1. A method for adaptive frame averaging within an ultrasound imaging system, comprising: storing in a memory a current frame image representing echo signals obtained from an ultrasound probe; using a characteristic obtaining unit to: divide the current frame image into a plurality of sub-blocks; and obtain a characteristic for each of the plurality of sub-blocks to obtain a characteristic image of the current frame image; determining a frame averaging coefficient for each of the plurality of sub-blocks using a coefficient determination unit based on the characteristic image of the current frame image and a characteristic image of a previous frame image; and frame-averaging a resultant frame-averaged image of the previous frame image and the current frame image using a frame averaging coefficient determination unit by using the frame averaging coefficient of each of the plurality of sub-blocks to obtain a resultant frame-averaged image of the current frame image.