This research will develop a new experimental technique to measure the complex properties of soft, fibrous materials, non-invasively and non-destructively. Muscle, tendons, and brain tissue are examples of such materials, which are found throughout the human body and in nature. Artificial fiber-reinforced, soft biomaterials are increasingly being used in engineering applications. It is critical to know the mechanical properties of such materials, such as their stiffness and ability to store and dissipate energy. In this project, a novel combination of magnetic resonance imaging and high-intensity focused ultrasound will be used to comprehensively measure the mechanical properties of soft fibrous materials, either inside the human body, or in a controlled environment. Benefits to society include the improved ability to diagnose injury and fibrotic disease, to design and evaluate artificial tissues, and to perform accurate computer simulations of traumatic brain injury. In terms of workforce development, researchers from engineering and imaging will work together to build, test, and demonstrate the new technology. Graduate students will develop a set of sophisticated research skills that span these two disciplines. Leveraging a summer research program, as well as summer and weekend workshops, this project will provide training and research experience at the intersection of engineering and imaging to undergraduate students and middle/high school students from diverse backgrounds.
MR imaging of harmonic ultrasound-induced motion will be developed and applied to accurately measure the complex, anisotropic, nonlinear behavior of soft tissue and fibrous biomaterials. First, parameters of linear, anisotropic (transversely isotropic and orthotropic), viscoelastic models of fibrous soft biomaterials will be estimated from slow (pure transverse) and fast (quasi-transverse) shear waves. Shear waves, with varying propagation and polarization directions relative to material symmetry axes, will be induced by focused ultrasound and imaged by MRI in artificially aligned biomaterials, in muscle, and in white matter brain tissue. Shear moduli and tensile moduli will be estimated by fitting speeds of directionally-filtered plane waves to analytical expressions and simulated results. Second, the nonlinear behavior of fibrous, soft materials will be characterized. Two types of nonlinearity will be explored. (i) Nonlinearity in the small-strain regime, arising from strong material nonlinearity, will be measured by quantifying the higher harmonic components of wave motion. (ii) Nonlinear, large-strain behavior will be characterized by imaging slow and fast shear waves superimposed on large deformations. This novel approach is expected to provide comprehensive characterization of anisotropy and nonlinearity, with unprecedented resolution, throughout these increasingly-important materials.
