Non-linear ultrasonic tomography of high-contrasted materials

International audienceThis study focuses on the ultrasonic characterization and imaging of elastic materials like cylinders or tubes. In this case, ultrasonic wave propagation is greatly perturbed by the difference in the acoustic impedance between the scatterer and the surrounding medium (soft tissues, water or coupling gel), which results in considerable parasite events such as the refraction, attenuation and scattering of the waves. The aim of this work is then to solve a non-linear inverse scattering problem. Analytical or algebraic approaches may be applied generally involving in a "classical" problem of minimization of the differences between modeling data and measurements. Several strategies can be used to model the forward problem and to solve the inverse problem simply, efficiently and accurately. The distorted diffraction tomography is an inversion iterative method and belongs to the class of algebraic reconstruction algorithms. This method was developed to increase the order of application of the Born approximation (in the case of weakly contrasted media) to higher orders. The iterations are performed numerically by solving the forward and inverse problems at every iteration after calculating an appropriate Green's function; the previous iteration serves in each case to define the surrounding medium with a variable background. This yields quantitative information about the scatterer, such as the speed of sound and the attenuation. Quantitative ultrasonic imaging techniques of this kind are of great potential value in fields such as medicine, underwater acoustics and non-destructive testing

Ultrasonic Computed Tomography

Ultrasonic Computed Tomography (UCT) is a full digital imaging technique, which consists in numerically solving the inverse scattering problem associated to the forward scattering problem describing the interaction of ultrasonic waves with inhomogeneous media. For weakly inhomogeneous media such as soft tissues, various approximations of the solution of the forward problem (straight ray approximation, Born approximation...), leading to easy-to-implement approximations of the inverse scattering problem (back-projection or back-propagation algorithms) can be used. In the case of highly heterogeneous media such as bone surrounded by soft tissues, such approximations are no more valid. We present here two non-linear inversion schemes based on high-order approximations. These methods are conceived like the prolongation of the methods implemented in the weakly inhomogeneous case for soft tissues. The results show the feasibility of this UCT approach to bones and its potential to perform measurements in vivo

Distorted Born diffraction tomography: limits and applications to inverse the ultrasonic field scattered by an non-circular infinite elastic tube

International audienceThis study focuses on the application of ultrasonic diffraction tomography to noncircular 2D-cylindrical objects immersed in an infinite fluid. The distorted Born iterative method used to solve the inverse scattering problem be longs to the class of algebraic reconstruction algorithms. This method was developed to increase the order of application of the Born approximation (in the case of weakly-contrasted media) to higher orders. This yields quantitative in formation about the scatterer, such as the speed of sound and the attenuation. Quantitative ultrasonic imaging techniques of this kind are of great potential value in fields such as medicine, under water acoustics and non destructive testing

Two-dimensional ultrasonic computed tomography of growth bone

International audienceUltrasonography is the main first-line imaging technique used to diagnose various pediatric pathologies. Pediatric radiologists are very keen on ultrasonography since it is a non-invasive, non-irradiant, painless, inexpensive imaging modality, which is also practicable on bedside. In bone diseases, this technique has also proved to be a very effective tool to assess congenital (i.e., hip dysphasia), infectious (i.e., sub-periostal abscess), inflammatory (i.e., chronic arthritis), and even traumatic (i.e., ankle sprain) processes. However, with standard devices, this exam is not suitable for diagnosis or monitoring bone tumors, classically explored by other more effective techniques, which presented various disadvantages (cost, irradiation, sedation, accessibility). Many authors have dealt with the ultrasonic imaging of bones. Their main aim has usually been to assess the thickness of the diaphysis and to calculate the speed of sound of a wave crossing the structure. Our group has been focusing on the cross-sectional radial imaging process, using ultrasonic computed tomography. Known to be a potentially valuable method of imaging objects with a similar acoustical impedance, difficulties arise when it is proposed to obtain quantitative tomograms of more highly contrasted media (hard bone tissues). Finding solutions here involves either using non-linear schemes and/or performing extensive studies on the limitation of the initial approximation. In this paper, we recall the limits of the ultrasonic computed tomography when dealing with highly contrasted scatterers, using a high-order tomographic method. The performances and limitations are evaluated in the basis of simulated and experimental data, obtained using an ultrasonic scanner, to one geometrical-mimicking phantom and to a real children fibula. Sample thicknesses and speed-of-sound varied as a function of radial position and the experimental procedure fulfilled the criteria of the diffraction tomography. Results are promising and suggest that the geometrical and acoustical characteristics of the bones, which are known to be one of the main factors contributing to the growth bone disease, can be determined using this ultrasonic computed tomography

Low Clinical Burden of 2009 Pandemic Influenza A (H1N1) Infection during Pregnancy on the Island of La Réunion

BACKGROUND: Pregnant women have been identified as a group at risk, both for respiratory complications than for the admissions to the Intensive Care Unit (ICU) during the 2009 H1N1 influenza pandemic (pdm). The purpose of this prospective register-based cohort-study was to characterize the clinical virulence of the pdm (H1N1/09)v during pregnancy in La Réunion. METHODS/PRINCIPAL FINDINGS: Over a twelve-week pdm wave (13 July to 3 October 2009), 294 pregnant women presented with an influenza-like illness (ILI) to one of the three maternity departments of the South Reunion area, Indian Ocean. Out of these, 278 were checked by RT-PCR for influenza viruses (157 positive and 121 negative, of whom, 141 with pdm flu and 132 with ILIs of non pdm origin, 5 untyped). The median body temperature was higher in women experiencing pdm flu than in those with non pdm ILI (38.9 degrees C versus 38.3 degrees C, P<0.0001), without evidence linked to circulating viremia. Oseltamivir was given for 86% of pdm flu cases in a median time inferior than 48 hrs (range 0-7 days). The hospitalization rate for pdm flu was of 60% and not associated with underlying conditions. Six viral pneumonia and fourteen asthma attacks were observed among 84 hospitalized pdm flu cases, of whom, only one led to the ICU for an acute lung injury. No maternal death occurred during the pdm wave. None adverse pregnancy outcome was associated with pdm flu. No congenital birth defect, nor early-onset neonatal influenza infection was attributable to pdm flu exposure. CONCLUSIONS/SIGNIFICANCE: This report mitigates substantially the presumed severity of pandemic H1N1/09 influenza infection during pregnancy. The reasons for which the clinical burden of H1N1/09 influenza virus may differ worldwide raise questions about a differential local viral-strain effect and public health preparedness, notably in timely access to special care and antiviral treatments

Imagerie ultrasonore d'objets à fort contraste par une méthode d'équations intégrales de domaine

National audienceCette étude concerne l'imagerie ultrasonore d'objets cylindriques (2D) de section quelconque, immergés dans un fluide infini. La méthode de Born itératif distordu utilisée fait partie de la classes des algorithmes algébriques de reconstruction (ART). Cette méthode a été développée afin de permettre de s'affranchir des conditions expérimentales

Identification du rayonnement à partir de mesures de pression in situ

National audienceIl existe de nombreuses techniques d'identification du champ rayonné par une source de bruit, destinées à en localiser le mécanisme ou à en déterminer le niveau et la répartition spatiale. L'approche présentée ici met l'accent sur la réduction du nombre de points de mesures et la possibilité de les effectuer dans un environnement acoustique ordinaire. Elle permet alors de reconstruire en 3D la répartition du champ qui aurait été mesuré en environnement anéchoïque ou semi-anéchoïque, donc d'en déduire la puissance et la directivité, d'effectuer des post-traitements plus spécifiques, ou d'inclure la source mesurée dans des simulations. Le principe mis en oeuvre est d'identifier la décomposition du rayonnement sur une série de termes, comme pour l’holographie de champ proche (NAH) oo les méthodes de séparation de champ (FSM), mais ici la série est celle des modes de rayonnement calculés d'après la forme générale de l'objet mesuré - ce qui réduit considérablement le nombre de termes à identifier

Quantitative non-linear ultrasonic imaging of targets with high acoustic impedance contrast—Application to bone imaging

International audienceThis study focuses on the ultrasonic imaging of high impedance acoustical contrast targets. The aim is to obtain information about shape, dimensions, and sound speed profile of the studied objects. One domain of application is the characterization of long bones. Quantitative information about the acoustic properties of bones tissues are of great interest for diagnosing or treatment monitoring of bone diseases. Inverse scattering problems of this kind are non-linear and various approximations can be used to linearize the scattering equations. Classical methods based on the first-order Born approximation give good results for weakly scattering targets but fail when it comes to give a quantitative information especially for high impedance contrast targets such as bones. In the inversion algorithm proposed here, Green's theorem is used to obtain a domain integral representation of the scattered field. An iterative non-linear algorithm minimizing the discrepancy between the measured and computed scattered fields is used to reconstruct the sound speed profile in the region of interest. The minimization process is performed using a conjugated-gradient method. An experimental study was performed with targets made of paraffin and with lamb bones. Images of the sound speed profile obtained by inversion of experimental data are presented and discussed in both cases

Quantitative ultrasonic tomography of high acoustic impedance contrast targets

International audienceThis study is concerned whith the ultrasonic imagery of elastic materials like cylinders or tubes by a diffraction tomography technic. Green's representaton is used to obtain an integral representation of the scattered field, and a discrete formulation of the inverse problem is obtained using a moment method. An iterative non-linear algorithm minimizing the discrepancy between the measured and computed scattered fields is used to reconstruct the sound speed profile in the target area. The minimization process is performed using a conjugated-gradient method. An experimental study with elastic targets immersed in water was performed. Cylindrical targets of various cross-sections have been selected. Inversions of both numerical and experimental data are presented and compared