Viscoelastic Response (VisR) assessment of longitudinal dystrophic degeneration in clinical Duchenne muscular dystrophy

Viscoelastic Response (VisR) imaging is an acoustic radiation force (ARF)-based ultrasonic technique for estimating the viscoelastic properties of tissue. It has been proposed as a method for monitoring degeneration in the skeletal muscles of boys with Duchenne muscular dystrophy (DMD). DMD causes progressive inflammation, necrosis, fibrosis and fatty deposition in muscle, all of which will alter the elasticity and viscosity of the tissue. The motivation of this work is to investigate VisR´s potential as method for monitoring dystrophic muscle degeneration, in vivo, in boys with DMD. In an ongoing longitudinal clinical study, muscles in the lower limbs of boys affected with DMD and age-matched healthy control boys are imaged using VisR thrice yearly for four years. A case study of serial imaging results in the Medial Gastrocnemius (GM) muscle of one boy with DMD is herein presented. Beginning at age 6.2 years, parametric VisR images of τ, or the ratio of viscosity to elasticity, show a growing region of high τ (>1.2 ms) over the span of one year. This result is consistent with expected progressive inflammation and fatty deposition early in the GM´s degenerative cycle. Over the course of the next four months, the area of high τ decreases, which is in agreement with the expected onset of muscle fibrosis. Then, at age 8.3 years, small and diffusely distributed high τ regions are observed in the muscle, consistent with expected distributed fatty depositions. These results suggest that VisR, a noninvasive ultrasound imaging method, may be clinically viable for monitoring local muscular compositional and structural changes associated with dystrophic degeneration, in vivo.