Calibration of a focused passive cavitation detector using bubble shock waves

Abstract

In microbubble-mediated therapeutic ultrasound, a focused passive cavitation detector (PCD) is often used to measure the bubbles’ acoustic emissions, providing useful signals for treatment monitoring. However, calibrating a spherically focused PCD is challenging, due to the difficulty of generating a spherical wave that matches the PCD’s surface curvature. Here, a PCD was calibrated using broadband shock waves generated by inertial collapses of single microbubbles. Microbubbles were diluted to a very low concentration, flowed through a wall-less gel channel, and sonicated using single-cycle, 0.5-MHz-centre-frequency, 1-MPa acoustic pulses. The focused PCD to be calibrated and a reference needle hydrophone captured their emissions. The sensitivity and phase response of the PCD relative to the reference hydrophone was calculated from the single bubble signals. For comparison, the PCD was also calibrated using a focused emitter as a sound source (Rich and Mast, JASA, 2015). The nominal PCD sensitivities obtained using the two methods agreed within 1% ± 14% within the PCD’s bandwidth (2–10 MHz). The calibration data from the bubble method was then used to correct the PCD’s signal distortions. Our method recovered the impulse waveform of the bubble-generated shock wave from the raw PCD signal, where such a waveform was not previously observed.