1. An ultrasound system, comprising: an input interface configured to receive, from a user, a user input of setting a sample volume and a pulse rate frequency (PRF) on an ultrasound image; an ultrasound probe configured to transmit ultrasound signals to a living body based on the PRF, and receive ultrasound echo signals from the living body; and a controller configured to determine a constant delay value based on position of the sample volume by using a mid-point algorithm, form digital signals based on the ultrasound echo signals, focus the digital signals based on the constant delay value throughout depths, and form a high pulse rate frequency Doppler image of the sample volume based on the focused digital signals, wherein a round trip time of the ultrasound signals to the sample volume is larger than a pulse repetition period corresponding to the PRF.