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

Abstract

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 (31) P-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 < .01) and perfusion (p = .03) were strongly associated with T2,inc , and linear mixed model analysis revealed that changes in PCr and perfusion both affect T2,inc (p < .001). These findings support previous assumptions that T2 changes are not only an intra-cellular phenomenon resulting from metabolic stress but are also affected by increased perfusion in loaded muscles.

Effects of perfusion on DTI and DKI estimates in the skeletal muscle

“…Simulated data for Experiment I was generated using Equation [] and two biologically feasible values of f : 1) f = 5 % , that is, the average value we observed in vivo (see the MRI experiments section); and 2) f = 15%, that is, the highest value observed in our invivo data in muscle. Noteworthy, these values are in line with previous studies . Simulations for Experiment II were generated using Equation [] and four sets of biologically feasible values of f and K : 1) f = 5% and K = 3; 2) f = 15% and K = 3; 3) f = 5% and K = 3.5; and 4) and f = 15% and K = 3.5.…”

“…For this, we employed an extensive acquisition protocol, which could be shortened to be clinically compatible while still avoiding the bias of the tensor fit and quantify f and D*. These IVIM parameters can be of interest when investigating the dynamics of perfusion after exercises (17,38,39) or in presence of disease (55)(56)(57). Nonetheless, effects of perfusion can be minimized in alternative ways, including choosing different b-values.…”

In-vivo 31P-MRS of skeletal muscle and liver: A way for non-invasive assessment of their metabolism

“…However, due to their costs and/or specific application demands, there are many more built-in-the-lab devices used worldwide [20], [104], [131], [132], [133], [134], [135], [136], [137], [138], [139]. Basic designs apply mechanical or pneumatic workload settings.…”

In vivo measurement of membrane permeability and myofiber size in human muscle using time‐dependent diffusion tensor imaging and the random permeable barrier model

“…Vascular effects, with estimated perfusion fractions in muscle between 3 and 7% from intravoxel incoherent motion (IVIM) studies (39)(40)(41)(42)(43), are also not explicitly modeled in RPBM. On a qualitative level, neglecting perfusion should lead to overestimation of D 0 (free diffusivity in tissue is less than the measured weighted average between tissue and IVIM), as well as underestimation of the parameter ζ (the net effect of restrictions, a combination of S/V and κ), since the overall relative change of D(t) with t should be achieved by a greater relative change in the Table 2.…”