An ultrasound system , comprising: an ultrasound data acquisition unit configured to transmit ultrasound beams to a target object, receive ultrasound echoes reflected from the target object and provide a plurality of ultrasound frame data sets for a plurality of frames, said plurality of ultrasound frame data being acquired at different steering angles of scan lines; and a processing unit configured to form a plurality of ultrasound images, wherein the processing unit includes: a mask forming section configured to form masks corresponding to the respective frames based on the ultrasound frame data sets, a mask setting section configured to determine pixel values of pixels of each of the mask based on intensities of the ultrasound frame data sets, an image forming section configured to form a plurality of mask images based on the masks and the plurality of ultrasound images based on the ultrasound frame data sets, and a spatially compounding section configured to spatially compound the plurality of ultrasound image based on the plurality of mask images to form an ultrasound spatial compound image, wherein the spatial compounding section is configured to sum pixel values of pixels overlapped at the ultrasound images to obtain a first summation value, sum pixel values of pixels overlapped at the mask images to obtain a second summation value and divide the first summation value by the second summation value to determine a pixel value at a corresponding pixel of the ultrasound spatial compound image. an ultrasound data acquisition unit configured to transmit ultrasound beams to a target object, receive ultrasound echoes reflected from the target object and provide a plurality of ultrasound frame data sets for a plurality of frames, said plurality of ultrasound frame data being acquired at different steering angles of scan lines; and a processing unit configured to form a plurality of ultrasound images, wherein the processing unit includes: a mask forming section configured to form masks corresponding to the respective frames based on the ultrasound frame data sets, a mask setting section configured to determine pixel values of pixels of each of the mask based on intensities of the ultrasound frame data sets, an image forming section configured to form a plurality of mask images based on the masks and the plurality of ultrasound images based on the ultrasound frame data sets, and a spatially compounding section configured to spatially compound the plurality of ultrasound image based on the plurality of mask images to form an ultrasound spatial compound image, a mask forming section configured to form masks corresponding to the respective frames based on the ultrasound frame data sets, a mask setting section configured to determine pixel values of pixels of each of the mask based on intensities of the ultrasound frame data sets, an image forming section configured to form a plurality of mask images based on the masks and the plurality of ultrasound images based on the ultrasound frame data sets, and a spatially compounding section configured to spatially compound the plurality of ultrasound image based on the plurality of mask images to form an ultrasound spatial compound image, wherein the spatial compounding section is configured to sum pixel values of pixels overlapped at the ultrasound images to obtain a first summation value, sum pixel values of pixels overlapped at the mask images to obtain a second summation value and divide the first summation value by the second summation value to determine a pixel value at a corresponding pixel of the ultrasound spatial compound image. <EOS>