Estimating degree of mechanical anisotropy in dystrophic and control rectus femoris in boys using VisR ultrasound, in vivo

Previous work has shown that the degree of anisotropy in tissue can be assessed using Acoustic Radiation Force Impulse (ARFI) imaging performed with a geometrically asymmetric excitation. In this work, we investigate the clinical relevance of mechanical anisotropy in Duchenne muscular dystrophy (DMD). Anisotropy assessment was performed using relative elasticity (RE) and relative viscosity (RV) parameters measured by Viscoelastic Response (VisR) ultrasound. These parameters were evaluated as the ratio of their respective values obtained with two orthogonal transducer orientations, i.e. parallel and perpendicular to the muscle fiber direction, to represent the transverse over the longitudinal RE or RV. In a pilot clinical feasibility study performed in six, 7.9-10.4 year-old boys with (DMD) and five age-matched boys with no known neuromuscular disorders, VisR anisotropy imaging was performed in vivo in the rectus femoris (RF) serially for a total of 22 time points in DMD and 12 time points in control boys. Both the RE and RV anisotropy ratios in the RF muscles of boys with DMD were significantly higher (Wilcoxon, p<;0.05) than those of healthy control boys (Median [IQR] - RE: DMD = 1.51 [0.87], Control = 0.88 [0.69], RV: DMD = 1.04 [0.71], Control = 0.74 [0.25]). Higher RE and RV ratios suggested increased transverse relative to longitudinal elastic and viscous moduli in the DMD versus the control cohort, which is consistent with expected muscle fiber fragmentation, fibrosis, and fatty deposition in DMD. These results suggest that VisR RE and RV anisotropy ratios are clinically relevant for noninvasively assessing muscle degeneration due to DMD and other causes.