-
[show abstract]
[hide abstract]
ABSTRACT: An ultrasonic hydrophone based on the Lorentz force is introduced. When a metallic wire is moved by ultrasound while submitted to a magnetic field, the Lorentz force induces an electrical current proportional to the integral of pressure along the wire. 2D pressure field mapping is achieved by performing a tomography through wire translations and rotations in the imaging plane. Performances of this hydrophone are assessed in this study. Signal is linear over pressure from 10 kPa to at least 10 MPa with a determination coefficient R(2) above 0.997. Excellent resistance to cavitation has been observed. Frequency bandwidth was measured against three different wire diameters: 70 μm, 100 μm, and 210 μm. Results showed that upper cut-off frequency decreases with increasing wire diameter. Additional measurements showed that wire tension has no visible effect on the signal. Such characteristics are potentially of great interest for high-intensity focused ultrasound and shockwave transducers calibration.
The Journal of the Acoustical Society of America 05/2013; 133(5):3230. · 1.55 Impact Factor
-
[show abstract]
[hide abstract]
ABSTRACT: This paper focuses on the experimental and numerical studies of acoustic cavitation induced micro bubbles in a standing waveguide filled with water. It is shown that the cylindrical geometry of the system used in this study allows the micro bubbles to self organize into particular patterns. At high pressure amplitudes, the cavitation bubbles tend to aggregate into well known cluster patterns and at relatively low pressure amplitudes, the cavitation micro bubbles aggregate into ring patterns. This study highlights that the shape of these ring patterns is directly related to the Bjerknes force distribution in the resonator. It is also shown both experimentally and numerically that cavitation bubbles may exhibit spiraling behavior around this ring pattern. This spiraling phenomenon is numerically studied and the conditions for which a single cavitation bubble follows an orbital trajectory in the cylindrical waveguide are established, and the influences of the acoustic pressure amplitude and the initial bubble radius are investigated.
The Journal of the Acoustical Society of America 05/2013; 133(5):3277. · 1.55 Impact Factor
-
[show abstract]
[hide abstract]
ABSTRACT: The velocity and pressure of an ultrasonic wave can be measured by an
electromagnetic hydrophone made of a thin wire and a magnet. The ultrasonic
wave vibrates the wire inside a magnetic field, inducing an electrical current.
Previous articles reported poor spatial resolution of comparable hydrophones
along the axis of the wire. In this study, submillimetric spatial resolution
has been achieved by using a tomographic method. Moreover, a physical model is
presented for obtaining absolute measurements. A pressure differential of 8%
has been found between piezoelectric and electromagnetic hydrophone
measurements. These characteristics show this technique as an alternative to
standard hydrophones.
07/2012;
-
[show abstract]
[hide abstract]
ABSTRACT: A tomographic method based on the Lorentz force for the measurement of the pressure of an ultrasound transducer is presented. When a metal wire is vibrating under the influence of a pressure field created by an ultrasound transducer while submitted to a magnetic field, the Lorentz force induces an electrical current. This current is considered proportional to the integral of pressure along the wire. By moving the wire perpendicular to the ultrasound axis, and rotating it around the same axis, a sinogram of the pressure field can be elaborated. Then an inverse Radon transform of the signal gives the pressure field spatial distribution. An experiment was conducted where a 1 MHz transducer generated an ultrasound wave with a focal point at 4 cm. A 100 μm in diameter shielded copper wire was placed perpendicular to the ultrasound propagation axis, and inside a 300 mT magnetic field created by a permanent magnet. The main advantages of the hydrophone created by the wire-magnet system are the large frequency bandwidth and the resistance to high pressure, parameters still under investigation. Possible disadvantages are the sensibility to electromagnetic noise and the possible distortion of the pressure field when using a too thick wire.
The Journal of the Acoustical Society of America 04/2012; 131(4):3526. · 1.55 Impact Factor
-
[show abstract]
[hide abstract]
ABSTRACT: Previous experiments on the influence of waveform excitation on ultrasonic cavitation threshold and activity showed that the combination of two neighbouring frequency components modifies cavitation threshold and stimulates cavitation activity beyond threshold in comparison to a classical monofrequency waveform. For high monofrequency thresholds, the bifrequency excitation reduces the threshold value. For low monofrequency thresholds, the bifrequency threshold is higher, which shows that nonlinear effects are involved in the process. In-situ measurements of the amplitudes of the difference frequency component show that this low frequency is too weak to trigger cavitation alone. Numerical simulations of the dynamics of a single bubble in a bifrequency oscillating pressure field were done. They show that the bifrequency excitation modifies the cavitation threshold for specific initial bubble radius but is not sufficient to explain experimental observations. To simulate first-order nonlinear propagation effect, the difference frequency component was superimposed to the excitation field. The reduction of cavitation threshold is observed on a wider range of initial bubble radius. Despite this larger effect, the experimental observations seem to be rather related to the spectral broadening of the excitation due to higher order nonlinear combinations of primary frequencies.
International Congress on Ultrasonics, Gdansk; 01/2012
-
[show abstract]
[hide abstract]
ABSTRACT: A bifrequency excitation consisting of two neighboring frequency components can reduce intensities needed to achieve strong inertial cavitation activities. We present in-vitro experimental results aiming at testing such a bifrequency excitation for extracorporeal ultrasound thrombolysis. In a first set of experiments, human blood clots were inserted in small tubes filled with saline and placed at the focus of a piezoelectric transducer. The efficiencies of mono- (550 kHZ) and bifrequency (535 and 565 kHz) excitations were compared for (spta) intensities ranging from 50 to 160 W/cm(2), and a passive recording of the cavitation activity was performed during treatment. A modified setup enabled to measure the size distribution of the debris resulting from thrombolysis experiments realized under flow. A comparison of the spatial temperature distribution for each type of excitation was performed in another set of experiments using MR temperature imaging. Under the conditions of the experiments, 80% of thrombolysis was achieved with a monofrequency intensity of 150 W/cm(2), while 80 W/cm(2) were sufficient with a bifrequency excitation. Mean debris size was reduced by the use of a bifrequency excitation, and MR temperature imaging showed that, for a given intensity, the spatial temperature distributions are the same for both types of excitation.
The Journal of the Acoustical Society of America 10/2011; 130(4):2501. · 1.55 Impact Factor
-
[show abstract]
[hide abstract]
ABSTRACT: 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.
Ultrasonics Sonochemistry 03/2011; 18(2):589-94. · 3.57 Impact Factor
-
[show abstract]
[hide abstract]
ABSTRACT: L'utilisation des ultrasons pour des applications thérapeutiques et diagnostiques est devenue fréquente. Il a été montré que l'utilisation d'ondes acoustiques focalisées permet de détruire des caillots de sang sans utiliser d'agent thrombolytique. Cependant, la technique se heurte à un certain nombre d'obstacles ou d'inconnues, tels que la taille des fragments libérés dans la circulation lors du processus. Dans ce cadre, nous nous proposons d'étudier et de quantifier, grâce à une technique optique basée sur le traitement d'images, la distribution de tailles de fragments libérés. Des expériences in-vitro ont été effectuées en utilisant des cibles de sang humain coagulé et placées dans une section d'essai siège d'un écoulement pulsé. les visualisations montrent qu'à partir de certaines intensités acoustiques, il est possible de percer le caillot et qu'on libère des fragments dont la distribution de tailles dépend de l'intensité d'excitation et du temps de séjours des résidus dans le champ ultrasonore.
20ème Congrès Français de Mécanique, Besançan, France; 01/2011
-
[show abstract]
[hide abstract]
ABSTRACT: A numerical model was developed to predict the dynamics of a solid particle in a poststenotic blood vessel region. The flow through a 3D axisymmetric stenosis with 75% reduction in cross-section area was considered for inlet Reynolds numbers of 500 and 1000, which corresponds to typical values for the blood flow in human large arteries. Spherical particles were injected in the flow from the stenosis and tracked using the Discrete Phase Model (DPM) based on a Lagrangian approach. Within the scope of the development of ultrasound thrombolysis methods, the hydrodynamical forces predicted were used to evaluate the residence time of the particle and the minimal ultrasonic intensity required to keep it in the treatment region. For particle sizes larger than 400 μm, the intensity required appeared to be compatible with extracorporeal therapeutic ultrasound.
Medical Engineering & Physics. 01/2011;
-
[show abstract]
[hide abstract]
ABSTRACT: 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.
Ultrasonics 01/2011; 51(1):94-101. · 1.84 Impact Factor
-
[show abstract]
[hide abstract]
ABSTRACT: Applications involving cavitation mechanisms, such as sonoporation, are irreproducible in the case of a fixed-intensity sonication, due to the non-stationary behavior of cavitation. We then propose to work at a fixed-cavitation level instead of under fixed-intensity sonication conditions. For this purpose a regulated cavitation generator has been developed in a stationary wave field configuration, which allows regulation of the cavitation level during sonication by modulating the applied acoustic intensity with a feedback loop based on acoustic cavitation measurements. The cavitation level indicator was quantified by the broadband spectrum noise level relative to inertial cavitation events. This generated inertial cavitation was characterized by both acoustic and chemical measurements, quantifying hydroxyl radicals produced by water sonolysis. While the cavitation level is obtained with a 40% standard deviation for fixed applied acoustic intensities in the range [0.01 3.44] W/cm(2), the regulated generator reproduces the cavitation level with a standard deviation of 3%. The results show that the hydroxyl radical production is better correlated with the cavitation level setting than with the applied acoustic intensity, highlighting the fact that broadband noise is a good indicator of inertial cavitation, with greatest interest for cavitation monitoring. In summary, the regulated device generates a cavitation level that is reproducible, repeatable and stable in time. This system produces reproducible effects that allow consideration of biological applications such as sonoporation to be independent of the experimental ultrasound device, as confirmed by transfection efficiency and cell cytotoxicity studies. Thus, this feedback loop process presents interesting perspectives for monitoring and controlling in-vivo cavitation.
10TH INTERNATIONAL SYMPOSIUM ON THERAPEUTIC ULTRASOUND, Tokyo; 01/2011
-
[show abstract]
[hide abstract]
ABSTRACT: 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.
Physical Review E 03/2008; 77(2 Pt 1):021302. · 2.26 Impact Factor
-
[show abstract]
[hide abstract]
ABSTRACT: Within the scope of an application to transcutaneous ultrasonic thrombolysis, this study compares inertial cavitation thresholds obtained at the focus of a transducer for two types of ultrasonic excitations: a classical one frequency signal and an excitation combining two slightly different frequencies. A cepstral filtering method is used to measure inertial cavitation activity for these signals. A 30% reduction of the threshold is observed when using bifrequency excitation.
Applied Physics Letters 09/2006; · 3.84 Impact Factor
-
[show abstract]
[hide abstract]
ABSTRACT: 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.
Physical Review Letters 06/2004; 92(20):204301. · 7.37 Impact Factor
-
[show abstract]
[hide abstract]
ABSTRACT: 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.
Physical Review Letters 06/2003; 90(17):174302. · 7.37 Impact Factor
-
[show abstract]
[hide abstract]
ABSTRACT: Bon nombre d'études ont montré que la cavitation inertielle jouait un rôle important en thérapie ultrasonore. Par exemple, pour une application à la thrombolyse ultrasonore, l'implosion de bulles de cavitation peut aboutir à la lyse d'un caillot sanguin sans ajout d'agents thrombolytiques. Dans une précédente étude, nous avons montré que l'utilisation d'une excitation bifréquentielle, constituée de deux fréquences proches (535kHz et 565 kHz); émises à l'aide d'un seul transducteur piézoélectrique focalisé, permet de réduire le seuil d'apparition de la cavitation inertielle et d'en augmenter l'activité. Ceci s'avère particulièrement intéressant dans le cadre d'une application à la thrombolyse ultrasonore, puisque cela permet de réduire les puissances utilisées en vue de limiter au maximum les échauffements des tissus sains environnants, tout en stimulant les effets mécaniques sur les caillots. La présente étude vise à comparer l'efficacité thrombolytique d'une excitation pulsée traditionnelle à celle d'une telle excitation bifréquentielle sur un modèle de caillots sanguins in vitro. Les caillots ont un poids moyen de 750 mg. Le signal d'excitation est une séquence de tirs pulsés de 27 ms avec un rapport cyclique de 1 :10. Les intensités (Isppa) utilisées vont de 500W/cm² à 1700W/cm². L'efficacité thrombolytique est obtenue en faisant le rapport des poids du caillot avant et après insonification. L'efficacité thrombolytique de l'excitation bifréquentielle est comparée à celle obtenue avec une excitation monofréquentielle (550kHz) de même intensité dans les mêmes conditions expérimentales. Les résultats obtenus montrent que l'excitation bifréquentielle permet de réduire de moitié la puissance nécessaire à la thrombolyse : 1500W/cm² sont névessaires en monofréquentiel pour atteindre 80% d'efficacité thrombolytique, alors que 800W/cm² suffisent en excitation bifréquentielle.
10ème Congrès Français d'Acoustique.
-
[show abstract]
[hide abstract]
ABSTRACT: Des gels de protéines translucides devenant opaques au-delà d'une certaine élévation de température sont utilisés en laboratoire afin de simuler la génération de lésions dans les tissus biologiques par l'exposition à des ultrasons. On s'intéresse dans cette étude aux lésions obtenues soit dans des échantillons de gels translucide contenant un pourcentage variable de protéines BSA, soit dans des gels bicouche constitués de deux tranches de gel aux concentrations de protéines différentes. L'étude est limitée à l'effet d'une excitation par ultrasons non focalisés, dont la propagation reste linéaire, configuration qui nécessite un temps d'exposition de plusieurs dizaines de secondes. Des travaux récents ont mis en évidence que la profondeur des lésions obtenues dans le cas d'un gel bicouche peut être plus importante que dans le cas d'un gel monocouche. Afin d'étudier l'influence des paramètres permettant d'augmenter la taille des lésions, la propagation des ultrasons dans les gels a été modélisée. Les paramètres caractéristiques du milieu de propagation (masse volumique, célérité du son, absorption) ont d'abord été mesurés en faisant varier la température, la fréquence d'excitation, et la concentration en protéines. La propagation des ultrasons a ensuite été modélisée et le champ acoustique calculé. La dose thermique dans différents types d'échantillons a été calculée pour des temps d'exposition de 80 secondes et pour différents niveaux d'excitation afin d'étudier les conditions d'apparition des lésions. Il apparait que la prise en compte de la variation locale de la célérité et de l'absorption en fonction de l'élévation de température est nécessaire pour expliquer la formation des lésions telles qu'elles sont observées expérimentalement.
10ème Congrès Français d'Acoustique.
-
[show abstract]
[hide abstract]
ABSTRACT: La cavitation ultrasonore peut conduire à différents types d'effets biologiques en fonction de son intensité. Ces effets, très destructeurs, présents en lithotritie, en hyperthermie par HIFU ou en histotritie, peuvent aussi réaliser, pour des intensités moindres, des pores cellulaires de taille suffisante pour permettre l'entrée de particules, de matériels génétiques ou de produits chimiques. Dans ces dernières applications, la cavitation n'apparait pas systématiquement et les expérimentations nécessitent souvent des adjuvants comme les produits de contraste utilisés comme germes de cavitation. Nous avons développé un générateur continu de cavitation, travaillant dans la plage d'intensité acoustique de 0.2 à 3.5 W/cm2, et régulé (cadencement : 5 ms) sur un indice de cavitation (CI) établi à partir du signal acoustique émis par la cavitation. Ce dispositif, de part la reproductibilité du processus de cavitation générée, a permis de caractériser en sonodynamothérapie les effets potentialisateurs d'un photosensibilisant (Photofrin©) et de transfecter des cellules non-adhérentes ( RL du lymphome folliculaire ou LLC de Leucémie Lymphoïde Chronique) très difficiles à transfecter de par ailleurs (électroporation, lipofection). Le Photofrin, additionné au milieu de culture (incubation de 30 min), potentialise l'effet cytotoxique induit par cavitation de sa phase stable à transitoire avec une liaison très forte avec le CI (r2>0.95). Nos travaux ont montré une plus grande sensibilité des membranes cellulaires aux contraintes mécaniques induite par le Photofrin. Dans l'étude du dysfonctionnement des mécanismes régulateurs de l'apoptose des LLC par le ciblage des gènes responsables de la chimiorésistance, la transfection de siRNAs correspondants a pu être réalisée. Les taux de transfection sont reproductibles de 10% pour le plasmide PeGFP à près de 60% pour un SiRNA (fluoroscein) avec une mortalité résiduelle réduite à 10%. Ce dispositif a aussi permis d'élaborer un modèle de tumeur fluorescent par une transfection « stable » de plasmides pour des études de tumorogénèse.
10ème Congrès Français d'Acoustique.
