Temperature Dependence of Tissue Thermal Parameters Should Be Considered in the Thermal Lesion Prediction in High-Intensity Focused Ultrasound Surgery

Coupled thermo-mechanical interaction on a multi-layered skin tissue with temperature-dependent physical properties irradiated by a pulse laser

A detailed understanding of the coupled thermo-mechanical interaction on the biological tissue irradiated by a pulse laser is essential for the existed therapeutic methods constructed on the photo-thermal effect, which will contribute to the design, characterization and optimization of strategies for delivering better treatment. The aim of present work is to explore the coupled thermo-mechanical behavior of a multi-layered skin tissue with temperature-dependent physical properties under the pulsed laser irradiation. A layered theoretical model involved variable physical parameters with temperature has been proposed firstly according to the generalized theory of thermo-elasticity with dual-phase lag mechanism. The numerical method based on an explicit finite difference scheme is then employed to predict the temporal and spatial distributions of the temperature, thermal deformation and stresses experienced to a short-pulse laser irradiation. On this basis, the effect of variable thermal and mechanical physical parameters of skin tissue on the coupled thermo-mechanical behavior and relative thermal damage has been evaluated.

Bio-thermal response and thermal damage in biological tissues with non-equilibrium effect and temperature-dependent properties induced by pulse-laser irradiation

Comprehension of thermal behavior underlying the living biological tissues helps successful applications of current heat therapies. The present work is to explore the heat transport properties of irradiated tissue during tis thermal treatment, in which the local thermal non-equilibrium effect as well as temperature-dependent properties arose from complicated anatomical structure, is considered. Based on the generalized dual-phase lag (GDPL) model, a non-linear governing equation of tissue temperature with variable thermal physical properties is proposed. The effective procedure constructed on an explicit finite difference scheme is then developed to predict numerically the thermal response and thermal damage irradiated by a pulse laser as a therapeutic heat source. The parametric study on variable thermal physical parameters including the phase lag times, heat conductivity, specific heat capacity and blood perfusion rate has been performed to evaluate their influence on temperature distribution in time and space. On this basis, the thermal damage with different laser variables such as laser intensity and exposure time are further analyzed.

Thermometry using entropy imaging of ultrasound radio frequency signal time series

Low intensity focused ultrasound (LIFU) is a novel approach that could activate drug release and considerably improve the delivery of anticancer drug. LIFU treatment has some features like is able to penetrate deep into the tissue and being non-invasive, as a consequence LIFU displays great capability for controlling the drug release and improving the chemotherapy treatment efficiency. The goal of this study is to research the feasibility of the entropy parameter of RF time series of ultrasound backscattered signals for measuring the changes in temperature induced by a LIFU device. Entropy Imaging is a technique for reconstructing ultrasound images based on the average uncertainty of time-series in a signal. Furthermore, the Shannon Entropy can quantify the uncertainty of a random process and is usually used as a measure for the information content of probability distributions. In this study, we use the Entropy Imaging method for measuring the LIFU-induced temperature changes in the deep region of ex vivo porcine tissue samples. The results obtained show that the changes of entropy parameter of RF time series signal are proportional to temperature changes recorded by a calibrated thermocouple in the temperature range of 37–47°C. In conclusion, in this study we show that Shannon entropy of RF time series signal possesses promising features like succinctly capturing the available information in a system by considering the uncertainty in a given data that can be used, as a new method, to measure temperature changes non-invasively and quantitatively in the deep region of tissue.

Development of a Numerical Model of High-Intensity Focused Ultrasound Treatment in Mobile and Elastic Organs: Application to a Beating Heart

High-intensity focused ultrasound (HIFU) is a promising method used to treat cardiac arrhythmias, as it can induce lesions at a distance throughout myocardium thickness. Numerical modeling is commonly used for ultrasound probe development and optimization of HIFU treatment strategies. This study was aimed at describing a numerical method to simulate HIFU thermal ablation in elastic and mobile heart models. The ultrasound pressure field is computed on a 3-D orthonormal grid using the Rayleigh integral method, and the attenuation is calculated step by step between cells. The temperature distribution is obtained by resolution of the bioheat transfer equation on a 3-D non-orthogonally structured curvilinear grid using the finite-volume method. The simulation method is applied on two regions of the heart (atrioventricular node and ventricular apex) to compare the thermal effects of HIFU ablation depending on deformation, motion type and amplitude. The atrioventricular node requires longer sonication than the ventricular apex to reach the same lesion volume. Motion considerably influences treatment duration, lesion shape and distribution in cardiac HIFU treatment. These results emphasize the importance of considering local motion and deformation in numerical studies to define efficient and accurate treatment strategies.