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高强度聚焦超声(HIFU)治疗过程中剧烈的空化效应可能损伤靶区周围健康组织, 因此, 亟需开发可对生物组织内部声空化效应进行高精度时空定量监测的新型技术手段, 方能确保临床安全和有效. 相对于传统的商用超声灰度值信号, 超声射频(RF)信号可以更好地保留声波散射信号更多的细节信息. 而信息熵作为非基于数学函数模型的统计参数, 可以表征由声空化效应引发的组织内部散射体无序度演变状态. 因此, 本文提出了一种基于超声RF信号熵分析的声空化实时监测成像系统, 在此基础上实时评估HIFU引发的超声空化区域时空演化行为. 首先, 通过改制后的B超系统获取凝胶生物仿体内部由HIFU引发的空化泡群产生的散射回波原始RF信号, 利用二维均值滤波方法抑制HIFU强声束对声空化监测成像回波信号的干扰后, 通过数据标准化处理扩展RF信号的动态变化范围, 再基于滑动窗信息熵分析重建熵值图像, 经过二值化处理后即可实现对HIFU作用下组织内部声空化区域的时空监测. 实验结果表明, 相比于传统B超灰度成像法, 基于RF信号熵分析的声空化监测成像算法可以更灵敏且精确地确定空化发生的起始时间和空间位置, 有助于更好地保障HIFU临床治疗的安全性和有效性. 本文的工作为HIFU治疗过程中组织内部声空化区域的时空监测提供一种具有极大应用前景的技术手段, 为声空化生物效应量-效评估体系的建立奠定了良好的理论和实验基础.The violent inertial cavitation effect generated during high intensity focused ultrasound (HIFU) treatment may damage healthy tissues around the target area. Therefore, it is urgent to develop new technical approaches that can quantitatively monitor the acoustic cavitation motions in biological tissues with high precision in space and time, so as to ensure clinical safety and effectiveness. Compared with the traditional commercial ultrasonic gray value signal, the ultrasonic radio frequency (RF) signal can well retain more detailed information about the acoustic scattering signal. As a statistical parameter not based on mathematical function model, the information entropy can characterize the spatiotemporal evolution state of disorder of scatters inside tissues resulting from acoustic cavitation. Therefore, this paper proposes a real-time monitoring system for spatiotemporal evolution of acoustic cavitation based on the entropy analysis of ultrasonic RF signals. First, the original RF signal of scattered echoes generated by HIFU-induced cavitation bubbles inside the gel phantom is obtained by using a modified B-ultrasound system, and the two-dimensional mean filtering method is used to suppress the HIFU-induced strong interferences overlapping with cavitation monitoring imaging signals. Then, the dynamic variation range of the RF signal is expanded through data standardization processing, and the entropy image is reconstructed based on the sliding window information entropy analysis to demonstrate the spatiotemporal evolution status of the HIFU-induced cavitation behanviors. The experimental results indicate that the acoustic cavitation imaging algorithm based on RF signal entropy analysis should be more sensitive and accurate than the B-model gray scale imaging method for determining the onset time and spatial position of cavitation activities, which is helpful in ensuring the safety and efficacy of HIFU clinical treatment. Thepresent work will provide a useful tool for the spatiotemporal monitoring of the acoustic cavitation generated in tissues during HIFU treatment, and lays a solid theoretical and experimental foundation to establish an effective quantity-effect evaluation system for the cavitation related biological effect.
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Keywords:
- acoustic cavitation/
- high intensity focused ultrasound/
- ultrasound radio frequency signal/
- information entropy
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