Effect of focused ultrasound on shearwave production in a hyperelastic media

Focused ultrasound (FUS) is an emerging noninvasive modality for treating various medical conditions. It encompasses both therapeutic and diagnostic applications, utilizing ultrasound waves at different intensities. In diagnostic modalities, ultrasound energy is deposited at the focus to generate acoustic radiation force (ARF), resulting in the generation of shear stress and waves, which are utilized in elastography to evaluate the mechanical properties of tissue. However, therapeutic modalities utilizing higher intensities may lead to elevated shear stress levels. The shear stress induced in the focal region during FUS procedures can potentially affect biological processes, such as cell membrane permeability and gene regulation. To better understand the mechanical stress generated during FUS procedures, we developed a finite element model (FEM) to simulate sonication using a single-element FUS transducer. We modeled soft tissue using a neo-Hookean hyperelastic constitutive behavior, offering a more realistic representation of tissue behavior compared to the linear elasticity assumptions commonly employed in ultrasound-based elastography techniques. Operational parameters were varied to simulate different acoustic powers of the transducer by applying mechanical surface pressure at various operating frequencies. The model depicted FUS wave propagation with amplified surface pressure at the focus, generating relevant focal pressures consistent with clinical setups. The focal beam size within the soft tissue material was characterized and exhibited dependency on the operating frequency of the transducer. As the FUS wave converged at the focus, an ARF was exerted, resulting in displacement and induced shear stress around the focal region, which were quantified. The displacement and shear stress that were analyzed were dependent on the applied transducer surface pressure. These findings deepen the understanding of the mechanics of low-intensity FUS and provide valuable insights into its shear-related effects due to displacement and deformation of the media.