1. An ultrasound system, comprising: a frame data forming unit for forming 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 for forming displacement maps of the interest frames by comparing the interest frames with the reference frame; a strain calculating unit for calculating the strains of interest frames with the use of the displacement maps; a local information processing unit for analyzing the strain at each pixel to determine boundary pixels, wherein the local processing unit analyzes the strains of a n th interest frame and sets up first boundary pixels in the n th interest frame, and obtaining second boundary pixels by moving the first boundary pixels of a n−1 th interest frame to corresponding position of the first boundary pixels of the n th interest frame; and an image processing unit for reconstructing 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.