Dipolar resonance frequency shifts in 1H MR spectra of skeletal muscle: confirmation in rats at 4.7 T in vivo and observation of changes postmortem.
Non-isotropic contributions to 1H MR spectra from human skeletal muscle in vivo have recently been observed in the 0- to 5-ppm region. One pair of peaks has been identified to be subject to dipolar couplings. The corresponding changes in resonance frequency are related to the orientation of muscle fibers with respect to the external magnetic field and are analogous to the behavior of small molecules dissolved in liquid crystals. Image-guided localized spectroscopy based on the STEAM method has been applied to verify these phenomena in rat skeletal muscle in vivo and to investigate the effect postmortem. Residual dipolar couplings and anisotropic contributions to 1H MR spectra of skeletal muscle have been confirmed in animals and at a higher field strength--albeit with a slightly different spectral pattern compared to the human study. The most prominent dipolar doublet due to creatine and/or phosphocreatine vanishes postmortem with a rate similar to the disappearance of phosphocreatine, and is no longer observable 2 h postmortem.