THEORETICAL DERIVATION OF SNR, CNR AND SPATIAL RESOLUTION FOR A LOCAL ADAPTIVE STRAIN ESTIMATOR FOR ELASTOGRAPHY

Conventional techniques in elastography estimate the axial strain as the gradient of the displacement (time-delay) estimates obtained using cross-correlation of pre- and temporally stretched postcompression radiofre- quency (RF) A-line segments. The use of a constant stretch factor for stretching the postcompression A-line is not adequate in the presence of heterogeneous targets that are commonly encountered. This led to the development of several adaptive strain estimation techniques in elastography. Yet, a theoretical framework for the image quality of adaptive strain estimation has not been established. In this work, we develop theoretical expressions for the image quality (measured in terms of the signal-to-noise ratio (SNR), contrrast-to-noise ratio (CNR) and spatial resolution) of elastograms obtained using an adaptive strain estimator developed by Alam et al. (1998). We show a linear trade-off between the SNR and axial resolution of the strain elastogram with respect to the window length used for strain estimation. The CNR shows a quadratic tradeoff with the axial resolution with respect to the window length. The SNR, CNR and axial resolution are shown to improve with the ultrasonic bandwidth. (E-mail: jonathan.ophir@uth.tmc.edu) © 2004 World Federation for Ultrasound in Medicine & Biology.