Dipolar coupling and ordering effects observed in magnetic resonance spectra of skeletal muscle.
Skeletal muscle is a biological structure with a high degree of organization at different spatial levels. This order influences magnetic resonance (MR) in vivo-in particular 1H-spectra-by a series of effects that have very distinct physical sources and biomedical applications: (a) bulk fat (extramyocellular lipids, EMCL) along fasciae forms macroscopic plates, changing the susceptibility within these structures compared to the spherical droplets that contain intra-myocellular lipids (IMCL); this effect leads to a separation of the signals from EMCL and IMCL; (b) dipolar coupling effects due to anisotropic motional averaging have been shown for 1H-resonances of creatine, taurine, and lactate; (c) aromatic protons of carnosine show orientation-dependent effects that can be explained by dipolar coupling, chemical shift anisotropy or by relaxation anisotropy; (d) limited rotational freedom and/or compartmentation may explain differences of 1H-MR-visibility of the creatine/phosphocreatine resonances; (e) lactate 1H-MR resonances are reported to reveal information on tissue compartmentation; (f) transverse relaxation of water and metabolites show multiple components, indicative of intra-, extracellular and/or macromolecular-bound pools, and in addition dipolar or J-coupling lead to a modulation of the signal decay, hindering straightforward interpretation; (g) diffusion weighted 31P-MRS has shown restricted diffusion of phosphocreatine; (h) magnetization transfer (MT) indicates that there is a motionally restricted proton pool in spin-exchange with free creatine; reduced availability or restricted motion of creatine is particularly important for an estimation of ADP from 31P-MR spectra, and in addition MT effects may alter the signal intensity of creatine 1H-resonances following water-suppression pulses; (i) transcytolemmal water-exchange can be studied in 1H-MRS by contrast-agents applied to the extracellular space; (k) transport of glucose across the cell membrane has been studied in diabetes patients using a combination of 13C- and 31P-MRS; and l residual quadrupolar interaction in 23Na MR spectra from human skeletal muscle suggest that sodium ions are bound to ordered muscular structures.
Boesch
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Department of Clinical Research, MR Spectroscopy and Methodology, University of Bern, Switzerland. chris.boesch@insel.ch
Kreis
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D016880:Anisotropy; D001692:Biological Transport; D003401:Creatine / Q000737:chemistry; D004058:Diffusion; D006801:Humans; D019344:Lactic Acid / Q000737:chemistry; D009682:Magnetic Resonance Spectroscopy; D008280:Magnetics; D018482:Muscle, Skeletal / Q000737:chemistry / Q000378:metabolism / Q000648:ultrastructure; D010725:Phosphocreatine / Q000737:chemistry; D012964:Sodium / Q000737:chemistry; D013654:Taurine / Q000737:chemistry
D010725:Phosphocreatine; D013654:Taurine; D019344:Lactic Acid; D012964:Sodium; D003401:Creatine