Interrelations of muscle functional MRI, diffusion-weighted MRI and 31P-MRS in exercised lower back muscles

Exercise-induced changes of transverse proton relaxation time (T-2), 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 P-31-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 (Delta pH approximate to-0.05). Baseline T-2 values were similar across all investigated muscles (approximately 27 ms at 3 T), but revealed right-left asymmetric increases (T-2,T-inc) after the exercise (right ES/MF: T-2,T-inc = 11.8/9.7%; left ES/MF: T-2,T-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 < .01) and perfusion (p = .03) were strongly associated with T-2,T-inc, and linear mixed model analysis revealed that changes in PCr and perfusion both affect T-2,T-inc (p < .001). These findings support previous assumptions that T-2 changes are not only an intra-cellular phenomenon resulting from metabolic stress but are also affected by increased perfusion in loaded muscles. Copyright (C) 2014 John Wiley & Sons, Ltd.