A method for aligning spatially different subvolumes of ultrasonic data of a blood vessel comprising: acquiring temporally discrete signals of a blood vessel with elements of a two dimensional array of ultrasonic transducer elements from spatially different depths of scanning opposed by each transducer element, said array being located in a first position with respect to the blood vessel during the acquiring; Doppler processing the temporally discrete signals received from each transducer element to produce spectral Doppler data of the scanning depth opposed by each transducer element; producing a first three dimensional map of the spectral Doppler data in spatial relationship to the position of the array with respect to the blood vessel; acquiring temporally discrete signals of the blood vessel with elements of the two dimensional array of ultrasonic transducer elements from spatially different depths of scanning opposed by each transducer element, said array being located in a second position with respect to the blood vessel during the acquiring; Doppler processing the temporally discrete signals received from each transducer element to produce spectral Doppler data of the scanning depth opposed by each transducer element; producing a second three dimensional map of the spectral Doppler data in spatial relationship to the position of the array with respect to the blood vessel; aligning the first three dimensional map with the second three dimensional map on the basis of one or more regions of matching spectral Doppler data of the two map; and producing a combined three dimension map of the blood flow of the vessel from the aligned first and second three dimensional maps.