An interleaved sampling strategy for MR spectroscopy in vivo: Applications on human calf musculature☆

Assessment of relaxation times, magnetization transfer rates, or apparent diffusion coefficients by volume selective 1H MR spectroscopy requires data from several single spectra with variable sequence parameters. Unintentional movements during the examination lead to inaccuracies, especially if the spatial distribution of concentrations is inhomogeneous. Improved comparability of the single spectra in a series recorded in vivo were obtained using a modified spectroscopic technique with INTerleaved ACquisiTion of multiple SPECtra (INTACTSPEC). INTACTSPEC series of spectra from the tibialis anterior muscle (m. tib. ant.), soleus muscle (m. soleus), and tibial bone marrow of 20 healthy volunteers were analyzed. Transverse relaxation times T2 of methylene signals in muscular lipid stores ranged from 77 ms (intramyocellular methylene component in m. tib. ant.) to 88 ms (intramyocellular methylene component in m. soleus) and were similar to those from yellow tibial bone marrow (T2 = 84 ms). Echo time-dependent signal intensities of choline and creatine deviated markedly from a monoexponential behavior in m. tib. ant., but were nearly exponential in m. soleus. Results from water diffusion measurements parallel and perpendicular to the axis of the lower leg showed significant differences between m. tib. ant. and m. soleus, probably due to the spatial orientation of the muscle fibers. Apparent diffusion coefficients along the leg axis were found to be higher in m. tib. ant. (2.10 ± 0.08 × 10−3 mm2/s) compared to m. soleus (1.78 ± 0.11 × 10−3 mm2/s), but m. soleus showed less restricted diffusion in perpendicular orientation (1.59 ± 0.19 × 10−3 mm2/s versus 1.20 ± 0.08 × 10−3 mm2/s in m. tib. ant.). Magnetization transfer experiments with various RF preparation pulse amplitudes led to very similar results for m. tib. ant. and m. soleus.