in vivo, during rest, exercise and recovery, in a single experiment. Using this technique, the time courses of the concentrations of phosphocreatine, inorganic phosphate (Pi), ATP, total creatine, and lactate in human gastrocnemius muscle can be quantified directly, without using physiological model assumptions, while intracellular pH can be derived from the chemical shifts of Pi and PCr in 31P MR spectra. Quantifying intramuscular lactate by NMR spectroscopy is a challenging task, because the lactate signal is overlapped by strong lipid resonances and exhibits modulations which dependent on factors hard to control or measure, like muscle fibre orientation relative to the magnetic field, intra- and extracellular compartmentation and relaxation times.
The thesis is organised into an introduction to muscle physiology and methods for studying metabolism, followed by a theoretical chapter on localised NMR spectroscopy using STEAM and the localised double quantum filter (DQF) pulse sequence which was implemented for in vivo lactate detection. The theoretical sections are supplemented by an outline of the product operator formalism to describe the NMR spectroscopy experiments mathematically, in the appendix.
The second part describes the experiments which have been conducted at the 3 Tesla whole body NMR scanner installed at the "High Field MR Centre of Excellence", Vienna Medical University: The non-magnetic exercise rig constructed for activation of human calf muscle during NMR measurements is presented. Examples of basic muscle MR spectroscopy are given and the interleaved 1H and 31P STEAM sequence implemented on the NMR scanner is explained in detail along with its application during calf muscle exercise, employing the exercise rig.
Then the localised double quantum filter (DQF) pulse sequence is introduced which was developed for detection of the lactate CH3 resonance and suppression of overlaying lipid signals. Lactate quantification is only possible with this method due to the incorporation of knowledge about ordering effects published by other groups in the previous year. The final chapter of the experimental part comprises the time resolved interleaved localised DQF, 1H and 31P STEAM measurements during ischaemic calf muscle exercise using the exercise rig.
In conclusion, 31P MRS data and lactate concentrations measured simultaneously by 1H MRS help to confirm model assumptions about cellular proton buffering capacity. Questions on metabolic efflux can be addressed when studying lactate, pH and PCr changes during recovery from ischaemic exercise with high time resolution, as well as control of glycogenolysis and coordination of oxidative versus glycolytic ATP production in aerobic and ischaemic exercise. Perspectives are further improvement of specificity by multi nuclear spectroscopic imaging and an increase of sensitivity and time resolution by transferring the technique to an even higher field strength.