An ultrasound system with a matrix array (500) probe (10) operable in the biplane mode is used to assess stenosis of a blood vessel by simultaneously displaying two color Doppler biplane images (60a, 60b) of the vessel, one a longitudinal cross-sectional view (60a) and the other a transverse cross-sectional view (60b). The two image planes intersect along a Doppler beam line (68) used for PW Doppler. A sample volume graphic (SV) is positioned over the blood vessel at the peak velocity location in one image, then positioned over the blood vessel at the peak velocity location in the other image. As the sample volume location is moved in one image, the plane and/or sample volume location of the other image is adjusted correspondingly. Spectral Doppler data (62) is then acquired and displayed from the sample volume location.