Near-field effects in Green's function retrieval from cross-correlation of elastic fields: experimental study with application to elastography.

In a lossless system, the causal and acausal Green's function for elastic waves can be retrieved by cross-correlating the elastic field at two positions. This field, composed of converging and diverging waves, is interpreted in the frame of a time-reversal process. In this work, the near-field effects on the spatio-temporal focusing of elastic waves are analyzed through the elastodynamic Green's function. Contrary to the scalar field case, the spatial focusing is not symmetric preserving the directivity pattern of a simple source. One important feature of the spatial asymmetry is its dependency on the Poisson ratio of the solid. Additionally, it is shown that the retrieval of the bulk wave speed values is affected by diffraction. The correction factor depends on the relative direction between the source and the observed field. Experimental verification of the analysis is carried out on the volume of a soft-solid. A low-frequency diffuse-like field is generated by random impacts at the sample's free surface. The displacement field is imaged using ultrasound by a standard speckle tracking technique. One important application of this work is in the estimation of the shear elastic modulus in soft biological tissues, whose quantification can be useful in non-invasive diagnosis of various diseases.

Design and acoustic characterization of limited diffraction ultrasonic devices.

Limited diffraction ultrasonic transducers are devices that have a large depth of acoustic field without important effects of diffraction, which make them optimal in applications of medical images, among others. This report details how this special type of piezoelectric device was designed by means of a simple technology using three electrodes in the form of concentric rings in both faces of a ferroelectric ceramic disk, which were used to apply a profile of non-homogeneous polarization. Once designed, the radiation fields emitted by these resonators were characterized experimentally by electro-acoustic and acousto-optic techniques and were compared with those emitted by conventional devices. As shown in the experimental characterizations, ultrasonic transducers with optimal properties for use in medical applications such as good collimation of the ultrasound beam, high lateral resolution, as well as little effects of diffraction were obtained.

Shear elasticity estimation from surface wave: the time reversal approach.

In this work the shear elasticity of soft solids is measured from the surface wave speed estimation. An external source creates mechanical waves which are detected using acoustic sensors. The surface wave speed estimation is extracted from the complex reverberated elastic field through a time-reversal analysis. Measurements in a hard and a soft gelatin-based phantom are validated by independent transient elastography estimations. In contrast with other elasticity assessment methods, one advantage of the present approach is its low sound technology cost. Experiments performed in cheese and soft phantoms allows one to envision applications in the food industry and medicine.

Power spectral estimation of high-harmonics in echoes of wall resonances to improve resolution in non-invasive measurements of wall mechanical properties in rubber tube and ex-vivo artery.

The aim of this work is to develop a new type of ultrasonic analysis of the mechanical properties of an arterial wall with improved resolution, and to confirm its feasibility under laboratory conditions. MOTIVATION: it is expected that this would facilitate a non-invasive path for accurate predictive diagnosis that enables an early detection & therapy of vascular pathologies. In particular, the objective is to detect and quantify the small elasticity changes (in Young's modulus E) of arterial walls, which precede pathology. A submicron axial resolution is required for this analysis, as the periodic widening of the wall (under oscillatory arterial pressure) varies between ±10 and 20 µm. This high resolution represents less than 1% of the parietal thickness (e.g., << 7 µm in carotid arteries). The novelty of our proposal is the new technique used to estimate the modulus E of the arterial walls, which achieves the requisite resolution. It calculates the power spectral evolution associated with the temporal dynamics in higher harmonics of the wall internal resonance f0. This was attained via the implementation of an autoregressive parametric algorithm that accurately detects parietal echo-dynamics during a heartbeat. Thus, it was possible to measure the punctual elasticity of the wall, with a higher resolution (> an order of magnitude) compared to conventional approaches. The resolution of a typical ultrasonic image is limited to several hundred microns, and thus, such small changes are undetected. The proposed procedure provides a non-invasive and direct measure of elasticity by doing an estimation of changes in the Nf0 harmonics and wall thickness with a resolution of 0.1%, for first time. The results obtained by using the classic temporal cross-correlation method (TCC) were compared to those obtained with the new procedure. The latter allowed the evaluation of alterations in the elastic properties of arterial walls that are 30 times smaller than those being detectable with TCC; in fact, the depth resolution of the TCC approach is limited to ≈20 µm for typical SNRs. These values were calculated based on echoes obtained using a reference pattern (rubber tube). The application of the proposed procedure was also confirmed via "ex-vivo" measurements in pig carotid segments.

Non-invasive temperature prediction of in vitro therapeutic ultrasound signals using neural networks.

In this paper, a novel black-box modelling scheme applied to non-invasive temperature prediction in a homogeneous medium subjected to therapeutic ultrasound is presented. It is assumed that the temperature in a point of the medium is non-linearly related to some spectral features and one temporal feature, extracted from the collected RF-lines. The black-box models used are radial basis functions neural networks (RBFNNs), where the best-fitted models were selected from the space of model structures using a genetic multi-objective strategy. The best-fitted predictive model presents a maximum absolute error less than 0.4 degrees C in a prediction horizon of approximately 2 h, in an unseen data sequence. This work demonstrates that this type of black-box model is well-suited for punctual and non-invasive temperature estimation, achieving, for a single point estimation, better results than the ones presented in the literature, encouraging research on multi-point non-invasive temperature estimation.

Transfer and Green functions based on modal analysis for Lamb waves generation

This work presents an easy way to deduce the tensorial transfer and Green functions for Lamb waves generated in isotropic elastic plates. These functions could be applied to obtain the response of each propagating mode in the ensemble of excited modes arising from any sort of pulsed excitation (wedge transducers, lasers, etc.). The transfer function is based on modal analysis development. Not only is it easy to manipulate but also allows the avoidance of laborious calculations for each kind of Lamb waves source. Theoretical predictions are compared with those of Viktorov [I. A. Viktorov, Rayleigh and Lamb Waves (Plenum, New York, 1967)] and with experimental measurements of Lamb waves generated by the wedge-transducer method.

Optical detection of evanescent ultrasound waves in water.

Acoustical perturbation by targets smaller than the wavelength can generate evanescent waves, which decay with the distance of propagation. By putting such targets immediately under the free surface of water, the diffracted acoustical field can excite the surface membrane before evanescence, and it produces a static ripple because of the radiation pressure. A collimated beam of light reflects at the perturbed surface, and it becomes modulated in phase. In this work we show experimental results where we achieve an optical image of the acoustical evanescent waves produced by an array-like target. Arising from the theory, we present a numerical calculus of the optical image produced by the ultrasonic field diffracted from the target in order to verify the experimental results. With this method, we look for a spatial resolution smaller than acoustical wavelength, for normal incidence and plane waves. In our experimental device, we use a sound wavelength in water greater than 1.5 mm, generated by a PZT transducer. We can resolve an array of 1.0 mm of periodicity.

Avoiding diffraction grid effect in ultrasonic fields of 1-3 PZT polymer piezocomposite transducers.

Improvement of sensitivity in ultrasonic fields of piezocomposite transducers is limited because of diffraction effects arising from the periodic structure. We present a theoretical and experimental study of ultrasonic fields of 1-3 PZT polymer piezocomposite transducers. The relation between the composite grid periodicity and the wavelength is critical in order to obtain a far field similar to that of homogeneous transducers. If diffraction grid effect is present, it results in an important energy loss in the higher diffraction orders. These higher orders perturb the central field of the transducer because of multipath contributions to the main lobe caused by reflections from the boundaries of the test sample.

Elasticity assessment of electrospun nanofibrous vascular grafts: a comparison with femoral ovine arteries.

Development of successful small-diameter vascular grafts constitutes a real challenge to biomaterial engineering. In most cases these grafts fail in-vivo due to the presence of a mechanical mismatch between the native vessel and the vascular graft. Biomechanical characterization of real native vessels provides significant information for synthetic graft development. Electrospun nanofibrous vascular grafts emerge as a potential tailor made solution to this problem. PLLA-electrospun nanofibrous tubular structures were prepared and selected as model bioresorbable grafts. An experimental setup, using gold standard and high resolution ultrasound techniques, was adapted to characterize in vitro the poly(L-lactic acid) (PLLA) electrospun structures. The grafts were subjected to near physiologic pulsated pressure conditions, following the pressure-diameter loop approach and the criteria stated in the international standard for cardiovascular implants-tubular vascular prostheses. Additionally, ovine femoral arteries were subjected to a similar evaluation. Measurements of pressure and diameter variations allowed the estimation of dynamical compliance (%C, 10(-2) mmHg) and the pressure-strain elastic modulus (E(Pε), 10(6) dyn cm(-2)) of the abovementioned vessels (grafts and arteries). Nanofibrous PLLA showed a decrease in %C (1.38±0.21, 0.93±0.13 and 0.76±0.15) concomitant to an increase in EPε (10.57±0.97, 14.31±1.47 and 17.63±2.61) corresponding to pressure ranges of 50 to 90 mmHg, 80 to 120 mmHg and 100 to 150 mmHg, respectively. Furthermore, femoral arteries exhibited a decrease in %C (8.52±1.15 and 0.79±0.20) and an increase in E(Pε) (1.66±0.30 and 15.76±4.78) corresponding to pressure ranges of 50-90 mmHg (elastin zone) and 100-130 mmHg (collagen zone). Arterial mechanics framework, extensively applied in our previous works, was successfully used to characterize PLLA vascular grafts in vitro, although its application can be directly extended to in vivo experiences, in conscious and chronically instrumented animals. The specific design and construction of the electrospun nanofibrous PLLA vascular grafts assessed in this work, showed similar mechanical properties as the ones observed in femoral arteries, at the collagen pressure range.

An hemodynamic work bench for in-vitro measurements in arterial bifurcations: experimental results and comparison with the output of a simplified CFD model.

To quantify fluid-structure interactions in arterial walls, from a biomechanical standpoint, a complete characterization of blood flow, shear stress in the interface between blood and endothelium, wall elasticity and wall stresses distribution are needed.

Possible patient early diagnosis by ultrasonic noninvasive estimation of thermal gradients into tissues based on spectral changes modeling.

To achieve a precise noninvasive temperature estimation, inside patient tissues, would open promising research fields, because its clinic results would provide early-diagnosis tools. In fact, detecting changes of thermal origin in ultrasonic echo spectra could be useful as an early complementary indicator of infections, inflammations, or cancer. But the effective clinic applications to diagnosis of thermometry ultrasonic techniques, proposed previously, require additional research. Before their implementations with ultrasonic probes and real-time electronic and processing systems, rigorous analyses must be still made over transient echotraces acquired from well-controlled biological and computational phantoms, to improve resolutions and evaluate clinic limitations. It must be based on computing improved signal-processing algorithms emulating tissues responses. Some related parameters in echo-traces reflected by semiregular scattering tissues must be carefully quantified to get a precise processing protocols definition. In this paper, approaches for non-invasive spectral ultrasonic detection are analyzed. Extensions of author's innovations for ultrasonic thermometry are shown and applied to computationally modeled echotraces from scattered biological phantoms, attaining high resolution (better than 0.1 °C). Computer methods are provided for viability evaluation of thermal estimation from echoes with distinct noise levels, difficult to be interpreted, and its effectiveness is evaluated as possible diagnosis tool in scattered tissues like liver.

Improvement of artery radii determination with single ultra sound channel hardware & in vitro artificial heart system.

In several clinical and experimental circumstances, it is widely necessary to characterize the bio-mechanical changes induced by atherosclerosis to the arterial wall. In this context, the purpose of this paper is twofold. Firstly, to propose a low cost ultrasound setup to improve artery radii determination in elasticity experiments, based on two transducers using a single channel ultrasound hardware. Secondly, to present an in vitro artificial heart system developed in our laboratory, which provides a wide range of hemodynamic parameters in arterial elasticity assessment experiments. It can be used in a liquid, stand alone mode or blowing air to a Jarvik device. This system will be integrated in future works with the proposed ultrasound setup to provide real time elasticity measurements.

Feasibility of a transient elastography technique for in vitro arterial elasticity assessment.

The early detection of biomechanical modifications in the arterial wall could be used as a predictor factor for various diseases, for example hypertension or atherosclerosis. In this work a transient elastography technique is used for the in vitro evaluation of the arterial wall elasticity. The obtained Young modulus is compared with the one obtained by a more classical approach: pressure-diameter relationships. As a sample an arterial phantom made of PolyVinyl Alcohol (PVA) gel was used. Diameter variation due to pressure variation inside the phantom was recorded by means of ultrasound. Through both techniques similar Young modulus estimations are obtained showing in this way the feasibility of applying transient elastography for the arterial wall elasticity assessment.

Changes in wall viscosity and filtering as determinant of carotid and femoral atherosclerotic plaque vulnerability: theoretical analysis.

Atherosclerotic plaque complication is a major cause of vascular accidents. Although a variety of factors have been proposed as key factors in these process, the mechanism that contribute to this problem remain to be characterized. Previously we demonstrated that changes in arterial wall viscous and elastic properties and/or in the filtering function (FF) could be part of the arterial wall alterations basis. If these properties are altered in arteries with atherosclerotic plaques remains to be analyzed. Our aims were 1) to analyze the arterial wall visco-elasticity and FF of carotid and femoral segments with atherosclerotic plaques, 2) to compare them with the mechanical behavior of segments without plaques (from the same artery) and of healthy arteries studied non-invasively. To this end, in each arterial segment, pressure and diameter signals were obtained, in vitro (circulation mock) and in vivo (non-invasive recordings). In atherosclerotic arteries recordings were performed on plaques and near regions without plaques. In each segment, the elasticity, the viscosity, and the wall FF were quantified. Atherosclerotic vessels, and particularly plaque regions, showed a reduced viscosity and FF. At the light of our results, hypothetical links between plaque events and changes in visco-elasticity and FF were discussed.

Time reversal of elastic waves in soft solids.

When a scalar far-field wave is time reversed, it starts to converge toward its initial point source location, then collapses and finally diverges. Without evanescent waves, the symmetric focus spot is limited by the Rayleigh criterion. We present an experimental observation of a time-reversal elastic wave in a soft solid cavity using the transient elastography technique. It is observed that the time-reversed far field wave collapses and gives birth to near fieldlike effects. Elastodynamic Green's functions computation confirms the experimental conclusions: the diffraction limit implies a direction dependant Rayleigh criterion.

Temperature models of a homogeneous medium under therapeutic ultrasound

A modelagem da temperatura em tecidos humanos, quando os mesmos são sujeitos a ultra-som de terapia, é um aspecto essencial para um correto controle e calibração da instrumentação de terapia. A existência de modelos precisos possibilitaria um uso mais seguro e eficiente das terapias térmicas. O objetivo principal deste trabalho é a comparação entre a performance de um modelo linear e de um modelo não linear, na estimação pontual da temperatura num meio homogêneo. O objetivo final do trabalho é a construção de modelos para estimação in-vivo da temperatura. Os modelos lineares aplicados foram "autoregressive models with exogenous inputs" (ARX), enquanto que os modelos não-lineares aplicados foram "radial basis functions neural networks" (RBFNN). As melhores estruturas para as RBFNN foram selecionadas usando o "multi-objective genetic algoritm" (MOGA). A melhor estrutura RBFNN apresentou um erro máximo absoluto de 0,2ºC, que é inferior em uma ordem de grandeza ao erro cometido pelo melhor modelo ARX.

Analisis por ultrasoundo de un medio fibrosdo aplicacion a tejidos musculares / Ultrasound analysis of application fiber medium the muscles tissue

Con el propósito de ser aplicado al estudio de elasticidad de tejidos musculares, se analiza un medio conformado por fibras. Algunos autores han estudiado una distribución aleatoria de fibras. Nuestro interés es encontrar algún parámetro experimental que contenga información sobre la separación entre fibras, es por eso que se analiza una distribución periódica de las mismas. Es un resultado conocido la relación existente entre la función de coherencia espacial del campo ultrasónico retrodifundido por un medio y la función de autocorrelación de dicho medio. Ese resultado es la base de la técnica denominada "Composición espacial". Utilizando esta técnica, eligimos una direción de barrido y trabajamos en modo pulso-eco para obtner un conjunto de señales que finalmente correlacionaremos entre si para formar la función de coherencia espacial. De la experiencia surge la información esperada: el gráfico de coherencia presenta picos separados entre si en el valor de la separación entre fibras.

Transductores piezocomposites 1-3 para diagnóstico clínico / Piezocomposite transducers 1-3 for clinical diagnostic

Los transductores piezocomposites 1-3, realizados a partir de "palitos" cerámicos dispuestos em configuraciones periódicas en un substrato de material polímero, son objeto de numerosas investigaciones recientes. Su baja impedancia acústica, comparada con la de las cerámicas piezoeléctricas puras, su alta flexibilidad y la conservación de un alto factor de acoplamiento electromecánico hacen que, en el rango de frecuencias del orden de los MHz, sean muy eficientes para obtención de imágenes clínicas en medicina...

Ultrassom de terapia / Ultrasound therapy

Temperature modeling of human tissue subjected to ultrasound for therapeutic use is essential for an accurate instrumental assessment and calibration. Prior studies with a homogeneous medium are hereby reported. Nonlinear punctual temperature modeling is proposed by means of Radial Basis Functions Neural Network (RBFNN) structures...