Passive cavitation detection with a needle hydrophone array

Abstract

Therapeutic ultrasound and microbubble technologies seek to drive systemically administered microbubbles into oscillations that safely manipulate tissue or release drugs. Such procedures often listen to and then utilize microbubble emissions to control the microbubble activity. However, most sensors reported introduce large distortions to the acoustic signal. Acoustic shockwaves, a key emission from microbubbles, are largely absent in reported recordings. Here, we present a needle hydrophone array better suited for monitoring ultrasound-driven microbubble activities. The array consisted of eight polyvinylidene fluoride (PVDF, diameter: 2 mm) needle hydrophones. Each needle had a flexible coaxial wire at its end allowing flexibility to fit into a 3D-printed scaffold. Using this array, we monitored microbubbles exposed to ultrasound pulses (center frequency: 0.5 MHz, peak-rarefactional pressure: 130–-597 kPa, pulse length: 4 cycles). Our tests revealed that each needle had a broadband frequency response (1–15 MHz) and was able to capture shock waveforms generated by bubbles. The signal-to-noise ratio of the array was approximately 2 times higher than individual hydrophones. Also, the array could localize microbubble activities and determine the cavitation threshold. Thus, the array accurately monitored and localized microbubble activities, and may be an important technological step towards safer and more effective treatments.