Muscle elastography: Stress versus stiffness

Abstract

Dynamic elastography imaging, whether based on magnetic resonance, ultrasound or optical modalities, attempts to reconstruct quantitative maps of the viscoelastic properties of biological tissue, properties that are altered by disease and injury, as well as response to therapy. Reconstruction often assumes isotropy and homogeneity and neglects the effects of nonhomogeneous boundary conditions. Skeletal muscle violates all these assumptions, posing challenges and opportunities for developing better imaging biomarkers. Muscle activation and varying tensile loads can influence measurement interpretation. For bulk shear waves in an isotropic viscoelastic material not influenced by boundary conditions the measured wavelength correlates with the square root of the tissue’s shear elastic modulus and viscosity, important rheological parameters. But, under significant tensile loading the measured transverse wavelength will also be affected by the induced quasistatic stress field, making identification of the muscle’s inherent rheological properties seem impossible. Acoustoelastic theory, previous studies addressing this issue specifically for muscle, as well as new studies by our group are reviewed, identifying remaining challenges, potential strategies and opportunities for a more comprehensive understanding of both muscle structure and function based on noninvasive measurements. [Support acknowledged: NSF #1852691, NIH #AR071162.]