Influence of dynamic tissue properties on temperature elevation and lesions during HIFU scanning therapy: Numerical simulation*

Abstract

When large tumors are treated, ablation of the entire volume of tumors requires multiple treatment spots formed by high intensity-focused ultrasound (HIFU) scanning therapy. The heating effect of HIFU on biological tissue is mainly reflected in temperature elevation and tissue lesions. Tissue property parameters vary with temperature and, in turn, the distribution of temperature as well as the heating effects change accordingly. In this study, an HIFU scanning therapy model considering dynamic tissue properties is provided. The acoustic fields and temperature fields are solved combining the Helmholtz wave equation with Pennes bio-heat transfer equation based on the finite element method (FEM) to investigate the effects of various tissue properties (i.e., the attenuation coefficient, acoustic velocity, thermal conductivity, specific heat capacity, density, and blood perfusion rate) on heating performance. Comparisons of the temperature distribution and thermal lesions under static and dynamic properties are made based on the data of tissue property parameters varying with temperature. The results show that the dynamic changes of thermal conductivity, specific heat capacity, and acoustic velocity may account for the decrease of temperature elevation in HIFU treatment, while the dynamic changes of attenuation coefficient, density, and blood perfusion rate aggravate the increase of temperature on treatment spots. Compared with other properties, the dynamic change of attenuation coefficient has a greater impact on tissue temperature elevation. During HIFU scanning therapy, the temperature elevation and tissue lesions of the first treatment spot are smaller than those of the subsequent treatment spots, but the temperature on the last treatment spot drops faster during the cooling period. The ellipsoidal tissue lesion is not symmetrical; specifically, the part facing toward the previous treatment spot tends to be larger. Under the condition of the same doses, the temperature elevation and the size of tissue lesions under dynamic properties present significant growth in comparison to static properties. Besides, the tissue lesion begins to form earlier with a more unsymmetrical shape and is connected to the tissue lesion around the previous treatment spot. As a result, lesions around all the treatment spots are connected with each other to form a closed lesion region. The findings in this study reveal the influence of dynamic tissue properties on temperature elevation and lesions during HIFU scanning therapy, providing useful support for the optimization of treatment programs to guarantee higher efficacy and safety.