Ultrasonic speckle is an inherent consequence of the stochastic nature of the reflectivity of biological tissues. Speckle is a source of noise on envelope detected echoes as well as on the power spectra. A technique where the spatial coherence of the transducer is controlled by moving a random phase screen (RPS) through the ultrasonic beam is described. An RPS that is a rough surface generating phase shifts that are randomly distributed between 0 and 2pi is presented. Measurements with unfocused and focused transducers working in broadband pulse-echo mode were performed. Experimental results on the directivity pattern and the ability of the system to obtain uncorrelated speckle patterns are presented. The performance of the RPS in SNR improvements is discussed. Experimental results are in good agreement with theoretical predictions.
Ultrasound speckle is a consequence of the stochastic nature of the reflectivity of scattering media (e.g., biological tissue) and of the coherent nature of piezoelectric transducers. This speckle noise can be reduced by the use of incoherent processing techniques (e.g., spatial compounding, incoherent summation, random phase and phase insensitive transducers). We present a unified framework that explains the limitations of incoherent processing in terms of the information grain theory. This theory predicts the gains in SNR as well as the losses in directivity. We also present the random phase transducer approach to incoherence to total coherence. We present applications to speckle reduction, detection of specular reflectors, attenuation estimation and ultrasound imaging. We show that totally incoherent transducers completely remove diffraction effects. They might be used in attenuation estimation, in which case, correction for diffraction is no longer required, in order to obtain unbiased estimates. Partially coherent transducers might also be useful in imaging to reduce speckle noise.
Paris 7-Tour 23-2 ,place Jussieu-75251 Paris cedex 05-FRANCE 2 Lab.Biophysique-uA CNRS 593-CHU Cochin Port Royal 24 rue du Fbg Saint Jarques-75674 Pari s cedex 1 4 FRANCE ABSTRACT The authors present a new method for mcreasing the spallal resolullon m attenuation measurements a random phase screen (RPS) is placed in front of the transducer. The RPS Introduces a random phase on the elementary waves scattered by individual scatterers located in the insonified volume. Different locations of the RPS produce different pattern of Interference at the front face of the probe . Uncorrelated echographlc lines are obtained by shifting the RPS in the ultrasoniC field. Thus, Without moving the transducer, a SIgnificant decrease of the statistical variance on the attenuaUon measurement is reached. We present the effect of the RPS on the beam profUe and show that under optimal experimental condl11ons the beam IS weakly spread. We give quantitative results on the increase of the SNR The reduction of the SiZe of the resolution cell gamed with a RPS shows that the RPS is prOmiSing for lOcal attenuatton measurements
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