Weighting the Passive Acoustic Mapping Technique With the Phase Coherence Factor for Passive Ultrasound Imaging of Ultrasound-Induced Cavitation.

Ultrasound (US) cavitation is currently being explored for low-invasive therapy techniques applied to a wide panel of pathologies. Because of the random behavior of cavitation, a real-time spatial monitoring system may be required. For this purpose, the US passive imaging techniques have been recently investigated. In particular, the passive acoustic mapping (PAM) beamforming method enables the reconstruction of cavitation activity maps by beamforming acoustic signals passively recorded by an array transducer. In this paper, an optimized version of PAM, PAM weighted with a phase coherence factor (PAM-PCF), is considered. A general validation process is developed including simulations on a point source and experiments on a wire. Furthermore, using a focused regulated US-induced cavitation generator, reproducible cavitation experiments are conducted in water and in agar gel. The spatial behavior of a bubble cavitation cloud is determined using the PAM-PCF beamforming method to localize the focal cavitation point in two perpendicular imaging planes.

Sonoporation of adherent cells under regulated ultrasound cavitation conditions.

A sonoporation device dedicated to the adherent cell monolayer has been implemented with a regulation process allowing the real-time monitoring and control of inertial cavitation activity. Use of the cavitation-regulated device revealed first that adherent cell sonoporation efficiency is related to inertial cavitation activity, without inducing additional cell mortality. Reproducibility is enhanced for the highest sonoporation rates (up to 17%); sonoporation efficiency can reach 26% when advantage is taken of the standing wave acoustic configuration by applying a frequency sweep with ultrasound frequency tuned to the modal acoustic modes of the cavity. This device allows sonoporation of adherent and suspended cells, and the use of regulation allows some environmental parameters such as the temperature of the medium to be overcome, resulting in the possibility of cell sonoporation even at ambient temperature.

Lorentz-force hydrophone characterization.

A Lorentz-force hydrophone consists of a thin wire placed inside a magnetic field. When under the influence of an ultrasound pulse, the wire vibrates and an electrical signal is induced by the Lorentz force, which is proportional to the pulse amplitude. In this study, a compact prototype of such a hydrophone is introduced and characterized, and the previously developed hydrodynamic model is refined. It is shown that the wire tension has a negligible effect on the measurement of pressure. The frequency response of the hydrophone reaches 1 MHz for wires with diameters between 70 and 400 µm. The hydrophone exhibits a directional response such that the signal amplitude differs by less than 3 dB as the angle of the incident ultrasound pulse varies from -20° and +20°. The linearity of the measured signal is confirmed across the 50 kPa to 10 MPa pressure range, and an excellent resistance to cavitation is observed. This hydrophone is of interest for high-pressure ultrasound measurements including high-intensity focused ultrasound (HIFU) and ultrasonic measurements in difficult environments.

Control of inertial acoustic cavitation in pulsed sonication using a real-time feedback loop system.

Owing to the complex behavior of ultrasound-induced bubble clouds (nucleation, linear and nonlinear oscillations, collapse), acoustic cavitation remains a hardly controllable phenomenon, leading to poorly reproducible ultrasound-based therapies. A better control of the various aspects of cavitation phenomena for in vivo applications is a key requirement to improve emerging ultrasound therapies. Previous publications have reported on systems performing regulation of acoustic cavitation in continuous sonication when applied in vitro, but the main challenge today is to achieve real-time control of cavitation activity in pulsed sonication when used in vivo. The present work aims at developing a system to control acoustic cavitation in a pulsed wave condition using a real-time feedback loop. The experimental setup consists of a water bath in which is submerged a focused transducer (pulsed waves, frequency 550 kHz) used for sonication and a hydrophone used to listen to inertial cavitation. The designed regulation process allows the cavitation activity to be controlled through a 300 µs feedback loop. Without regulation, cavitation exhibits numerous bursts of intense activity and large variations of inertial cavitation level over time. In a regulated regime, the control of inertial cavitation activity within a pulse leads to consistent cavitation levels over time with an enhancement of the reproducibility.