Shear strain rate from phase contrast velocity encoded MRI: Application to study effects of aging in the medial gastrocnemius muscle

Background: Strain rate (SR) is a measure of the rate of regional deformation that can be computed by analyzing velocity-encoded phase-contrast 2D images. Recent studies have explored the changes in normal components of the strain tensor in aging muscle, while shear strain may also provide valuable information. Purpose: To compute the shear SR from velocity-encoded MRI of the lower leg and to study the correlation of SR parameters measured in the medial gastrocnemius (MG) to muscle force in a cohort of young and senior subjects. Study Type: Prospective cohort study. Subjects: Six young (26.1 ± 2.3 years) and six senior (76.7 ± 8.3 years) healthy females; two other subjects were scanned on three separate occasions for repeatability studies. Field Strength/Sequence: 1.5T using a single oblique sagittal slice with velocity-encoding in three directions (velocity-encoded phase contrast gradient echo sequence). Assessment: Age-related and regional differences in the SR eigenvalues (SR _fiber, SR _in-plane), normal SRs (SR _ff, SR _cc), and shear SRs (SR _fc, SR _{fc_max}) were statistically analyzed. Statistical Tests: Difference between young and senior cohorts were assessed using two-way analysis of variance (ANOVAs). The coefficient of variation and repeatability coefficient were calculated from repeat studies. Univariate and stepwise multivariable linear regression was performed to identify predictors of force. Results: During isometric plantarflexion contraction, SRs in the principal basis (SR _fiber, SR _in-plane) and maximum shear SR (SR _{fc_max}) was significantly lower in the senior cohort (P < 0.05). On multiple variable regression, maximum shear SR (SR _{fc_max}) and normal SR in the fiber cross-section (SR _cc) were significantly associated with force (R = 0.681, F = 14.034, P < 0.001). Data Conclusion: This study establishes that computation of shear strain is feasible and is a significant predictor of force variability with age. Level of Evidence: 2. Technical Efficacy: Stage 2. J. Magn. Reson. Imaging 2018;47:1351–1357.