[Cardiac imaging by nuclear magnetic resonance. II. Spectroscopy].

Nuclear magnetic resonance (NMR) spectroscopy is a new technique to study myocardial metabolism in living tissues by noninvasive means. The biochemical studies on intact hearts are performed in multinuclear NMR spectrometers using Fourier transform techniques for data acquisition. The biological NMR experiments preserve the tissues and can be repeated with a temporal resolution of seconds to several minutes in a reproducible fashion. As a marker of the intermediary metabolism P-31, C-13, H-1, F-19, N-14, Na-23, and K-39 isotopes are commonly used. NMR spectrum analysis permits the identification of several important substrates of the myocardial metabolism and their concentrational changes. In addition, the biophysical parameters of magnetic relaxation properties are measured. In some instances enzyme kinetics can be assessed. The disadvantage of NMR spectroscopy is the low sensitivity: only substrates with a intracellular concentration of greater than or equal to 0.5 mM can be detected. Improvements in sensitivity can be achieved, if the number of scans per spectrum and magnetic field strength are increased. The application of NMR spectroscopy in cardiovascular medicine is new and systematic studies on myocardial metabolism in vivo are not yet available. However, using P-31 MR spectroscopy several important results concerning the changes of the high energy phosphates and the intracellular pH changes during myocardial ischemia, as well as interesting insights into the regulatory principles of the cellular respiration were obtained. Similarly, C-13 NMR spectroscopy successfully described some aspects of glycogen metabolism and the kinetics of citric acid cycle in the myocardium. The clinical application of NMR spectroscopy appears feasible in a near future. The practical importance of this promising technique in clinical cardiology will depend on availability of the whole-body MR spectrometers, on the development of pertinent techniques for spatial MR signal resolution, and on our ability to uncover and to understand the biochemical principles of cardiac diseases. However, it is already today evident that MR spectroscopy successfully shifted the research interests towards biochemical processes at the cellular level as important causes and markers of cardiac diseases and extended our knowledge of the pathophysiology of the myocardium.