Shock waves and cavitation are key phenomena in high-intensity therapeutic ultrasound and blast-induced traumatic brain injury (bTBI), as their dynamics may lead to tissue damage, either deliberately in pathogenic tissue ablation (histotripsy), unintentionally in bTBI, or collaterally in kidney stone treatment (lithotripsy). While shocks and bubbles in water are relatively well-understood hydrodynamic phenomena, this knowledge cannot readily be extended to soft tissue because the viscoelasticity and heterogeneity at the macroscopic scales of injury (¡Â0.01−10 mm) and the wide range of loading frequencies in these medical applications (1− 1,000,000 Hz) strongly affect the physics. Furthermore, direct observations are hindered by the opacity of tissue and the difficulties of in vivo measurements. Recently, numerical simulations have emerged as a tool to complement and expand the scope of the traditional "theory and experiment" paradigm. However, cavitation and shocks are difficult to represent in current approaches for tissue simulations (e.g., finite element modeling for solid mechanics or nonlinear acoustics), while compressible flow codes are typically for inviscid materials. These physical and numerical drawbacks undermine efforts to ascertain tissue damage, thus inhibiting timely progress in therapeutic ultrasound and bTBI research. The objective of this project is to understand and predict the response of soft tissue to shocks and cavitation with realistic material and pulse properties. The hypothesis is that energy is transferred from the incoming shock and dissipated through different processes, including cavitation. In a departure from traditional tissue mechanics simulations, the PI will leverage his experience in compressible multiphase flows, transport phenomena, and high-performance computing to develop a numerical framework integrating (i) theoretical/ numerical modeling for spherical bubble dynamics (Rayleigh-Plesset-like equations) and (ii) direct simulations of the full equations of motion to accurately represent shocks and bubbles in general viscoelastic media. This high-order accurate shock- and interface-capturing framework will be validated against experiments, and uncertainties in the pulse and material properties will be quantified. Detailed simulations and analysis of relevant canonical problems (isolated shock propagation and bubble dynamics, freely collapsing bubbles, and shock interactions) in a medium exhibiting viscosity, elasticity and relaxation will be conducted. The relationship between the energy transfer and dissipation processes and the pulse and tissue properties will be established, and the injury mechanisms will be identified under realistic conditions.
In terms of the broader impacts, the results of the project will advance the general field of fluid mechanics and will guide medical researchers in their quest for safer, cost-effective procedures. The education and outreach program, closely integrated with the research, seeks to engage the general public and train future leaders in science, technology, engineering and mathematics (STEM). Undergraduate students will work primarily on one of the key research thrusts. In cooperation with the University of Michigan Natural History Museum, a series of demonstrations on acoustics and viscoelasticity will be developed for hands-on exhibits, culminating in workshops and campus visits for middle schools students from Southeast Michigan in the Museum's Science for Tomorrow program. The proposed research on elucidating the mechanical outcomes of shocks and cavitation in tissue will provide the foundation for subsequent studies of biological effects, e.g., changes in cell function caused by transient and highly variable pressures and displacements.
