Elucidating viscoelastic effects on focused ultrasound thermal therapy with acoustic-solid-thermal coupling analysis

Focused ultrasound (FUS) therapy generates sufficient heat for medical interventions like tumor ablation by concentrating energy at the focal point. The complex viscoelastic properties of biological tissues pose challenges in balancing focusing precision and penetration depth, impacting the safety of surrounding tissues and treatment efficacy. This study develops an acoustic-solid-thermal coupling computational model to elucidate the dynamic mechanical response and energy dissipation mechanisms of soft tissue during FUS thermal therapy using a hyper-viscoelastic constitutive model. Results indicate that the high compressibility and low shear resistance of biological tissues result in a unique shear dissipation mechanism. Energy dissipation efficiency per area is indirectly influenced by load frequency via its effect on the dynamic shear modulus and is directly proportional to load amplitude. Focusing precision, represented by the focal zone width, is inversely controlled by frequency via wavelength. A mathematical model for evaluating temperature rise efficiency is proposed, and an optimal frequency for efficient FUS thermal therapy in brain-like soft materials is identified. This research elucidates the link between viscoelastic tissue behavior and FUS treatment outcomes, offering insights for optimizing FUS applications in various medical fields.