CHARACTERIZATION OF ELASTOGRAPHIC NOISE USING THE ENVELOPE OF ECHO SIGNALS

A theoretical formulation characterizing the noise performance of stra in estimation using envelope signals is presented for the cross-correl ation based strain estimator in elastography, using a modified strain filter approach. The strain filter describes the relationship among th e elastographic signal-to-noise ratio (SNRe), sensitivity, contrast-to -noise ratio and dynamic range for a given resolution in the elastogra m, as determined by the cross-correlation window length and window ove rlap. Theoretical results indicate that the envelope strain filter noi se performance (SNRe level) is about half that obtained in the radio f requency (RF) case (f(o) = 7.5 MHz). Simulation results corroborate th e trend predicted using the strain filter. Experimental SNRe vs. strai n plots presented in this article illustrate the same trend as the the oretical results. These plots allow a quantitative comparison of the e lastograms obtained with RF and envelope signal processing. For small strains, the performance obtained using RF signals is superior to that obtained for envelope signals (since jitter errors are smaller due to the utilization of phase information in RF signals). However, for lar ge tissue strains, envelope analysis provides an accurate estimate of the tissue strain (since envelope signal decorrelation is smaller than RF signal decorrelation at large strains), An algorithm that combines the low-noise characteristics of the crosscorrelation analysis using RF signals at small strains and envelope signals for estimation of lar ge tissue strains is proposed to improve the dynamic range in the elas togram. (C) 1998 World Federation for Ultrasound in Medicine & Biology .