Real-Time Heterogeneous Helical Wave Spectrum Method for Transabdominal Passive Acoustic Mapping

In focused ultrasound (FUS) therapy, passive acoustic mapping (PAM) with convex arrays is often employed to monitor the cavitation activity in the human abdomen, given their large acoustic window. However, phase aberration caused by the speed-of-sound (SoS) heterogeneity in the abdominal wall degrades the image quality, leading to inaccurate localization of the cavitation source and, hence, deteriorates the safety and efficacy level of the therapy. In this work, we derive the general solution to the wave equation in SoS heterogeneous media in polar coordinates. With the solution, we propose the real-time heterogeneous helical wave spectrum (HHWS) method to account for SoS heterogeneity in transabdominal PAM with convex arrays. In both the in silico and in vitro experiments mimicking transabdominal PAM imaging of single and multiple microbubble (MB) cavitation source(s) in humans, the results clearly demonstrated the phase aberration-correction capability of the HHWS method with improved image quality and source localization accuracy, compared with the helical wave spectrum (HWS) method for homogeneous media. A parallel implementation of the HHWS method realized a several tens of milliseconds image reconstruction speed for phase-aberration-corrected PAM in a large field of view (FOV). Combined with B-mode imaging, real-time dual-mode monitoring of MB cavitation activity in the heterogeneous medium mimicking the human abdominal wall with a single probe has been realized. These results well demonstrated the potential of the HHWS method for transabdominal PAM with convex arrays, for safe, effective, and controlled cavitation-based FUS therapies.