IEEE Trans Ultrason Ferroelectr Freq Control. 2021 Apr;68(4):910-925. doi: 10.1109/TUFFC.2020.3032345. Epub 2021 Mar 26.
Passive ultrasound imaging is of great interest for cavitation monitoring. Spatiotemporal monitoring of cavitation bubbles in therapeutic applications is possible using an ultrasound imaging probe to passively receive the acoustic signals from the bubbles. Fourier-domain (FD) beamformers have been proposed to process the signals received into maps of the spatial localization of cavitation activity, with reduced computing times with respect to the time-domain approach, and to take advantage of frequency selectivity for cavitation regime characterization. The approaches proposed have been mainly nonadaptive, and these have suffered from low resolution and contrast, due to the many reconstruction artifacts. Inspired by the array-processing literature and in the context of passive ultrasound imaging of cavitation, we propose here a robust estimation of the second-order statistics of data through spatial covariance matrices in the FD or cross-spectral density matrices (CSMs). The benefits of such formalism are illustrated using advanced reconstruction algorithms, such as the robust Capon beamformer, the Pisarenko class beamformer, and the multiple signal classification approach. Through both simulations and experiments in a water tank, we demonstrate that enhanced localization of cavitation activity (i.e., improved resolution and contrast with respect to nonadaptive approaches) is compatible with the rapid and frequency-selective approaches of the FD. Robust estimation of the CSM and the derived adaptive beamformers paves the way to the development of powerful passive ultrasound imaging tools.
被动式超声成像是对空化监测很感兴趣。在治疗应用中，通过使用超声成像探头被动接收来自气泡的声信号，可以对气泡的空化进行时空监测。傅里叶域（FD）波束形成器已被提议用于处理接收到的信号，将其转换为空化活动空间定位的图谱，与基于时域的方法相比，计算时间减少，并利用频率选择性进行空化状态表征。所提出的方法主要是非自适应的，由于存在许多重建伪影，这些方法的分辨率和对比度都很低。受阵列处理文献的启发，并在被动式超声空化成像的背景下，我们在这里通过 FD 中的空间协方差矩阵或互谱密度矩阵（CSM）对数据的二阶统计量进行稳健估计。通过先进的重建算法（例如稳健的 Capon 波束形成器、Pisarenko 类波束形成器和多信号分类方法）来说明这种形式的优势。通过在水箱中的模拟和实验，我们证明了空化活动的增强定位（即与非自适应方法相比提高了分辨率和对比度）与 FD 的快速和频率选择性方法兼容。CSM 的稳健估计和由此产生的自适应波束形成器为开发强大的被动式超声成像工具铺平了道路。
