Pahk Ki Joo, Gélat Pierre, Kim Hyungmin, Saffari Nader
Center for Bionics, Biomedical Research Institute, Korea Institute of Science and Technology (KIST), Seoul, 02792, Republic of Korea.
Department of Mechanical Engineering, University College London, London, WC1E 7JE, UK.
Ultrasound Med Biol. 2018 Dec;44(12):2673-2696. doi: 10.1016/j.ultrasmedbio.2018.07.025. Epub 2018 Sep 15.
Boiling histotripsy is a non-invasive, cavitation-based ultrasonic technique which uses a number of millisecond pulses to mechanically fractionate tissue. Though a number of studies have demonstrated the efficacy of boiling histotripsy for fractionation of solid tumours, treatment monitoring by cavitation measurement is not well studied because of the limited understanding of the dynamics of bubbles induced by boiling histotripsy. The main objectives of this work are to (a) extract qualitative and quantitative features of bubbles excited by shockwaves and (b) distinguish between the different types of cavitation activity for either a thermally or a mechanically induced lesion in the liver. A numerical bubble model based on the Gilmore equation accounting for heat and mass transfer (gas and water vapour) was developed to investigate the dynamics of a single bubble in tissue exposed to different High Intensity Focused Ultrasound fields as a function of temperature variation in the fluid. Furthermore, ex vivo liver experiments were performed with a passive cavitation detection system to obtain acoustic emissions. The numerical simulations showed that the asymmetry in a shockwave and water vapour transport are the key parameters which lead the bubble to undergo rectified growth at a boiling temperature of 100°C. The onset of rectified radial bubble motion manifested itself as (a) an increase in the radiated pressure and (b) the sudden appearance of higher order multiple harmonics in the corresponding spectrogram. Examining the frequency spectra produced by the thermal ablation and the boiling histotripsy exposures, it was observed that higher order multiple harmonics as well as higher levels of broadband emissions occurred during the boiling histotripsy insonation. These unique features in the emitted acoustic signals were consistent with the experimental measurements. These features can, therefore, be used to monitor (a) the different types of acoustic cavitation activity for either a thermal ablation or a mechanical fractionation process and (b) the onset of the formation of a boiling bubble at the focus in the course of HIFU exposure.
沸腾组织粉碎术是一种基于空化作用的非侵入性超声技术，它使用多个毫秒级脉冲对组织进行机械破碎。尽管许多研究已经证明了沸腾组织粉碎术对实体瘤破碎的有效性，但由于对沸腾组织粉碎术所诱导气泡的动力学了解有限，通过空化测量进行治疗监测的研究并不充分。这项工作的主要目标是：（a）提取由冲击波激发的气泡的定性和定量特征；（b）区分肝脏中热诱导或机械诱导损伤的不同类型的空化活动。基于考虑了热质传递（气体和水蒸气）的吉尔摩方程，开发了一个数值气泡模型，以研究在不同高强度聚焦超声场作用下，组织中单个气泡的动力学随流体温度变化的情况。此外，还使用被动空化检测系统进行了离体肝脏实验，以获取声发射信号。数值模拟表明，冲击波的不对称性和水蒸气传输是导致气泡在100°C沸腾温度下发生整流生长的关键参数。整流径向气泡运动的开始表现为：（a）辐射压力增加；（b）在相应的频谱图中突然出现高阶多重谐波。通过检查热消融和沸腾组织粉碎术暴露产生的频谱，观察到在沸腾组织粉碎术声辐射期间出现了高阶多重谐波以及更高水平的宽带发射。发射声信号中的这些独特特征与实验测量结果一致。因此，这些特征可用于监测：（a）热消融或机械破碎过程中不同类型的声空化活动；（b）在高强度聚焦超声暴露过程中焦点处沸腾气泡形成的开始。
