• Vadim Malis, Usha Sinha, Robert Csapo, Marco Narici, Edward Smitaman, and Shantanu Sinha.
• Muscle Imaging and Modeling Lab, Department of Radiology, UC San Diego, San Diego, California, USA.
• J Magn Reson Imaging. 2019 Jun 1; 49 (6): 1655-1664.

BackgroundDiffusion tensor imaging (DTI) assesses underlying tissue microstructure, and has been applied to studying skeletal muscle. Unloading of the lower leg causes decreases in muscle force, mass, and muscle protein synthesis as well as changes in muscle architecture.PurposeTo monitor the change in DTI indices in the medial gastrocnemius (MG) after 4-week unilateral limb suspension (ULLS) and to explore the feasibility of extracting tissue microstructural parameters based on a two-compartment diffusion model.Study TypeProspective cohort study.SubjectsSeven moderately active subjects (29.1 ± 5.7 years).Field Strength/Sequence3T, single-shot fat-suppressed echo planar spin echo sequence.AssessmentSuspension-related changes in the DTI indices (eigenvalues: λ1 , λ2 , λ3 , fractional anisotropy; coefficient of planarity) were statistically analyzed. Changes in model-derived tissue parameters (muscle fiber circularity and diameter, intracellular volume fraction, and residence time) after suspension are qualitatively discussed.Statistical TestsChanges in the DTI indices of the MG between pre- and postsuspension were assessed using repeated-measures two-way analysis of variance (ANOVA).ResultsAll the eigenvalues (λ1 : P = 0.025, λ2 : P = 0.035, λ3 : P = 0.049) as well as anisotropic diffusion coefficient (P = 0.029) were significantly smaller post-ULLS. Diffusion modeling revealed that fibers were more circular (circularity index increased from 0.55 to 0.95) with a smaller diameter (diameter decreased from 82-60 μm) postsuspension.Data ConclusionWe have shown that DTI indices change with disuse and modeling can relate these voxel level changes to changes in the tissue microarchitecture.Level Of Evidence2 Technical Efficacy: Stage 1 J. Magn. Reson. Imaging 2018.© 2018 International Society for Magnetic Resonance in Medicine.

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