Doppler passive acoustic mapping for monitoring microbubble velocities in ultrasound therapy

Abstract

The success of microbubble-mediated ultrasound treatments, such as blood-brain barrier disruption and sonothrombolysis, is determined by whether the correct cavitation dynamics are produced at the correct locations. Passive acoustic mapping (PAM) can track the location, magnitude, type, and duration of microbubble-seeded cavitation produced during sonication. Using a single element passive cavitation detector (PCD), we recently showed that microbubble velocities within the PCD listening volume can be determined by analysing the Doppler shifts in the microbubble acoustic emissions. Here, we developed a PAM-based algorithm to passively track microbubble velocities using a linear array. Microbubbles embedded within a vessel were sonicated using a 1 MHz focused ultrasound transducer (pulse length: 50 ms, peak-negative pressure: 200-600 kPa). Acoustic emissions were captured by a co-aligned L7-4 linear array. PAM using Capon beamforming was used to localize the acoustic emissions. We spectrally analyzed the time traces in order to derive position-dependent Doppler shifts and estimate axial velocities at each location. Doppler PAM imaged the axial microbubble velocities along the ultrasound propagation direction, at different time points during sonication. Microbubbles moved at peak velocities of 1-2 m/s due to acoustic radiation forces, producing a time dependent velocity profile. Doppler PAM allowed estimation of microbubble translation within an imaging plane, enabling enhanced monitoring of therapeutic ultrasound applications.