A system for improving the flow estimation sensitivity of an ultrasonic Doppler flow imaging system includes an ultrasonic wave transmitter and receiver for sequentially transmitting ultrasonic waves toward and into a living body and for receiving the reflected Doppler-shifted echo signals. A series of echo signals received from each of a number of points within the body are processed in a velocity estimator, using maximum entropy techniques, to produce an output signal representative of an estimated velocity of the flow at each of said points within the body. The processing means detects the series of echo signals for each point and calculates an average power spectrum based on the amplitude-versus-frequency distribution of the series of received echo signals. The power spectrum is calculated by fitting a first order polynomial to the series of echo signals, although higher order polynomials can be used. A peak center frequency shift associated with the data detected for each point is derived from the power spectrum. The resulting peak center frequency shift information is then processed to produce a velocity estimate for each of the points at which velocity is being measured, and the resulting velocity estimate information is then processed further for imaging in a two-dimensional Doppler color flow imaging display.