Development of combined systems of ultrasonic pulsed-Doppler flow meter and real-time B-mode scanner gives
an impact for the practical use of non-invasive Doppler blood flow measurement. However, there are several problems.
Major problems in relation to the combined systems are as follows: (1) The maximum measurable velocity in the simultaneo us B/D-mode is reduced to a half of that in D-mode (detecting only Doppler signal), (2) Noise is increased in the simultaneous B/D-mode, (3) Doppler shifted frequency spectrum spreads even if velocity of a reflector is
unity, and the center frequency of the Doppler shifted frequency is shifted due to frequency dependent attenuation of human body, (4) D-mode is recorded with a time lag of several milliseconds to M-mode. In order to remove disadvantages of (1) and (2), blood flow is recorded in D-mode after the position is confirmed in B/D-mode.
Larger ultrasonic pulse width should be used for Doppler flow measurement. The time lag which leads to misdiagnosis must be corrected.