The relationship between bubble concentration and the acoustic emission energy of separate frequency bands

This letter presents the relationship between bubble concentration and the energy ratio of low to high frequency bands of their acoustic emissions. Two sensors, placed perpendicular and concentric to a transmitter, captured the emissions from sonicated microbubbles. Emissions from different bubbles arrived at the perpendicular sensor with small time differences. Low frequencies with periods longer than the time differences interfered constructively, while higher frequencies interfered both constructively and destructively. The low-frequency (2nd–3rd harmonics) to high-frequency (7th–12th harmonics) energy ratio increased with the bubble concentration. The relationship was not observed with the concentric sensor, where the time differences were larger

Imaging with therapeutic acoustic wavelets–short pulses enable acoustic localization when time of arrival is combined with delay and sum

—Passive acoustic mapping (PAM) is an algorithm that reconstructs the location of acoustic sources using an array of receivers. This technique can monitor therapeutic ultrasound procedures to confirm the spatial distribution and amount of microbubble activity induced. Current PAM algorithms have an excellentlateral resolution but have a poor axial resolution, making it difficult to distinguish acoustic sources within the ultrasound beams. With recent studies demonstrating that short-length and low-pressure pulses—acoustic wavelets—have the therapeutic function, we hypothesizedthat the axial resolution could be improved with a quasi-pulse-echo approach and that the resolution improvement would depend on the wavelet’s pulse length. This article describes an algorithm that resolves acoustic sources axially using time of flight and laterally using delayand-sum beamforming, which we named axial temporal position PAM (ATP-PAM). The algorithm accommodates a rapid short pulse (RaSP) sequence that can safely deliver drugs across the blood–brain barrier. We developed our algorithm with simulations (k-wave) and in vitro experiments for one-, two-, and five-cycle pulses, comparing our resolution against that of two current PAM algorithms. We then tested ATP-PAM in vivo and evaluated whether the reconstructed acoustic sources mapped to drug deliver

Angular dependence of the acoustic signal of a microbubble cloud

© 2020 Acoustical Society of America. Microbubble-mediated ultrasound therapies have a common need for methods that can noninvasively monitor the treatment. One approach is to use the bubbles' acoustic emissions as feedback to the operator or a control unit. methods interpret the emissions' frequency content to infer the microbubble activities and predict therapeutic outcomes. However, different studies placed their sensors at different angles relative to the emitter and bubble cloud. Here, it is evaluated whether such angles influence the captured emissions such as the frequency content. In computer simulations, 128 coupled bubbles were sonicated with a 0.5-MHz, 0.35-MPa pulse, and the acoustic emissions generated by the bubbles were captured with two sensors placed at different angles. The simulation was replicated in experiments using a microbubble-filled gel channel (0.5-MHz, 0.19-0.75-MPa pulses). A hydrophone captured the emissions at two different angles. In both the simulation and the experiments, one angle captured periodic time-domain signals, which had high contributions from the first three harmonics. In contrast, the other angle captured visually aperiodic time-domain features, which had much higher harmonic and broadband content. Thus, by placing acoustic sensors at different positions, substantially different acoustic emissions were captured, potentially leading to very different conclusions about the treatment outcome

Passive cavitation detection with a needle hydrophone array

Therapeutic ultrasound and microbubble technologies seek to drive systemically administered microbubbles into oscillations that safely manipulate tissue or release drugs. Such procedures often detect the unique acoustic emissions from microbubbles with the intention of using this feedback to control the microbubble activity. However, most sensor systems reported introduce distortions to the acoustic signal. Acoustic shockwaves, a key emission from microbubbles, are largely absent in reported recording, possibly due to the sensors being too large or too narrowband, or having strong phase distortions. Here, we built a sensor array that countered such limitations with small, broadband sensors and a low phase distorting material. We built 8 needle hydrophones with polyvinylidene fluoride (PVDF, diameter: 2 mm) then fit them into a 3D-printed scaffold in a two-layered, staggered arrangement. Using this array, we monitored microbubbles exposed to therapeutically-relevant ultrasound pulses (center frequency: 0.5 MHz, peak-rarefactional pressure: 130-597 kPa, pulse length: 4 cycles). Our tests revealed that the hydrophones were broadband with the best having a sensitivity of -224.8± 3.2 dB re 1 V/μPa from 1 to 15 MHz. The array was able to capture shockwaves generated by microbubbles. The signal-to-noise (SNR) ratio of the array was approximately 2 times higher than individual hydrophones. Also, the array could localize microbubbles (-3dB lateral resolution: 2.37 mm) and determine the cavitation threshold (between 161 kPa and 254 kPa). Thus, the array accurately monitored and localized microbubble activities, and may be an important technological step towards better feedback control methods and safer and more effective treatments