ERROR ANALYSIS IN ACOUSTIC ELASTOGRAPHY .2. STRAIN ESTIMATION AND SNRANALYSIS

Accurate displacement estimates ate required to obtain high-quality st rain estimates in elastography. In this paper the strain variance is d erived from the statistical properties of the displacement field to de fine a point signal-to-noise ratio for elastography (SNR(0)). Displace ments caused by compressional forces applied along the axis of the tra nsducer beam are modeled by scaling and shifting the axial reflectivit y profile of the tissue. The strain variance is given as a function of essential experimental parameters, such as the amount of tissue compr ession, echo waveform window length, and the amount of window overlap. SNR(0) is defined in terms of applied compression and strain variance and normalized by the input signal-to-noise ratio (SNR,) for echo sig nals, to formulate the performance metric SNR(0)/SNR(i). This quantity characterizes the noise properties, dynamic range, and sensitivity of strain images based on the spatial resolution requirements. The resul ts indicate that low noise, high sensitivity, and limited dynamic rang e strain images are obtained for high-frequency bandpass signals when the applied strain is small. For large strains, however, one strategy for low-noise strain imaging employs base-band signals to obtain image s with large dynamic range but limited peak sensitivity and noise figu re. A better strategy includes companding, which eliminates the averag e strain in the echo signal before cross-correlation to reduce the dyn amic range requirement and increase peak sensitivity for strain estima tes. (C) 1997 Acoustical Society of America.