An ultrasonic diagnostic imaging system is described which quantifies regurgitant flow through a mitral valve. Echo signals received by an ultrasound probe (10) are used to produce an image of a regurgitant flow region, and are processed by a wall filter having a response characteristic which peaks at an intermediate sampling rate between zero and the Nyquist limits. This response characteristic is thus highly sensitive to lower flow rates which may be anticipated in a flow velocity field proximal a regurgitant orifice. Echo signals passed by the wall filter are Doppler processed and used to quantify the flow through the regurgitant orifice.