Mapping fiber orientation in human muscle by proton MR spectroscopic imaging.

Proton magnetic resonance spectroscopic imaging ((1)H-MRSI) was used to determine muscle fiber orientations in human calf muscles. The method is based on the fact that some resonances show orientation-dependent dipolar splitting, caused by incomplete motional averaging. This leads to proton spectra that depend strongly on the angle between muscle fibers and the magnetic field B(0). The orientation-dependent dipolar splittings were mapped using a fit with a basis set of predefined coupling patterns reflecting the fiber orientation. The fitted coupling patterns were displayed as images and assigned to different muscles based on segmented MR images. They showed gross differences in fiber orientation between some muscles, including m. soleus and m. tibialis anterior, for all subjects. In addition, smaller but significant differences between subjects were detected, which could be due to localization differences or real interindividual differences. Since dipolar splitting affects metabolite intensities, it is important to take this effect into account when calculating metabolite concentrations from MR spectra in muscle tissue. Spatial maps of the MR signals of trimethyl-ammonium groups and creatine/ phosphocreatine revealed significant differences in intensity between muscles.