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High-intensity focused ultrasound (HIFU) can produce cavitation, which requires monitoring for specific applications such as sonoporation, targeted drug delivery, or histotripsy. Passive acoustic mapping has been proposed in the literature as a method for monitoring cavitation, but it lacks spatial resolution, primarily in the axial direction, due to the absence of a time reference. This is a common issue with passive imaging compared to standard pulse-echo ultrasound. In order to improve the axial resolution, we propose an adaptation of the cross spectral matrix fitting (CMF) method for passive cavitation imaging, which is based on the resolution of an inverse problem with different regularizations that promote sparsity in the reconstructed cavitation maps: Elastic Net (CMF-ElNet) and sparse Total Variation (CMF-spTV). The results from both simulated and experimental data are presented and compared to state-of-the-art approaches, such as the frequential delay-and-sum (DAS) and the frequential robust capon beamformer (RCB). We show the interest of the method for improving the axial resolution, with an axial full width half maximum (FWHM) divided by 3 and 5 compared to RCB and DAS, respectively. Moreover, CMF-based methods improve contrast-to-noise ratio (CNR) by more than 15 dB in experimental conditions compared to RCB. We also show the advantage of the sparse Total Variation (spTV) prior over Elastic Net (ElNet) when dealing with cloud-shaped cavitation sources, that can be assumed as sparse grouped sources.

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With the development of promising cavitation-based treatments, the interest in cavitation monitoring with passive acoustic mapping (PAM) is significantly increasing. While most of studies regarding PAM are performed in 2-D, 3-D imaging modalities are getting more attention relying on either custom-made or commercial matrix probes. Unless specific phased-arrays are used for a specific application, limitations due to probe apertures often results in poor performances of the 3-D mapping, due to the use of a delay-and-sum (DAS) classic beamformer, which results in strong artifacts and large main lobe sizes. In this article, 3D-PAM is achieved by performing adaptive beamforming in the frequency domain (FD) in 3-D, and using a random sparse apodization of a commercial matrix array driving only 256 elements among the 1024 available. It reduces the computation time and makes use of only one 256-channel research platform. Three beamformers have been implemented in 3-D and in the FD: the DAS beamformer, which corresponds to the beamformer used in previous 3D-PAM studies, the robust capon beamformer (RCB), an adaptive algorithm widely used in 2D-PAM for its high performances, and the MidWay (MW) beamformer, an adaptive algorithm with a computation complexity equivalent to the one of DAS. These algorithms are evaluated both in simulations and experiments with a harmonic source at different positions, and are also applied to real cavitation signals. The results show that, in the case of matrix arrays of small aperture such as generic commercial matrix probes, the DAS beamformer leads to large main lobe sizes, while adaptive beamformers largely improve the performances of the mapping. The low computation time and its parameter-free character make MW beamformer a good compromise for 3D-PAM applications. It thus appears that a random sparse apodization combined with adaptive beamforming is a good solution to achieve high-performance 3D-PAM with manageable devices.

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Steady and pulsatile aortic stenotic flows through stenosis tubes were experimentally and numerically investigated. The objective was the understanding of the fluid dynamics in arterial geometries most relevant in the context of atherosclerosis. Axisymmetric phantoms corresponding to significant artery stenosis of 50% in diameter and severe aortic stenosis of 75% were respectively machined from silicon. A water flow circuit was established, a steady flow was provided by gravity and a pulsed flow by a pulsatile pump. At inlet Reynolds numbers in the range of 85 to 1125, flows at the stenosis region were investigated using two-component Particle Image Velocimetry (PIV). For the unsteady flow, three different heartbeats (60, 69 and 90 beats per minute) were considered. The k-ω shear-stress-transport first-order turbulence model in Computational Fluid Dynamics (CFD) commercial software was adopted for simulations. Experimental measurements of the velocity fields show good agreements with CFD for both steady and pulsed flows. Recirculation regions were found near the stenosis in both cases. Reverse flow through the stenosis was also observed in pulsatile flow during the end diastolic phase of the cycle. CFD simulations allowed us to accurately assess wall shear stress in the stenotic region where the optical measurements are very noisy. High values of wall shear stress (with high variations both in space and time), are observed, which are indicators of possible future aortic wall damage.

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Passive ultrasound imaging is of great interest for cavitation monitoring. Spatiotemporal monitoring of cavitation bubbles in therapeutic applications is possible using an ultrasound imaging probe to passively receive the acoustic signals from the bubbles. Fourier-domain (FD) beamformers have been proposed to process the signals received into maps of the spatial localization of cavitation activity, with reduced computing times with respect to the time-domain approach, and to take advantage of frequency selectivity for cavitation regime characterization. The approaches proposed have been mainly nonadaptive, and these have suffered from low resolution and contrast, due to the many reconstruction artifacts. Inspired by the array-processing literature and in the context of passive ultrasound imaging of cavitation, we propose here a robust estimation of the second-order statistics of data through spatial covariance matrices in the FD or cross-spectral density matrices (CSMs). The benefits of such formalism are illustrated using advanced reconstruction algorithms, such as the robust Capon beamformer, the Pisarenko class beamformer, and the multiple signal classification approach. Through both simulations and experiments in a water tank, we demonstrate that enhanced localization of cavitation activity (i.e., improved resolution and contrast with respect to nonadaptive approaches) is compatible with the rapid and frequency-selective approaches of the FD. Robust estimation of the CSM and the derived adaptive beamformers paves the way to the development of powerful passive ultrasound imaging tools.

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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.

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Chemical vapor deposition (CVD) on metals is so far the best suited method to produce high-quality, large-area graphene. We discovered an unprecedentedly large family of small size-selective carbon clusters that form together with graphene during CVD. Using scanning tunneling microscopy (STM) and density functional theory (DFT), we unambiguously determine their atomic structure. For that purpose, we use grids based on a graphene moiré and a dilute atomic lattice that unambiguously reveal the binding geometry of the clusters. We find that the observed clusters bind in metastable configurations on the substrate, while the thermodynamically stable configurations are not observed. We argue that the clusters are formed under kinetic control and establish that the evolution of the smallest clusters is blocked. They are hence products of surface reactions in competition with graphene growth, rather than intermediary species to the formation of extended graphene, as often assumed in the literature. We expect such obstacles to the synthesis of perfect graphene to be ubiquitous on a variety of metallic surfaces.

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An asymptotic method is applied to analyze the nonlinear oscillations of a gas bubble driven by a dual-frequency excitation. More specifically, the latter is considered as a combination of two neighboring, incommensurate frequencies and is treated as a nonstationary excitation. This implies that both amplitude and phase of the bubble response are slowly oscillating at the time scale of the frequency difference, thus leading to a regime of aperiodic oscillations. The approximate solution is successfully compared with numerical simulations and reveals the possibility of achieving larger bubble response amplitude compared to the monofrequency excitation for sufficiently high driving pressure and specific values of the frequency difference. The asymptotic theory captures the generation of additional spectral components coming from the nonlinear mixing of both driving frequencies. This mechanism is responsible for a global enhancement of the dual-frequency bubble response as it enables an energy transfer towards more efficient components which are successively tuned with the nonlinearly shifted resonance frequency of the bubble, thus limiting the saturation due to softening.

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This research evaluates the use of Particle Image Velocimetry (PIV) technique for characterizing acoustic streaming flow generated by High Intensity Focused Ultrasound (HIFU). PIV qualification tests, focusing on the seeding particle size (diameter of 5, 20 and 50µm) were carried out in degassed water subjected to a focused field of 550kHz-frequency with an acoustic pressure amplitude of 5.2, 10.5 and 15.7bar at the focus. This study shows that the ultrasonic field, especially the radiation force, can strongly affect seeding particle behavior. Large particles (50µm-diameter) are repelled from the focal zone and gathered at radiation pressure convergence lines on either side of the focus. The calculation of the acoustic radiation pressure applied on these particles explains the observed phenomenon. PIV measurements do not, therefore, properly characterize the streaming flow in this case. On the contrary, small particles (5µm-diameter) velocity measurements were in good agreement with the Computational Fluid Dynamics (CFD) simulations of the water velocity field. A simple criterion approximating the diameter threshold below which seeding particles are qualified for PIV in presence of focused ultrasound is then proposed.

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We report observations of strong nonlinear interactions between the spherical, translational, and shape oscillations of micrometer-size bubbles. This is achieved through high-speed recordings of single bubble dynamics driven by amplitude-modulated ultrasound. The features of mode coupling are highlighted through (i) the exponential growth of the parametrically excited mode (n=3) triggered by the spherical oscillations followed by a saturation due to energy transfer towards the translation and even modes, (ii) the excitation of modes well below their parametric pressure threshold, and (iii) clear modification of the breathing mode R(t). These results are compared to recent theories accounting for nonlinear mode coupling, providing predictions in agreement with the observed bubble dynamics.

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Focused ultrasound involving inertial cavitation has been shown to be an efficient method to induce thrombolysis without any pharmacological agent. However, further investigation of the mechanisms involved and further optimization of the process are still required. The present work aims at studying the relevance of a bifrequency excitation compared to a classical monofrequency excitation to achieve thrombolysis without any pharmacological agent. In vitro human blood clots were placed at the focus of a piezoelectric transducer. Efficiency of the thrombolysis was assessed by weighing each clot before and after sonication. The efficiencies of mono- (550 kHz) and bifrequency (535 and 565 kHz) excitations were compared for peak power ranging from 70 W to 220 W. The thrombolysis efficiency appears to be correlated to the inertial cavitation activity quantified by passive acoustic listening. In the conditions of the experiment, the power needed to achieve 80% of thrombolysis with a monofrequency excitation is reduced by the half with a bifrequency excitation. The thermal effects of bifrequency and monofrequency excitations, studied using MR thermometry measurements in turkey muscle samples where no cavitation occurred, did not show any difference between both types of excitations when using the same power level.

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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.

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Acoustic cavitation-induced microbubbles in a cylindrical resonator filled with water tend to concentrate into ring patterns due to the cylindrical geometry of the system. The shape of these ring patterns is directly linked to the Bjerknes force distribution in the resonator. Experimental observations showed that cavitation bubbles located in the vicinity of this ring may exhibit a spiraling behavior around the pressure nodal line. This spiraling phenomenon is numerically studied, the conditions for which a single cavitation bubble follows an orbital trajectory are established, and the influences of the acoustic pressure amplitude and the initial bubble radius are investigated.

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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.

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Applications involving acoustic cavitation mechanisms, such as sonoporation, are often poorly reproducible because of the unstationary behavior of cavitation. For this purpose, this study proposes to work at a fixed cavitation level instead of a fixed acoustic intensity. A regulated cavitation generator has been developed in an in vitro configuration of standing wave field. This system implements the regulation of the cavitation level during sonication by modulating the applied acoustic intensity with a feedback loop based on acoustic measurements. The experimental setup consists of a plane piezoelectric transducer for sonication (continuous wave, frequency 445 kHz) and a hydrophone pointing to the sonicated medium. The cavitation level is quantified every 5 ms from a spectral analysis of the acoustic signal. The results show that the regulation device generates reproducible mean cavitation levels with a standard deviation lower than 1.6% in the applied intensity range (from 0.12 to 3.44 W/cm(2)), while this standard deviation can reach 76% without regulation. The feedback loop process imposes precise cavitation level even in low applied acoustic intensity.

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Enhancing cavitation activity with minimal acoustic intensities could be interesting in a variety of therapeutic applications where mechanical effects of cavitation are needed with minimal heating of surrounding tissues. The present work focuses on the relative efficiency of a signal combining two neighbouring frequencies and a one-frequency signal for initiating ultrasound inertial cavitation. Experiments were carried out in a water tank, using a 550kHz piezoelectric composite spherical transducer focused on targets with 46µm roughness. The acoustic signal scattered, either by the target or by the cavitation bubbles, is filtered using a spectral and cepstral-like method to obtain an inertial cavitation activity measurement. The ultrasound excitations consist of 1.8ms single bursts of single frequency f(0)=550kHz excitation, in the monofrequency case, and of dual frequency f(1)=535kHz and f(2)=565kHz excitation, in the bifrequency case. It is shown that depending on the value of the monofrequency cavitation threshold intensity the bifrequency excitation can increase or reduce the cavitation threshold. The analysis of the thresholds indicates that the mechanisms involved are nonlinear. The progress of the cavitation activity beyond the cavitation threshold is also studied. The slope of the cavitation activity considered as a function of the acoustic intensity is always steeper in the case of the bifrequency excitation. This means that the delimitation of the region where cavitation occurs should be cleaner than with a classical monofrequency excitation.

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We report on acoustic wave propagation in a regular array of nominally identical beads under isotropic static stress. The weak polydispersity of the beads makes the contact lattice random. Time-frequency analysis of the acoustic signal is performed and allows measurement of the full lattice dispersion relation. Comparison with the theoretical prediction for a perfect triangular lattice gives an indication of the level of randomness in the contact lattice. The results extend, in a consistent way, a previous study restricted to long wavelength propagation [B. Gilles and C. Coste, Phys. Rev. Lett. 90, 174302 (2003)]: The contact lattice is ordered by increasing the stress, and the smaller the wavelength, the higher the stress required to get regular lattice behavior. Measurements involving ballistic propagation of the coherent wave, whatever its frequency, evidence reversible lattice behavior under compression and/or decompression. Nevertheless, correlations of short wavelength incoherent waves are a sensitive probe of disorder, and allow us to exhibit a small irreversible evolution of the lattice.

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Weight measurements at the bottom of a quasi-2D vertical sheet of static cohesionless grains are carried out. The grains are held between two coaxial cylinders. This peculiar setup allows us to set either periodic or fixed lateral boundary conditions. Huge relative fluctuations in weight measurements appear in case of fixed lateral walls. This may be related to some indetermination in the mobilization state of friction forces on lateral walls. This argument would hold for any piling, but would lead to huge fluctuations in 2D systems only, because of averaging effects in 3D.

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We study sound propagation in a triangular lattice of spherical beads under isotropic stress. Polydispersity of real beads breaks some contacts, creating a disordered lattice of contacting beads. At large stress, the sound velocity behaves according to Hertz contact law and departs from it at lower stress. This evolution is reversible, with the same crossover when increasing or decreasing the stress, for a given piling. Correlations are much more sensitive to disorder. When calculated with signals propagated in the same lattice, they evolve reversibly with the stress, being much higher at large stress when the contact lattice is more regular. This leads to an interpretation of the non-Hertzian behavior in terms of progressive activation of contacts, in discrepancy with previous models involving buckling of force chains.