1. An ultrasound system, comprising: a signal acquisition unit configured to transmit and receive ultrasound signals to and from a target object to acquire first Doppler signals; and a processing unit configured to form color Doppler mode images including power images and velocity images, to analyze the power images to set a references power image whose mean power is greater than a predetermined threshold, to compound the reference power image with power images whose mean power is less than the predetermined threshold to form compound images, to set a mask for detecting a region of interest by using the reference power image and the compound images, and to detect contours of the mask, to perform smoothing upon the detected contours, and to perform filling upon the smoothed mask, the processing unit being further configured to mask the power images and the velocity images by using the mask.