Pulsed Ultrasound Assisted Thermo-Therapy for Subsurface Tumor Ablation: A Numerical Investigation

High-intensity focused ultrasound (HIFU) is a promising therapy for thermal ablation and hyperthermia, characterized by its non-invasiveness and high penetration depth. Effective HIFU thermo-therapy requires the ability to accurately predict temperature elevation and corresponding thermal dose distribution in target tissues. We report a parametric numerical study of the thermal response and corresponding of thermal dose in a soft tissue in response to ultrasound. We compared the predictions of tissue models with two, three, and seven layers, to ultrasound-induced heating at duty cycles ranging from 0.6 and 0.9. Further, two tumor sizes and transducer powers (10 W and 15 W) were considered. The inhomogeneous Helmholtz equation was coupled with the Pennes bio-heat equation to predict heating in response to pulsed ultrasound. Necrotic lesion size was calculated using the cumulative equivalent minute (CEM) thermal dose function. In-vitro experiments were performed with agar-based tissue phantoms as a preliminary validation of the numerical results. The simulations conducted with the seven-layered model predicted up to 33.5% lower peak pressure amplitude than the three-layered model. As the ultrasound pulse width decreased with the equivalent sonication time fixed, the corresponding magnitude of the peak temperature and the rate of temperature rise decreased. Pulsed ultrasound resulted in the increased volume of necrotic lesions for an equivalent time of sonication. The findings of this study highlight the dependence of HIFU-induced heating on target geometry and acoustic properties and could help guide the choice of suitable ultrasound exposure parameters for further studies.