This paper presents a detailed simulation method to estimate Doppler power spectrum and mean blood velocity using real CW Doppler transducers with twin-crystal arrangement. The method is based on dividing the sample volume into small cells and using the statistics of the Doppler power spectrum with the same Doppler shift frequency, which predicts the mean blood velocity. The acoustic fields of semicircular transducers across blood vessels were calculated and the effects of acoustical and physiological factors on Doppler power spectrum and mean blood velocity were analyzed. Results show that nonuniformity of the acoustic field of the ultrasonic beam in the blood vessel and blood velocity profiles significantly affect Doppler power spectrum and mean blood velocity. However, Doppler angle, vessel depth, and sample volume length are not sensitive functions. Comparisons between simulation and experimental results illustrated a good agreement for parabolic flow profile. These results will contribute to a better understanding of Doppler power spectrum and mean blood velocity in medical ultrasound diagnostics.

A continuous-wave ultrasonic Doppler system using wide field ultrasound transducers was applied to telemeter blood velocity from the carotid artery of exercising subjects. Velocity spectrogram was obtained by Hanning windowed fast Fourier transformation of the telemetered data. Distortion caused by a high-pass filter and transducers in the telemetry system was discussed in the paper. As the maximum Reynolds number in our experiment was 1478 which is smaller than the critical level of 2000, the blood flow should be laminar. Spatial velocity profiles were then reconstructed from the velocity spectrogram. In this paper, we defined a converging index Q of the velocity spectrum to measure the bluntness of the spatial velocity distribution across the blood vessel. Greater Q, the blunter the velocity profile will be. Simulation results for spatial velocity distributions of theoretical parabolic flow and Gaussian-distribution spectra with varied Q value showed that the cut-off effect by a high-pass filter of cut-off frequency fc=200Hz in our system could be ignored when the axial velocity is larger than 0.30 m/s and Q is greater than 2.0. Our experimental results, in contrast to those obtained from phantom systems by us and by Hein and O'Brien, indicate that the distribution of blood velocity is much blunter than previously thought. The Q index exceeded 10 during systole, whereas it was 0.5 in parabolic flow. The peak of Q index lagged behind that of axial blood velocity by approximately 0.02s. The phase delay of the Q index curve might be due to the time needed for the red blood cells to form the non-homogeneous distribution.