A BC-mode image with an improved frame rate in an ultrasound system is formed. A transmit/receive unit forms first and second receive signals responsive to first and second control signals. An image processing unit is configured to form a B-mode image and a C-mode image based the first and second receive signals and to combine them to form a BC-mode image. A control unit is configured to divide scan lines into a plurality of scan line groups and to generate a third control signal for alternately selecting one of the scan line groups for a B-mode scan for forming scan line group receive signals. The control unit repeatedly generates the second and third control signals. The image processing unit is configured to update the first receive signals with the scan line group receive signals and form a B-mode image based on the updated first receive signals.