1. An ultrasound system, comprising: a frame data forming unit configured to form reference frame data with reflective signals from a target prior to applying pressure to the target and interest frame data of a plurality of interest frames with other reflective signals from the target undergoing the pressure; a displacement map forming unit configured to form displacement maps of the interest frames by comparing the interest frames with the reference frame; a strain calculating unit configured to calculate the strains of interest frames with the use of the displacement maps; a local information processing unit configured to analyze the strain at each pixel to determine pixels where the strain varies as first boundary pixels on each interest frame, wherein the local information processing unit is configured to analyze the strains of a n th interest frame and set up first boundary pixels in the n th interest frame, and to move first boundary pixels set up in a n−1 th interest frame to corresponding positions of the first boundary pixels of the n th interest frame to obtain second boundary pixels; and an image processing unit configured to reconstruct the n−1 th interest frame with the second boundary pixels and to perform persistence with the reconstructed n−1 th interest frame and the n th interest frame, wherein the processes performed by the local information processing unit and the image processing unit are iteratively performed for each of a plurality of n th interest frames and n−1 th interest frames.