Velocity vector reconstruction for color flow Doppler: experimental evaluation of a new geometrical method.

Ultrasonic color flow mapping (CFM) is employed for the diagnosis of a variety of cardiovascular abnormalities. But CFM permits only the analysis of the in-beam component of the velocity distribution. A reconstruction method based on a geometrical approach was developed to calculate the two-dimensional, in-plane velocity vector using the information of two independent CFM measurements obtained from two closely spaced transducers. However, with the CFM measurements being limited by the noise and quantified in a limited number of velocity values, the accuracy of the velocity vector reconstruction is not satisfactory. A simple filtering process was applied to improve the results. The method was tested with simulations and a series of in vitro measurements. The results of the simulations were compared with analytical solutions. The in vitro measurements were compared with the results of laser Doppler anemometry (LDA) and theoretical calculations. The results of in vivo measurements were compared with theoretical calculations based on Womersley's theory. The comparison showed good applicability of the method to different flow fields. The flow vector reconstructed without filtering had an error of more than 20%; whereas our method, including the filtering process, had an error of less than 10%, in comparison with the theoretical or the LDA results. The method could be extended to three-dimensional flows.

Compensation of the bias caused by the wall filter on the mean Doppler frequency.

In the presence of noise, any wall filter induces a bias on the mean Doppler frequency estimated by the algorithm based on the calculation of the phase of the autocorrelation function at lag 1. In this paper, it is shown that the bias results from the nonzero value of the autocorrelation function of the filtered noise at lag 1. A general method for compensating the bias was then deduced. It consists of a preliminary estimation of the filtered noise contribution and its subtraction from the autocorrelation function of the Doppler signal. The method is independent of the nature of the noise and wall filter. An evaluation of this method on simulated Doppler signals and on measurements from a moving-string phantom showed that effective compensation of the bias was obtained, but at the expense of a higher variance. Taking into account both the bias and the variance, however, it was shown that this method offers an improvement over the noncompensated method. Finally, the performance of this method was demonstrated using in vivo measurements taken from a human aorta.