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Conventional ultrasound echography (B-scan) displays the detected envelope of the echoes that originate in a tissue probed by an ultrasound beam. Since the main echoes originate at organ boundaries, these images mainly display contours. During the envelope detection process most of the information that is related to phase (and then to frequency) has been lost. Now this information can be related to physical parameters of the tissue, such as the atte-nuation the ultrasound beam experiences in it. And these parameters have proved to be good pathology discrimi-nants for some diffuse disease that are very difficulty diagnosed using other methods. Tissue characterization aims at restoring this lost information. Various methods, based on sophisticated signal processing techniques such as time-frequency distributions, are available to estimate attenuation. Methods using either the so-called spectral centroid or the intensity of the signal in narrow frequency bands are presented. The hypotheses they require and their capability to estimate true attenuation are discussed. A method to correct these estimations for the depth-dependent effect of acoustic diffraction is developed. This method is based on the derivation of the ultrasonic field in the case of unfocused transducers and on calibration measurements. After the agreement between the theoretical derivation and experimental calibration has been shown the method is extended to focused transducers and its effectiveness is demonstrated. As a by-product of these computations, new imaging modalities are presented. They involve as well spectral-centroid images as narrow band images. The first display an estimate of the instant frequency of the ultrasonic signal. They seem to provide more information about the inner structure of biological tissue than conventional images do. The latter enhance the frequency diversity of ultrasonic signals and provide a powerful tool for the reduction of the speckle-like noise that conventional reflectivity images evidence.
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