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We describe a novel and robust strain estimation method, which is capable of fast, accurate strain estimation even in the presence of large signal decorrelation. Global temporal stretching of post-compression signals compensating for the applied strain significantly improves the "quality" of strain estimates. In a natural extension of this approach (adaptive stretching), a search is performed at each data window for the stretch factor that maximizes the correlation between the pre- and post-compression echo signal segments. Adaptive stretching performs well under harsh signal environments (because the correlation is maximized at each location); however it is computation intensive because many iterations may be required at each location. In contrast, global stretching is a fast algorithm, but performs well only in areas where local strains are close to the applied strain. The proposed method strikes a balance between the speed of global stretching and the performance of adaptive stretching. In this method, global stretching is performed with a range of different stretch factors and strain maps are computed for each stretch factor. The correlation between the pre- and post-compression echo segments is the maximum when the stretch factor corresponds to the local strain. Thus, the area in each computed strain image with strain values closely corresponding to the uniform stretch factor will contain "good quality" strain estimates. (Naturally, this area is different in each image.) To produce a single elastogram at the end, these strain maps are combined as follows. Correlation values quantify the "quality" of strain estimates; thus, at each location we identify the strain map with the maximum correlation, and the strain value in that strain map at that location is chosen for the combined map. Results from data generated by finite-element simulation and phantom experiments show that the variable stretching strain estimator is fast and is significantly less susceptible to signal degradation than conventional strain estimators.
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