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Volume blood flow information is important to the diagnosis of cardiovascular diseases. In Doppler ultrasound, the mean velocity of the sampled flow is directly related to the mean frequency of the echo signal. Perhaps the major difficulty in velocity estimate is the accurate determination of the Doppler shift in a noisy environment. 
Because Doppler shift is a linear function of the flow velocity, signal processing in the frequency domain is natural. However, basing on FFT techniques, these methods are noise-sensitive and suffer from spectral leakage effects, due to windowing that are inherent in finite-length data records.
An autocorrelation velocity estimator operates in the time domain. It is based on using a  PW Doppler system that transmits a gated tone burst while the per-channel echo signals are independently range gated. The gated signals are phase aligned, summed and converted into baseband. Finally, the obtained I/Q components are integrated over the combined time interval of the all range gates and applied to the velocity estimator. 
The estimation of velocity can also be done by finding the time-shift between two consecutive RF signals directly.
Typically, the time-domain velocity estimators use the Kasai algorithm wherein  the mean frequency is directly derived from a pair of successive  I and Q signals as follows:


It is widely accepted that the Kasai technique is both accurate and computationally efficient. As seen below, its hardware implementation  is quite simple too. However, the signals from the blood is much weaker than the signal from the tissue. Furthermore, in the Doppler operating mode, the time period of the signal phase variations depends upon the blood flow velocity. In slower flows, the period of the Doppler signal becomes longer. It means that the phase increments between two adjacent samples N and N - 1 are reduced, i.e.,

Consequently, accurate estimation of  the numerator in Eq. 1 requires an excessive bit resolution or/and spatial averaging.
Unlike to conventional techniques, the proposed solution is based on a hybrid analog/digital architecture. This allows to implement lower-resolution and hence more cost-effective ADCs.
Kasai Estimator

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