Forward model for quantitative pulse-echo speed-of-sound imaging

Computed ultrasound tomography in echo mode (CUTE) allows determining the spatial distribution of speed-of-sound (SoS) inside tissue using handheld pulse-echo ultrasound (US). This technique is based on measuring the changing phase of beamformed echoes obtained under varying transmit (Tx) and/or receive (Rx) steering angles. The SoS is reconstructed by inverting a forward model describing how the spatial distribution of SoS is related to the spatial distribution of the echo phase shift. CUTE holds promise as a novel diagnostic modality that complements conventional US in a single, real-time handheld system. Here we demonstrate that, in order to obtain robust quantitative results, the forward model must contain two features that were not taken into account so far: a) the phase shift must be detected between pairs of Tx and Rx angles that are centred around a set of common mid-angles, and b) it must account for an additional phase shift induced by the error of the reconstructed position of echoes. In a phantom study mimicking liver imaging, this new model leads to a substantially improved quantitative SoS reconstruction compared to the model that has been used so far. The importance of the new model as a prerequisite for an accurate diagnosis is corroborated in preliminary volunteer results

Caractérisations thermiques de composites polymères dispersés dans du cristal liquide et de matériaux composites thermoélectriques à base de polymères avec les techniques photothermiques

Primarily, newly developed, high sensitive and accurate methods for thermal characterization of liquids using photothermal radiometry are presented. Two experimental configurations are suggested, tested and validated with usual liquid materials. These methods are used to study polymer dispersed liquid crystal samples. Dynamic thermal properties of samples are analysed verses amplitude varying applied electric field with constant frequency as well as versus frequency varying electric field with constant amplitude. Our results clearly show the thermal properties of the samples are prone to depolarizing field effects at the lower frequencies of the applied electric field. The experimental results are modeled against existing theories to predict electric properties of the sample composites. Second part of the manuscript describes the development of a novel photothermal technique based on thermoelectric effect. This technique is particularly useful for thermally characterizing thermoelectric materials without using a separate sensor for measuring induced temperature changes. A theoretical and experimental study is presented. The experiments are done on polyaniline - carbon nanotube composite pellets by measuring Seebeck voltage generated by the samples upon heating by a modulated laser beam. Additional infrared radiometry experiments are done on the same samples and the results are in good agreement with those previously found. Later on, the possibility of photothermoelectric materials to be used as sensors for finding thermal transport properties of materials with a thermal wave resonant cavity is suggested.Dans une première partie, une nouvelle méthodologie, précise et hautement sensible de caractérisation des paramètres thermiques de liquides par radiométrie photothermique est ici présentée. Deux configurations expérimentales sont proposées. Elles ont été testées et validées avec des matériaux liquides usuels aux paramètres thermiques connus. Par la suite, cette démarche a été utilisée pour l'étude de polymères dispersés dans des cristaux liquides. Les propriétés thermiques dynamiques de chaque échantillon ont été mesurées en fonction de l'amplitude du champ électrique appliqué à une fréquence donnée aussi bien qu'en fonction de la fréquence du champ électrique à une amplitude fixe. Cette étude a montré que les propriétés thermiques étaient sujettes aux effets du champ de dépolarisation aux basses fréquences. La seconde partie de ce manuscrit décrit la nouvelle technique photothermique basée sur l'effet thermoélectrique. Cette technique est utile pour caractériser thermiquement les matériaux thermoélectriques sans avoir à recourir à un capteur extérieur pour mesurer le changement de température. Une étude théorique et expérimentale est présentée. Ces expériences ont été réalisées avec des composites polyaniline/nanotubes de carbone par mesure de la tension générée par l'échantillon thermoélectrique chauffé par un faisceau laser. Des mesures additionnelles à l'aide de la radiométrie infrarouge sur ces mêmes échantillons ont été entreprises et les résultats sont en bon accord avec ceux précédemment trouvés. Enfin, la possibilité d'utiliser les matériaux thermoélectriques comme capteur photothermique au travers d'une cavité résonnante à ondes thermiques est évoquée

Caractérisations thermiques de composites polymères dispersés dans du cristal liquide et de matériaux composites thermoélectriques à base de polymères avec les techniques photothermiques

Dans une première partie, une nouvelle méthodologie, précise et hautement sensible de caractérisation des paramètres thermiques de liquides par radiométrie photothermique est ici présentée. Deux configurations expérimentales sont proposées. Elles ont été testées et validées avec des matériaux liquides usuels aux paramètres thermiques connus. Par la suite, cette démarche a été utilisée pour l'étude de polymères dispersés dans des cristaux liquides. Les propriétés thermiques dynamiques de chaque échantillon ont été mesurées en fonction de l'amplitude du champ électrique appliqué à une fréquence donnée aussi bien qu'en fonction de la fréquence du champ électrique à une amplitude fixe. Cette étude a montré que les propriétés thermiques étaient sujettes aux effets du champ de dépolarisation aux basses fréquences. La seconde partie de ce manuscrit décrit la nouvelle technique photothermique basée sur l'effet thermoélectrique. Cette technique est utile pour caractériser thermiquement les matériaux thermoélectriques sans avoir à recourir à un capteur extérieur pour mesurer le changement de température. Une étude théorique et expérimentale est présentée. Ces expériences ont été réalisées avec des composites polyaniline/nanotubes de carbone par mesure de la tension générée par l'échantillon thermoélectrique chauffé par un faisceau laser. Des mesures additionnelles à l'aide de la radiométrie infrarouge sur ces mêmes échantillons ont été entreprises et les résultats sont en bon accord avec ceux précédemment trouvés. Enfin, la possibilité d'utiliser les matériaux thermoélectriques comme capteur photothermique au travers d'une cavité résonnante à ondes thermiques est évoquée.Primarily, newly developed, high sensitive and accurate methods for thermal characterization of liquids using photothermal radiometry are presented. Two experimental configurations are suggested, tested and validated with usual liquid materials. These methods are used to study polymer dispersed liquid crystal samples. Dynamic thermal properties of samples are analysed verses amplitude varying applied electric field with constant frequency as well as versus frequency varying electric field with constant amplitude. Our results clearly show the thermal properties of the samples are prone to depolarizing field effects at the lower frequencies of the applied electric field. The experimental results are modeled against existing theories to predict electric properties of the sample composites. Second part of the manuscript describes the development of a novel photothermal technique based on thermoelectric effect. This technique is particularly useful for thermally characterizing thermoelectric materials without using a separate sensor for measuring induced temperature changes. A theoretical and experimental study is presented. The experiments are done on polyaniline - carbon nanotube composite pellets by measuring Seebeck voltage generated by the samples upon heating by a modulated laser beam. Additional infrared radiometry experiments are done on the same samples and the results are in good agreement with those previously found. Later on, the possibility of photothermoelectric materials to be used as sensors for finding thermal transport properties of materials with a thermal wave resonant cavity is suggested.DUNKERQUE-SCD-Bib.electronique (591839901) / SudocSudocFranceF

Dynamics of specific heat and other relaxation processes in supercooled liquids by impulsive stimulated scattering

Laser based impulsive stimulated scattering or transient grating excitation in a heterodyne diffraction scheme is a powerful method to extract information about different relaxing properties from different signal contributions. Longitudinal acoustic waves are detected simultaneously with thermal expansion and thermal diffusion. Careful fitting of the time-domain density response at different temperatures makes it possible to obtain the various relaxing physical parameters, and to construct Arrhenius plots for the respective relaxation processes. In this work we focus on the influence of the specific heat capacity C on the slower part of the density response function S(t), and, inversely, on the possibility to extract from experimental S(t) data the relaxation behaviour C(omega). The specific heat capacity is relevant for both the initially rising part of the impulsive stimulated scattering signal (together with the time and frequency dependent thermal expansion eta(t)), and for the thermal diffusion dominated decrease of the signal at later times after the excitation. By simulating S(t) data in different scenarios, we address the feasibility of unravelling the impulse response functions C(t) and eta(t) (and via Fourier transform also C(omega) and eta(omega)) by careful fitting of the signal. This approach offers a unique possibility to extend the 100 kHz bandwidth of current dynamic calorimetric techniques determining C(omega) (photopyroelectric spectroscopy) to the sub-GHz range.no ISBNstatus: publishe

Receive Beam-Steering and Clutter Reduction for Imaging the Speed-of-Sound Inside the Carotid Artery

Handheld imaging of the tissue’s speed-of-sound (SoS) is a promising multimodal addition to diagnostic ultrasonography for the examination of tissue composition. Computed ultrasound tomography in echo mode (CUTE) probes the spatial distribution of SoS, conventionally via scanning the tissue under a varying angle of ultrasound transmission, and quantifying—in a spatially resolved way—phase variations of the beamformed echoes. So far, this technique is not applicable to imaging the lumen of vessels, where blood flow and tissue clutter inhibit phase tracking of the blood echoes. With the goal to enable the assessment of atherosclerotic plaque composition inside the carotid artery, we propose two modifications to CUTE: (a) use receive (Rx) beam-steering as opposed to transmit (Tx) beam-steering to increase acquisition speed and to reduce flow-related phase decorrelation, and (b) conduct pairwise subtraction of data obtained from repetitions of the scan sequence, to highlight blood echoes relative to static echo clutter and thus enable the phase tracking of blood echoes. These modifications were tested in a phantom study, where the echogenicity of the vessel lumen was chosen to be similar to the one of the background medium, which allows a direct comparison of SoS images obtained with the different techniques. Our results demonstrate that the combination of Rx-steering with the subtraction technique results in an SoS image of the same quality as obtained with conventional Tx-steering. Together with the improved acquisition speed, this makes the proposed technique a key step towards successful imaging of the SoS inside the carotid artery

In situ imaging of the dynamics of photo-induced structural phase transition at high pressures by picosecond acoustic interferometry

International audiencePicosecond acoustic interferometry is used to monitor in time the motion of the phase transition boundary between two water ice phases, VII and VI, coexisting at a pressure of 2.15 GPa when compressed in a diamond anvil cell at room temperature. By analyzing the time-domain Brillouin scattering signals accumulated for a single incidence direction of probe laser pulses, it is possible to access ratios of sound velocity values and of the refractive indices of the involved phases, and to distinguish between the structural phase transition and a recrystallization process. Two-dimensional spatial imaging of the phase transition dynamics indicates that it is initiated by the pump and probe laser pulses, preferentially at the diamond/ice interface. This method should find applications in three-dimensional monitoring with nanometer spatial resolution of the temporal dynamics of low-contrast material inhomogeneities caused by phase transitions or chemical reactions in optically transparent media

Single-crystal elastic moduli Cij(P)and shear modulus G(P)of solid argon up to 64 GPa from time-domain Brillouin scattering

International audienceSolid argon, crystallizing above 1.3 GPa in the face-centered cubic (fcc) structure, is considered as the archetype of a simple classical solid [1] due to a highly symmetric distribution of electron density in the atoms having completely filled electron shells. For this reason, its behavior at high pressures was intensively investigated, also by theory, aiming understanding of evolution of solids upon a strong density increase. Symmetric electron shells and cubic structure of solid argon imply elastic isotropy of its crystals which, however, was not confirmed experimentally. Accordingly, a significant and growing-with-compression contribution of non-central interaction between the Ar-atoms was hypothesized as an explanation. Also, solid argon (as well as other solidified noble gases such as neon or helium) is extremely compressible and produces a quasi-hydrostatic environment, as has been concluded on the basis of sharpness of its X-ray diffraction peaks measured up to ~8 GPa [2]. However, the degree of its hydrostaticity at higher pressures was a subject of recent controversial discussions [3,4]. Due to a strong change of its lattice parameter with pressure, solid argon was also suggested and used as an internal pressure standard [5-7]. In the present work, we have measured the maximal and minimal values of the product of the refractive index n(P) with the longitudinal sound velocity V L (P), n(P)·V L (P), in single crystals of solid argon up to 64 GPa (Figure 1). For solid argon having cubic structure, the maximal sound velocity corresponds to that along the 111 direction in a single crystal, V L111 , and the minimal sound velocity to that along the 100 direction, V L100. The observed in this work strong deviation of V L111 (P) from V L100 (P) with pressure increase (Figure 1) indicated a strong elastic anisotropy of this solid, which deserves a special attention because, at the maximal pressure, the density of solid argon is more than twice higher than that just after its solidification at P=1.2 GPa [6]. It should be mentioned here that the detected strong elastic anisotropy of solid argon could not be falsified by the presence of the hcp phase, potentially coexisting with the main fcc phase at high pressures [8]. This is because our theoretical calculations showed that the hcp phase could not contribute to the measured-by-us strong deviation of the maximal and minimal n(P)·V L (P) values (Figure 1). In the present work we used the technique of time-domain Brillouin scattering (TDBS) [9] permiting 3D-scanning of the n·V L distribution in transparent samples with sub-μm resolution along the DAC axis, in addition to the micrometric lateral resolution [10-12]. Important is to mention that the spatial resolution of the TDBS technique did not degrade with pressure. These capabilities of the TDBS technique permitted us to pitch on the extremes of the n·V L values in the polycrystalline samples of fcc argon compressed in a DAC and to closely approach the n·V L111 and n·V L100 values up to the highest pressure of our work of 64 GPa (Figure 1). Calculation of the refractive index of the fcc argon as a function of pressure n(P) was another part of the work which allowed to derive V L (P) values from the the products n(P)·V L (P) obtained directly from the oscilating TDBS signals S(t). To obtain the reliable axially resolved data, we applied the same demanding time-frequency analysis of the raw TDBS signals S(t) as described in detail earlier [11]. Figure 1. Our experimental and theoretical data on longitudinal sound velocities of solid argon at high pressures. The experimental datapoints in terms of the product n·V L , obtained using the TDBS technique, are represented by triangles pointing up and down corresponding to n(P)·V L111 (P) and n(P)·V L100 (P), respectively. They are compared with the same theoretically-calculated values for the fcc phase (solid red lines) and hcp phase (dashed violet lines) Using the envelope method and our theoretical n(P) for the fcc phase we determined, with a high degree of confidence, pressure dependences of the fastest an

Picosecond laser ultrasonics for imaging of transparent polycrystalline materials compressed to megabar pressures

International audiencePicosecond laser ultrasonics is an all-optical experimental technique based on ultrafast high repetition rate lasers applied for the generation and detection of nanometric in length coherent acoustic pulses. In optically transparent materials these pulses can be detected not only on their arrival at the sample surfaces but also all along their propagation path inside the sample providing opportunity for imaging of the sample material spatial inhomogeneities traversed by the acoustic pulse. Application of this imaging technique to polycrystalline elastically anisotropic transparent materials subject to high pressures in a diamond anvil cell reveals their significant texturing/structuring at the spatial scales exceeding dimensions of the individual crystallites

Elastic anisotropy and single-crystal moduli of solid argon up to 64 GPa from time-domain Brillouin scattering

International audienceSingle-crystal elastic moduli Cij, shear modulus G, and Zener anisotropic ratio A of solid argon having the face-centered cubic (fcc) structure were determined at high pressures between 12 and 64 GPa using a combination of experimental and theoretical approaches. The experimental data, namely, the maximal and minimal values of the product of the longitudinal sound velocity and refractive index n·VL, were obtained from the 3D scanning of elastic inhomogeneities in compressed samples of argon using the technique of time-domain Brillouin scattering. These inhomogeneities, caused by elastic anisotropy of solid argon, were revealed to be about twice as strong as those reported in the earlier experiments using classical Brillouin light scattering (BLS). To derive the VL values, we used the refractive index obtained here from ab initio calculations which also permitted us to rule out any contribution to the amplitude of the observed elastic inhomogeneities of the hexagonal close-packed phase of argon, proposed to coexist with the fcc phase at high pressures. From the measured Ci j(P), we derived pressure dependence of shear modulus of the fcc argon GH (P) using the Voigt-Reuss-Hill approximation and found a very good agreement with the earlier G(P) obtained from shear sound velocities in the classical BLS measurements. Our results agree very well with the earlier predictions based on a relatively simple many-body model employing the Buckingham pair potential. Finally, our measurements show a much weaker change of the Cauchy discrepancy (C12 − C44 − 2P) of the fcc argon with pressure than reported in all earlier experimental and theoretical works