Interrelations of muscle functional MRI, diffusion-weighted MRI and31P-MRS in exercised lower back muscles

Exercise-induced changes of transverse proton relaxation time (T2), tissue perfusion and metabolic turnover were investigated in the lower back muscles of volunteers by applying muscle functional MRI (mfMRI) and diffusion-weighted imaging (DWI) before and after as well as dynamic 31P-MRS during the exercise. Inner (M. multifidus, MF) and outer lower back muscles (M. erector spinae, ES) were examined in 14 healthy young men performing a sustained isometric trunk-extension. Significant phosphocreatine (PCr) depletions ranging from 30% (ES) to 34% (MF) and Pi accumulations between 95% (left ES) and 120%–140% (MF muscles and right ES) were observed during the exercise, which were accompanied by significantly decreased pH values in all muscles (∆pH ≈ –0.05). Baseline T2 values were similar across all investigated muscles (approximately 27 ms at 3 T), but revealed right–left asymmetric increases (T2,inc) after the exercise (right ES/MF: T2,inc = 11.8/9.7%; left ES/MF: T2,inc = 4.6/8.9%). Analyzed muscles also showed load-induced increases in molecular diffusion D (p = .007) and perfusion fraction f (p = .002). The latter parameter was significantly higher in the MF than in the ES muscles both at rest and post exercise. Changes in PCr (p = .03), diffusion (p