Echo decorrelation from displacement gradients

摘要

Several ultrasonic techniques for the estimation of blood velocity and tissue motion and elasticity are based on the estimation of displacement through echo time-delay analysis. A common assumption is that tissue displacement is constant withinthe observation time. The precision of time delay estimation (TDE) is mainly limited by noise sources corrupting the echo signals. In addition to electronic and quantization noise, a substantial source of TDE error is the de-correlation of echo signals as a result of displacement gradients within the observation time.We present a theoretical model that describes the mean changes of the cross-correlation function as a function of observation time and displacement gradient. The gradient is assumed to be small and uniform; the de-correlation introduced by the lateral and elevational displacement components are assumed to be small compared to the de-correlation due to the axial component. The de-correlation model predicts that the expected value of the cross-correlation function is a low-pass filtered version of theauto-correlation (i.e., the cross-correlation obtained without gradients). The filter is a function of the axial gradient and the observation time. This theoretical finding is corroborated experimentally. Limitations imposed by and potential uses ofde-correlation in medical ultrasound are discussed.