Abstract
A high intensity focused ultrasound (HIFU) scanning approach is needed to obtain multiple treatment spots for the ablation of large volume tumors, but it will bring some problems such as longer treatment times, the inhomogeneity of temperature and thermal lesions in tissues. Although some optimal control methods have been proposed, it is difficult to take into account the uniformity, efficiency and entirety of thermal lesions. In this study, based on the Helmholtz equation and Pennes’ bio-heat transfer equation, a coupled acoustic-thermal field model is proposed to investigate the relationship between temperature elevation, thermal lesions and neighboring treatment spots, and to analyze the effects of the heating time and acoustic intensity on thermal lesions by the finite element method (FEM). Consequently, optimal control schemes for the heating time and acoustic intensity based on the contribution from neighboring treatment spots to thermal lesions are put forward to reduce treatment times and improve the uniformity of temperature and thermal lesions. The simulation results show that the peak historical temperature elevation on one treatment spot is related to the number, distance and time interval of its neighboring treated spots, and the thermal diffusion from the neighboring untreated spots can slow down the drop of temperature elevation after irradiation, thus both of them affect the final shape of the thermal lesions. In addition, increasing the heating time or acoustic intensity of each treatment spot can expand the overall area of thermal lesions, but it would aggravate the elevation and nonuniformity of the temperature of the treatment region. Through optimizing the heating time, the total treatment time can be reduced from 249 s by 17.4%, and the mean and variance of the peak historical temperature elevation can decrease from 44.64 °C by 13.3% and decrease from 24.6317 by 45%, respectively. While optimizing the acoustic intensity, the total treatment time remains unchanged, and the mean of the peak historical temperature elevation is reduced by 4.3 °C. Under the condition of the same thermal lesions, the optimized schemes can reduce the treatment time, lower the peak of the temperature on treatment spots, and homogenize the temperature distributions. This work is of practical significance for the optimization of a HIFU scanning therapy regimen and the evaluation of its treatment effect.
Highlights
High intensity focused ultrasound (HIFU), as a noninvasive method of tumor hyperthermia, has been widely used for treating various solid tumors in the past two decades, such as primary or metastatic liver, lung, breast, prostate, and malignant kidney tumors [1,2,3,4,5]
The purpose of this study is to reveal the influence of neighboring treatment spots on the temperature elevation and thermal lesions during HIFU scanning therapy, and to produce uniform thermal lesions by shorter treatment times and lower the peak of temperature based on the optimization of the heating time or acoustic intensity, which can provide a reference for the optimization of HIFU scanning therapy regimens and the evaluation of the treatment effect
This paper focuses on the formation of thermal lesions and its optimal control during HIFU
Summary
High intensity focused ultrasound (HIFU), as a noninvasive method of tumor hyperthermia, has been widely used for treating various solid tumors in the past two decades, such as primary or metastatic liver, lung, breast, prostate, and malignant kidney tumors [1,2,3,4,5]. When large volume tumors are ablated, an ultrasound phased array with electronic scanning or a single source with mechanical scanning is needed to produce multiple treatment spots which are independent or overlapping of each. The ultrasound phased array has the advantages of a short treatment time, multi-focus, and no mechanical movement, but it is still in the stage of experimental research on the prototype and rare applied in commercial HIFU systems [6,7]. Mechanical scanning has been widely used in clinical treatment for various tumors, where after one treatment spot is heated for a period of time, the treatment spot would be irradiated by moving the HIFU-sourced transducer or the biological tissue at a certain step size until all treatment spots are treated [8,9,10]. Several optimal control approaches have been investigated to obtain the desired temperatures and thermal responses in tissues
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