In an ultrasound imaging system, a displacement vector is estimated for a pattern of samples throughout an imaged region of interest (ROI) by comparing two successive B-mode frames. The displacement vector is preferably estimated using block matching. Once displacement vectors are estimated for samples throughout the ROI, corresponding strain values are estimated, which indicate the degree of elasticity of the respective tissue portions. An image is then displayed showing the strain distribution within the ROI as it is stressed, for example, by the user pressing the ultrasound transducer against the patient's body. The invention allows for both real-time and post-processed generation of elasticity displays, even based on the same body of acquired frame data. The real-time display is preferably generated using lower quality block matching whereas the post-processed elasticity calculations are carried out using high-quality techniques. Different techniques are included for adjusting the size and location of the search region of the block matching based on different measures of reliability of current displacement estimates.