The purpose of this study was to evaluate temporal stability, multi‐center reproducibility and the influence of covariates on a multimodal MR protocol for quantitative muscle imaging and to facilitate its use as a standardized protocol for evaluation of pathology in skeletal muscle. Quantitative T2, quantitative diffusion and four‐point Dixon acquisitions of the calf muscles of both legs were repeated within one hour. Sixty‐five healthy volunteers (31 females) were included in one of eight 3‐T MR systems. Five traveling subjects were examined in six MR scanners. Average values over all slices of water‐T2 relaxation time, proton density fat fraction (PDFF) and diffusion metrics were determined for seven muscles. Temporal stability was tested with repeated measured ANOVA and two‐way random intraclass correlation coefficient (ICC). Multi‐center reproducibility of traveling volunteers was assessed by a two‐way mixed ICC. The factors age, body mass index, gender and muscle were tested for covariance. ICCs of temporal stability were between 0.963 and 0.999 for all parameters. Water‐T2 relaxation decreased significantly (P < 10−3) within one hour by ~ 1 ms. Multi‐center reproducibility showed ICCs within 0.879–0.917 with the lowest ICC for mean diffusivity. Different muscles showed the highest covariance, explaining 20–40% of variance for observed parameters. Standardized acquisition and processing of quantitative muscle MRI data resulted in high comparability among centers. The imaging protocol exhibited high temporal stability over one hour except for water T2 relaxation times. These results show that data pooling is feasible and enables assembling data from patients with neuromuscular diseases, paving the way towards larger studies of rare muscle disorders.

Background The majority of sports‐related injuries involve skeletal muscle. Unlike acute trauma, which is often caused by a single traumatic event leading to acute symptoms, exercise‐induced microtrauma may remain subclinical and difficult to detect. Therefore, novel methods to detect and localize subclinical exercise‐induced muscle microtrauma are desirable. Purpose To assess acute and delayed microstructural changes in upper leg muscles with multiparametric quantitative MRI after running a marathon. Study Type Longitudinal; 1‐week prior, 24–48 hours postmarathon and 2‐week follow‐up Population Eleven men participants (age: 47–68 years). Field Strength/Sequence Spin‐echo echo planar imaging (SE‐EPI) with diffusion weighting, multispin echo, Dixon, and fat‐suppressed turbo spin‐echo (TSE) sequences at 3T. MR datasets and creatine kinase (CK) concentrations were obtained at three timepoints. Assessment Diffusion parameters, perfusion fractions, and quantitative (q)T 2 values were determined for hamstring and quadriceps muscles, TSE images were scored for acute injury. The vastus medialis and biceps femoris long head muscles were divided and analyzed in five segments to assess local damage. Statistical Tests Differences between timepoints in MR parameters were assessed with a multilevel linear mixed model and in CK concentrations with a Friedman test. Mean diffusivity (MD) and qT 2 for whole muscle and muscle segments were compared using a multivariate analysis of covariance (MANCOVA). Results CK concentrations were elevated (1194 U/L [166–3906], P < 0.001) at 24–48 hours postmarathon and returned to premarathon values (323 U/L [56–2216]) at 2‐week follow‐up. Most of the MRI diffusion indices in muscles without acute injury changed at 24–48 hours postmarathon and returned to premarathon values at follow‐up (MD, RD, and λ3; P < 0.006). qT 2 values ( P = 0.003) and perfusion fractions ( P = 0.003) were higher at baseline compared to follow‐up. Local assessments of MD and qT 2 revealed more pronounced changes than whole muscle assessment (2–3‐fold; P < 0.01). Data Conclusion Marathon running‐induced microtrauma was detected with MRI in individual whole upper leg muscles and even more pronounced on local segments. Level of Evidence 2 Technical Efficacy Stage 3 J. Magn. Reson. Imaging 2020;52:407–417.

Objectives Assessment of the repeatability of diffusion parameter estimations in the upper leg muscles of healthy adults over the time course of 2 weeks, from a simultaneous bilateral upper leg DTI measurement. Methods SE-EPI DTI datasets were acquired at 3 T in the upper legs of 15 active adults at a time interval of 2 weeks. ROIs were manually drawn for four quadriceps and three hamstring muscles of both legs. The following DTI parameters were analyzed: 1st, 2nd, and 3rd eigenvalue (λ 1 , λ 2 , and λ 3), mean diffusivity (MD), and fractional anisotropy (FA). DTI parameters per muscle were calculated with and without intravoxel incoherent motion (IVIM) correction together with SNR levels per muscle. Bland-Altman plots and within-subject coefficient of variation (wsCV) were calculated. Left-right differences between muscles were assessed. Results The Bland-Altman analysis showed good repeatability of all DTI parameters except FA for both the IVIM-corrected and standard data. wsCV values show that MD has the highest repeatability (4.5% IVIM; 5.6% standard), followed by λ 2 (4.9% IVIM; 5.5% standard), λ 1 (5.3% IVIM; 7.5% standard), and λ 3 (5.7% IVIM; 5.7% standard). wsCV values of FA were 15.2% for the IVIM-corrected data and 13.9% for the standard analysis. The SNR (41.8 ± 16.0 right leg, 41.7 ± 17.1 left leg) and wsCV values were similar for the left and right leg and no left-right bias was detected. Conclusions Repeatability was good for standard DTI data and slightly better for IVIM-corrected DTI data. Our protocol is suitable for DTI of the upper legs with overall good SNR. Key Points • The presented DTI protocol is repeatable and therefore suitable for bilateral DT imaging of the upper legs. • Additional B1 + calibrations improve SNR and repeatability. • Correcting for perfusion effects improves repeatability. Keywords Diffusion tensor imaging. Magnetic resonance imaging. Hamstring muscles. Quadriceps muscle Abbreviations BFL Left biceps femoris long head BFR Right biceps femoris long head IVIM Intravoxel incoherent motion MD Mean diffusivity RFL Left rectus femoris RFR Right rectus femoris ROI Region of interest SML Left semimembranosus SMR Right semimembranosus SNR Signal-to-noise ratio STL Left semitendinosus STR Right semitendinosus VIL Left vastus intermedius VIR Right vastus intermedius Electronic supplementary material The online version of this article (