Non-equally-spaced pulse transmission for non-aliasing ultrasonic pulsed Doppler measurement

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This paper presents a new method of measuring blood velocities and the Doppler frequencydistribution that is not limited by the Nyquist frequency. A non-equally-spaced pulse transmission(a staggered pulse repetition time)method is used calculate the second-order phase difference(velocity)between successive echoes from the blood flow, and the noise-induced fluctuation in the phase difference is suppressed by averaging the phase difference vectors. With conventional pulsed-Doppler equipment, Nyquist's sampling theorem and the relationship between frequency and distance limit the maximum measurable velocity to the reciprocal of twice the pulse repetition time(sampling time). We show theoretically that the proposed method has a larger maximum velocity. When the non-equally-spaced pulse repetition intervals are T and T+T_s, the maximum measurable velocity can be increased to P times the conventional value by selecting T_s equal to T/P(P>1). The results of computer simulation with a high signal-to-noise ratio are in good agreement with the results measured using the proposed method. In-vivo high-speed blood flow(at over the Nyquist-frequency)in the carotid artery and heart is unambiguously measured without aliasing. To illustrate the feasibility of the proposed method, we also compare the spectrum with that obtained using conventional pulsed-Doppler equipment.
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