Imagerie microonde : reconstruction quantitative du profil de permittivité complexe d'hétérogénéités enterrées.

Ce rapport propose plusieurs algorithmes d'inversion quantitative permettant la reconstruction des profils de permittivité et de conductivité d'objets enterrés à partir de la mesure d'un champ électromagnétique diffracté. Toutes les méthodes itératives développées dans le rapport utilisent les propriétés des approches multifréquence et multiincidence. Une technique de régularisation avec préservation des discontinuités de l'objet est enfin introduite dans le processus de reconstruction. Chaque partie est illustrée à l'aide d'images reconstruites à partir de données simulées

Les savanes d'Afrique centrale et les grands enjeux du développement

International audienceSynthèse générale du colloqu

Ultra-Wideband Microwave Imaging of Heterogeneities

International audienceThe technique of time-reversal acoustics was applied to image a bottle filled with saline, using an eight element Vivaldi antenna array with frequency bandwidth 2 to 8 GHz. At these short length scales, a smooth three-dimensional image of the bottle was obtained, with the usual limitations imposed by limited offset and frequency. Time snapshots of the wavefield evolution in reversed time are presented for two real data sets. The first, shows the focusing for the single target of the bottle, while the second demonstrates the principle for two targets

Radiofrequency conical emission from femtosecond filaments in air

International audienceWe show that the broadband conical emission associated with filaments in air extends down to the radiofrequency region. This rf emission which originates from the longitudinal oscillation of charged ions formed during filamentation is strongly enhanced by the presence of a longitudinal static electric field

Active microwave imaging of inhomogeneous bodies

A numerical method and experimental technique for microwave imaging of inhomogenous bodies is presented. This method is based on the interpretation of the diffraction phenomena and leads to tomographic reconstruction of the body under investigation. Various numerical examples are given on spatial impulse response, recognition of dielectric rods, inhomogeneous bodies, and simulated human arm. Different experimental results on dielectric rods and isolated animal organs are also given.Peer ReviewedPostprint (published version

Planar and cylindrical active microwave temperature imaging: numerical simulations

A comparative study at 2.45 GHz concerning both measurement and reconstruction parameters for planar and cylindrical configurations is presented. For the sake of comparison, a numerical model consisting of two nonconcentric cylinders is considered and reconstructed using both geometries from simulated experimental data. The scattered fields and reconstructed images permit extraction of very useful information about dynamic range, sensitivity, resolution, and quantitative image accuracy for the choice of the configuration in a particular application. Both geometries can measure forward and backward scattered fields. The backscattering measurement improves the image resolution and reconstruction in lossy mediums, but, on the other hand, has several dynamic range difficulties. This tradeoff between forward only and forward-backward field measurement is analyzed. As differential temperature imaging is a weakly scattering problem, Born approximation algorithms can be used. The simplicity of Born reconstruction algorithms and the use of FFT make them very attractive for real-time biomedical imaging systems.Peer Reviewe

Microwave diffraction tomography for biomedical applications

Peer ReviewedPostprint (published version

Numerical Modeling and High Speed Parallel Computing: New Perspectives for Tomographic Microwave Imaging for Brain Stroke Detection and Monitoring

This paper deals with microwave tomography for brain stroke imaging using state-of-the-art numerical modeling and massively parallel computing. Microwave tomographic imaging requires the solution of an inverse problem based on a minimization algorithm (e.g. gradient based) with successive solutions of a direct problem such as the accurate modeling of a whole-microwave measurement system. Moreover, a sufficiently high number of unknowns is required to accurately represent the solution. As the system will be used for detecting the brain stroke (ischemic or hemorrhagic) as well as for monitoring during the treatment, running times for the reconstructions should be reasonable. The method used is based on high-order finite elements, parallel preconditioners from the Domain Decomposition method and Domain Specific Language with open source FreeFEM++ solver

Detection of brain strokes using microwave tomography

Brain stroke is a major cause of disability and death worldwide. There are two types of stroke, ischemic or cerebral infarction (85% of cases) and hemorrhagic (15%). The diagnosis must be made quickly (within 3 to 4 hours after the onset of symptoms) to determine the nature of the stroke and proceed to treatment. Recent works have shown the modification of the complex permittivity according to the nature of stroke [1] in the microwave domain. We are interested here in the detection of brain strokes using microwave tomography. We present results obtained by electromagnetic simulations coupled to a realistic noise model of measurements. The forward problem is based on a massively parallel computing using domain decomposition method, and an inverse problem based on L-BFGS algorithm with a regularization based on total variation (TV)