在高強度聚焦超音波熱治療過程中空蝕氣泡的研究

Cavitation study in high-intensity focused ultrasound thermal therapy

辛艾卡(Ekaterina Zilonova)

指導教授 ： 許文翰

共同指導教授 ： 馬克沁(Maxim Solovchuk)

國立臺灣大學/工學院/工程科學及海洋工程學研究所/博士(2019年)

https://doi.org/10.6342/NTU201902181

摘要

無資料

關鍵字

空泡現象 ； 氣泡動力學 ； 高強度聚焦超音波 ； 熱沉積 ； 軟組織

並列摘要

In the present thesis, the cavitation phenomenon in soft tissue exposed to high intensity focused ultrasound (HIFU) has been studied. The accomplished research tasks included the development of new mathematical models, parametric studies and comparison with the corresponding experimental data.
In the first part of the dissertation, the investigation has been focused on the single-bubble cavitation. The mathematical models for the simulation of bubble dynamics in soft tissue and associated temperature elevation have been proposed. On the basis of the proposed models, the analysis has been done, which led to the following conclusions. First, it is crucial to simulate viscoelastic features of soft tissue, since they significantly affect both the bubble’s motion and temperature values. Namely, elasticity and viscosity diminish the amplitude of the bubble’s oscillations and decrease temperature values inside and outside the bubble. Relaxation time, conversely, enlarges the bubble’s size during its growth phase. Second, the continuous temperature distribution inside and outside the oscillating bubble has been presented and showed very high-temperature values (10^5 K inside the bubble and 10^3 K close to the bubble wall). The obtained temperature values emphasize the significance of the cavitation thermal contribution to the therapy. Moreover, the vapor diffusion within the bubble and vapor flux through the bubble wall have been incorporated in the mathematical model. If the vapor mass flux effect is taken into account in the simulations, the rectified growth effect has been observed, which leads to a decrease of the temperature values.
The research on the single bubble revealed the important patterns of the bubble’s radial motion and corresponding temperature values. In the second part of the thesis, based on the findings made regarding the single bubble, the present investigation has been extended to the multiple-bubble cavitation. This part of the dissertation included an examination of such phenomena as bubble-bubble interaction, bubbles’ translational movement and temperature elevation caused by multiple bubbles.
The bubble-bubble interaction has been studied in the viscoelastic medium (that simulates the biological fluid) for the first time. The translational motion of the bubbles in soft tissue is questionable, however, bubbles certainly can move in the cardiovascular system. New equations for calculating the viscoelastic drag exerted on bubbles during their translational motion in a viscoelastic medium have been derived. The drag equations have been incorporated in the bubble-bubble interaction model in which, thereby, both the translational and radial motions of the bubbles were affected by the viscoelastic features of the medium. The derived equations have been applied to investigate how the viscoelastic properties of the medium can impact the dynamics of multiple bubbles, as well as how the bubbles can affect each other. It was discovered that the bubble-bubble interaction can significantly influence the dynamics of a single bubble. As the distance between the bubbles increases, their effect on each other decreases, and at a distance of several millimeters, this effect can be neglected. Moreover, it was concluded that with increasing elasticity and viscosity of the medium, as well with decreasing relaxation time, the effects of other bubbles on the current bubble’s radial motion can become negligible. The translational motion of the bubbles was investigated for different viscoelastic models. The elasticity was revealed to resist the motion of bubbles in space, resulting in a dynamical steady state of the distance between the bubbles at high elasticity values. The relaxation time of the medium was also found to be important in terms of the bubbles’ translational movement.
Finally, the temperature elevation caused by the presence of multiple bubbles in soft tissue during tumor ablation procedure has been obtained. The obtained temperature distribution has been compared to the collected experimental data. The porcine muscle was exposed to an ultrasound heating for 30 s with the electric power ranged from 80 W to 200 W. The temperature map of tissue was obtained from the MRI data that was available continuously during the whole ablation procedure. It is known that in the absence of cavitation, the porcine muscle heating can be well described by the Pennes bioheat equation. However, in the vicinity of the focal point, the measurements showed the characteristic temperature peaks corresponding to the formation of multiple bubbles in that area. The classical Pennes bioheat equation does not take bubbles’ presence into account. Therefore, it is necessary to refine the theoretical formulation so as to model the cavitation effect. In the current research, the extra heat deposition caused by the bubble cloud was described by the addition of cavitation heat sources into the bioheat equation. Excellent agreement between the measured and simulated temperature rises was found. The temperature peaks equal to 75 − 100◦ C in the experimental data were demonstrated in the simulations with the consideration of the bubble cloud consisting of 50 − 65 bubbles per 2mm×2mm×2mm (≈ 6-8 bubbles/mm^3). The present work shows that the temperature elevation due to the cavitation can be well predicted by simulations.

並列關鍵字

Cavitation ； Bubble Dynamics ； HIFU ； Heat deposition ； Soft Tissue

參考文獻

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