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.