F. Patat

A finite difference model For cMUT devices

A finite difference method was implemented to simulate capacitive micromachined ultrasonic transducers (cMUTs) and compared to models described in the literature such as finite element methods. Similar results were obtained. It was found that one master curve described the clamped capacitance. We introduced normalized capacitance versus normalized bias voltage and metallization rate, independent of layer thickness, gap height, and size membrane, leading to the determination of a coupling factor master curve. We present here calculations and measurements of electrical impedance for cMUTs. An electromechanical equivalent circuit was used to perform simulations. Our experimental measurements confirmed the theoretical results in terms of resonance, anti-resonance frequencies, clamped capacitance, and electromechanical coupling factor. Due to inhomogeneity of the tested element array and strong parasitic capacitance between cells, the maximum coupling coefficient value achieved was 0.27. Good agreement with theory was obtained for all findings.

Very-High-Frequency Ultrasound Corneal Imaging as a New Tool for Early Diagnosis of Ocular Surface Toxicity in Rabbits Treated with a Preserved Glaucoma Drug

Aim: To evaluate very-high-frequency (VHF) ultrasound imaging as a new method to detect and quantify early corneal epithelium changes induced by chronic exposure to a benzalkonium-chloride-containing antiglaucoma drug. Methods: Timolol preserved with 0.01% benzalkonium chloride solution was applied b.i.d. in 1 eye of 10 rabbits for 56 days. Unpreserved timolol solution was used as control. Ocular surface changes were assessed weekly combining clinical examinations, in vivo 60-MHz ultrasound imaging and ex vivo histological analysis. Results: VHF ultrasound imaging allowed quantitative measurement of corneal epithelium thickness and qualitative imaging of toxic epithelial damage. It revealed significantly decreased epithelial thickness in vivo as early as the 21st day of treatment (40.75 ± 1.72 µm at D0 vs. 39 ± 2 at D21, vs. 31.9 ± 2.98 at D56; p = 0.017 and p = 0.005, respectively). The first clinical changes appeared from the 42nd day of treatment (conjunctival redness, conjunctival staining and corneal staining; D56 compared to D0: p = 0.005, 0.01 and 0.004, respectively) and then correlated with VHF ultrasound data. Epithelial thickness measured with VHF ultrasound was correlated with histological epithelial pachymetry (p < 0.001) and with the corneal damage score assessed with scanning electron microscopy (p = 0.038). Conclusion: VHF ultrasound imaging provided an early in vivo diagnosis of corneal epithelium pathology induced by chronic exposure to a preserved glaucoma drug, before the first clinical evidence of ocular toxicity. It could be a new reproducible method to detect the toxicity of glaucoma medication so that therapy can then be adapted.

Acoustic pressure measurement by acousto-optic tomography

Measurements of acoustic fields in liquids are generally made by means of hydrophones, which limits are clearly known: spatial resolution limited by the active element size and need for periodic calibration when one wants absolute measurements. This work describes how the acousto-optic effect can be used for precise ultrasonic beam mapping. The laser beam of an optical interferometer is launched in the liquid in a direction perpendicular to the ultrasonic beam, and is reflected back by a mirror. Since the optical index is modified by the ultrasonic pressure, the receiving photodiode will see a phase change proportional to the line integral of the acoustical pressure along the light path, i.e. across the ultrasonic beam. By moving the laser beam, one can get a set of projections of the acoustical map at different angles. Then a classical filtered backprojection algorithm is used to perform the tomographic reconstruction and recover the acoustical field values in absolute values of pressure if ones knows the piezo-optic coefficient of the liquid. Experiments have been made in water using a Thales SH 140 interferometer and a HIFU transducer. Optical beam deflection gives an upper limit for high intensities. In our experiment, a maximum pressure integral of 3.35 MPa.mm was measured. With reasonable averaging, a sensitivity of /spl sigma//sub v/=3.53 /spl mu/rd//spl radic/Hz can be obtained leading to a measurement dynamic range of at least 87.5 dB. The spatial resolution is linked to the laser beam size (a few tens of microns) and the measurement bandwidth is 40 MHz, so it is easy to follow the growing of ultrasonic harmonics.

P2P-9 Collective Behavior of cMUT Cells for the Prediction of Electroacoustic Response and Directivity Pattern

This paper describes the development of numerical model of a cMUT array element. The array is modeled in the same way than piezoelectric plates, i.e. using an equivalent linear matrix transfer. Radiation conditions of collective cMUT are discussed, in the baffle plane and far from the baffle. Finally, example of pulse is given and compared with experimental results

Évaluation in vivo par échographie à très haute fréquence des modifications épithéliales cornéennes induites par un bêta-bloquant avec 0,01 % de chlorure de benzalkonium

But Evaluation de l’apport de l’echographie tres haute frequence (ETHF) dans le suivi comparatif in vivo des modifications corneennes epitheliales induites par une toxicite medicamenteuse locale. Materiel et methode Une solution de timolol contenant 0,01 % de chlorure de benzalkonium (BAC) (Timoptol ® ) a ete instillee deux fois par jour dans l’œil test de 10 lapins pendant deux mois, tandis que l’œil temoin recevait la meme solution sans BAC (Timabak ® ). Nous avons utilise une ETHF a 60 MHz afin d’evaluer in vivo la toxicite corneenne du BAC. Cette etude prospective et comparative comportait une evaluation clinique et echographique hebdomadaire ainsi qu’une analyse histologique finale. Resultats Les examens cliniques ont mis en evidence une alteration de la surface oculaire des cornees exposees au BAC : hyperhemie conjonctivale, keratite et alteration du film lacrymal. L’ETHF a revele une decroissance dans le temps de l’epaisseur epitheliale des cornees exposees au BAC (de 40,9 ± 1,6 μm a J0, a 31,8 ± 3,4 μm a J56 ; p = 0,0006), alors que les cornees non-exposees presentaient une stabilite de l’epaisseur epitheliale. Les mesures quantitatives de l’epaisseur epitheliale corneenne par ETHF etaient correlees a l’atteinte corneenne clinique (a J34 et J56 ; p = 0,0025 et p = 0,0377 respectivement) ainsi qu’aux mesures morphometriques histologiques (p = 0,0176). De plus, l’ETHF a permis la visualisation echographique precoce et in vivo des alterations morphologiques epitheliales induites par le BAC. Conclusion L’ETHF a permis un suivi in vivo des modifications epitheliales induites par une exposition chronique a un conservateur. Cette technique d’imagerie in vivo pourrait permettre une evaluation objective, reproductible et precoce de la pathologie toxique de la surface oculaire.

Shear wave elastography: modeling of the shear wave propagation in heterogeneous tissue by pseudospectral method

Shear wave elastography is a promising approach to characterize mechanical properties of soft tissue. By using radiation pressure, it is possible to create deeply localized stress in biological tissue which generates shear waves. We have developed a numerical technique, based on a pseudospectral (PS) method, which solves the wave equation with a source term in an isotropic solid. Perfectly matched layers (PML) are used on the boundaries of the numerical grid to avoid reflections. Moreover, the temporal evolution is done with an Adams-Bashforth method. In the case of a point source in a homogeneous elastic medium, numerical results are favorably compared (directivity patterns, displacement shape, ...) with analytical results computed with the elastodynamic Greens function. The shear wave propagation has then been simulated in a 2D plane comprising a small inclusion with a shear elasticity coefficient different from the surrounding tissue. The shear wavefront deformation, as well as the diffraction and reflection of the shear wave, can be calculated and analyzed.

4J-6 In Vivo High Frequency Elastography for Mechanical Behavior of Human Skin Under Suction Stress: Elastograms and Kinetics of Shear, Axial and Lateral Strain Fields

The aim of this work was to show the potential of high-frequency elastography in depicting the mechanical behavior of human skin under suction stress in vivo. Our 20 MHz real time sonograph (DERMCUP2020trade), was combined with a circular suction device to measure, by elastography, different strain fields in the skin. The elastographic process was based on filtered SSE technique with 2D tracking. Elastograms were cumulated to reach high deformation percents with a good SNRe and to generate kinetic profiles. In-vivo experiments were conducted on the volar forearm healthy human skin. Shear, axial and lateral strain elastograms/kinetics were produced with cumulating small deformation steps. Axial, lateral and shear strain fields presented useful and complementary data to describe the mechanical behavior in different layer in the skin. The quantitative approach was to measure kinetic profiles of specific ROIs in elastograms and extract the useful information corresponding to each kind of strain

High frequency elastography for in-vivo study of the mechanical behavior of skin

The aim of this paper is to show the potential of high frequency elastography in the evaluation of the in-vivo mechanical behavior of human skin. In the first part we describe the filtered and staggered strain estimation (FSSE) which is a new elastographic algorithm based on the staggered strain estimation (SSE) method. This algorithm does not integrate the global stretching, commonly used in the case of global compression stress, but it uses a low-pass filtering to estimate the axial strain in a complex medium such as the skin. We present also a lateral displacement detection technique based on a parabolic interpolation. In the second part we expose a PVA-cryogel phantom and in-vivo experiments conducted with two combined devices: our 20 MHz real time sonographer (DERMCUP2020/spl reg/) used for ultrasound imaging and a patented device devoted to apply uniaxial stretching stress. The in-vivo mechanical behavior of human skin is illustrated using the multi-stretching FSSE technique and elastograms and lateral displacement images are shown in the results.

Optical observation of shear waves excited by focused ultrasound in a tissue-mimicking phantom

Acoustic radiation force imaging is a new approach to characterize mechanical properties of soft tissues. This method provides to the physician a virtual finger to probe the elasticity of internal regions. The tissue response to the impulse stress caused by the localized radiation force can be described by the elastodynamic Green function: the remotely induced displacements are mainly created by shear waves. This work describes how an optical interferometric method can be used to study both the displacements induced by the radiation force at the focal point and the axial shear displacement along the radial direction around the focal point of the transducer. A tissue-mimicking phantom including a I gm thick metallized sheet of Mylar is placed in a water tank. While the transient shear wave propagates, this reflective target moves with the tissue. The central plane of the phantom was positioned between the transducer and the laser beam, perpendicular to both ultrasonic and laser beams. The laser beam propagates in the optically transparent phantom and is reflected back by the Mylar. A 1 MHz transducer focalizes at the optical focal point. By moving the transducer and continuously measuring the displacement at the same location with the laser probe, we can reconstruct the temporal and spatial behavior of axial shear displacement along the radial direction around the focal point of the transducer. From these displacement measurements, attenuation and celerity distributions of this shear wave are extracted. By using this configuration (for pressures in the MPa range), displacements in the gm range can be measured with a noise level of 2 nm whereas the ultrasonic methods usually used to measure these tissue displacements have a noise level of 1 /spl mu/m.

3J-5 Concentration of Particles in Suspension in a Continuous Stream Using Ultrasound Radiation Force

Exposure of suspended microparticles to an ultrasonic standing-wave field subjects them to acoustic radiation forces that cause the particles to migrate into pressure nodal or anti-nodal planes of the sound field. The objective of our work is to design a continuous-flow microparticle filter to separate particle suspensions into a cleared phase (reduced particle concentration) and an enriched phase (increased particle concentration). Based on the principle of the h-shaped ultrasonic separator, we propose a device in which the radiation-force pattern is slowly moved into the cavity, by superimposing two standing waves with a linear phase modulation between them. One then obtains a slowly-progressive standing wave, which theoretically improves the ability of the system to concentrate a particle suspension. We favorably compare theoretical ultrasound pressure, computed with an analytical 1D transfer-matrix approach, and measurements realized by acousto-optic interactions inside the separation chamber