  High Intensity Focused Ultrasound (HIFU) is a promising therapy for thermal
ablation and hyperthermia, characterised by it noninvasiveness, 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 bio-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. Inhomogeneous Helmholtz equation was coupled with Penne's bioheat
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 increased the volume of necrotic lesions for 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.
