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Deconvolution in Low Frequency Ultrasonic Tomography Déconvolution en tomographie ultrasonore basse frequence

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Abstract:

Medical Imaging is, with Non Destructive Testing of materials, the main application of Ultnsonic Reflection Tomography (URT). This method results from a linearization of the Inverse Acoustic Scattering Problem, named Inverse Born Approximation (IBA) and allows small perturbations of a reference medium to be visualized. For media with weak inhomogeneities, such as biological media, one chooses the surrounding medium as reference. This leads to a "Constant Background" IBA method. For media with strong heterogeneities, the problem is quite non-linear and there is in general no single solution. In the case of small disturbance, one can use a "Variable Background" IBA method - the reference background being the water-specimen set - to reconstruct the perturbation. URT fails when strong multiple scattering occurs (strong contrast, large object with respect to wavelength). In this case, one would guess that Low Frequency Tomography (<1 MHz) will have a larger domain of validity than the classical one. But the usual algorithm leads to poor resolution images, inappropriate for material imagery. To improve low frequency ultrasonic reflection tomography, we used a deconvolution technique. Our enhancement procedure is based on Papoulis deconvolution. Papoulis proposed an extension of the generalized inversion in the complementary bandwidth of the electro-acoustic set-up. The resolution enhancement procedure was first tested on academic targets and on test targets, showing the possibility to obtain high resolution images with low-frequency transducers.

French
L'imagcric médicale est, avec le CND des matériaux, l'application principale de la tomographie ultrasonore. Cette méthode résulte de la linéarisation du probléme inverse de diffusion et permet de visualiser les petitcs perturbations d'un milieu de référence. Pour des milieux à faible contraste, comme les tissus biologiques, le milieu ambiant sert de référence homogène. C'est la méthode dite à "fond constant". Lorsque le milieu présente de plus fortes hétérogéneités, alors le probleme devient non linéaire et il n'existe plus de solution simple. En admettant que l'inhomogénéité (défaut par exemple) est assez petite, on considère l'ensemble eau/objet comme milieu de référence. C'est l'imagerie tomographique à "fond variable". La méthode est mise en défaut lorsque le phénomène de diffusion est amplifié. Ce problème devient encore plus gênant à des fréquences basses (<1 MHz). Si on gagne en profondeur de champ, on perd en résolution d'image. Pour contourner cette difficulté, nous avons recours à des méthodes de traitement du signal notamment la déconvolution des échogrammes par une réponse caractéristique de la fonction d'appareil. Neus avons utilisé pour cette étude l'algorithme de Papoulis qui permet de conserver toute l'information connue en fréquence et en temps. Après un exposé de l'algorithme, nous présentons des résultats obtenus sur des objets simples illustrant bien l'intérêt du travail de mise en œuvre.

Language: French

Document Type: Research Article

Publication date: May 1, 2000

More about this publication?
  • Acta Acustica united with Acustica, published together with the European Acoustics Association (EAA), is an international, peer-reviewed journal on acoustics. It publishes original articles on all subjects in the field of acoustics, such as general linear acoustics, nonlinear acoustics, macrosonics, flow acoustics, atmospheric sound, underwater sound, ultrasonics, physical acoustics, structural acoustics, noise control, active control, environmental noise, building acoustics, room acoustics, acoustic materials, acoustic signal processing, computational and numerical acoustics, hearing, audiology and psychoacoustics, speech, musical acoustics, electroacoustics, auditory quality of systems. It reports on original scientific research in acoustics and on engineering applications. The journal considers scientific papers, technical and applied papers, book reviews, short communications, doctoral thesis abstracts, etc. In irregular intervals also special issues and review articles are published.
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