Echo decorrelation from displacement gradients in elasticity andvelocity estimation

摘要

Several ultrasonic techniques for the estimation of bloodnvelocity, tissue motion and elasticity are based on the estimation ofndisplacement through echo time-delay analysis. A common assumption isnthat tissue displacement is constant within a short observation timenused for time delay estimation (TDE). The precision of TDE is mainlynlimited by noise sources corrupting the echo signals. In addition tonelectronic and quantization noise, a substantial source of TDE error isnthe decorrelation of echo signals because of displacement gradientsnwithin the observation time. The authors present a theoretical modelnthat describes the mean changes of the crosscorrelation function as anfunction of observation time and displacement gradient. The gradient isnassumed to be small and uniform within the observation time; thendecorrelation introduced by the lateral and elevational displacementncomponents is assumed to be small compared with the decorrelation causednby the axial component. The decorrelation model predicts that thenexpected value of the crosscorrelation function is a low-pass filterednversion of the autocorrelation function (i.e., the crosscorrelationnobtained without gradients). The filter is a function of the axialngradient and the observation time. This theoretical finding isncorroborated experimentally. Limitations imposed by decorrelation inndisplacement estimation and potential uses of decorrelation in medicalnultrasound are discussed