Monitoring holograms for therapeutic ultrasound using passive cavitation beamforming

Acoustic holograms can generate cavitation patterns of complex spatial distribution by shaping and steering the focal spot of therapeutic ultrasound systems. However, when monitoring these systems by passive cavitation detection, off-axis therapeutical targets and the receiver directivity may not be aligned. In this paper, we present passive cavitation beamforming to monitor a therapeutical ultrasound system using holograms targeted to arbitrary locations, in which both therapeutic and passive cavitation monitoring systems use 3D-printed acoustic lenses. The therapeutic system uses an acoustic hologram to focus the ultrasound beam on the target, which is off-axis. Then, a second lens is designed to beamform the cavitation signals which emerge from the therapeutic target, steering the directivity of the passive cavitation detector in the direction of the therapeutic focus and, in addition, compensating for skull aberrations. The system is experimentally tested with an ex-vivo macaque skull and a blood vessel phantom with microbubbles. In addition, results are compared with a standard confocal configuration and an off-axis configuration in the absence of the monitor lens. A parametric study is performed by varying the amplitude of the emitted signal and the impact on the behaviour of the microbubbles is analysed based on the cavitation index values. Results show that monitoring holograms align the passive cavitation detector response with the focal spot of the targeted therapeutic transducer. These holograms encode a fixed beamformer for cavitation signals in reception, increasing the sensitivity of cavitation emission at the target. In this way, cavitation doses can be used to locally monitor the cavitation activity of microbubbles, thus opening a new path to low-cost monitoring of therapeutic ultrasound systems.