A line flow of scatterers crossing the sound field produced by a focused circular transducer at uniform velocity originates a quasi-triangular Doppler spectrum. It is known that the spectrum shape and width depend on the line flow to beam axis angle, as well as on the transducer geometry. It has recently been theoretically predicted that this spectrum width is independent of the flow line location in the sound field. Experimental verification of the new theorem, based on the use of a thread phantom operated at various orientations, ranges, and offsets, with respect to the ultrasound transducer, is presented. The tests were made with a computerized pulsed Doppler system designed to perform optimal real-time spectral analysis of data obtained in this application. The prototype system and the experimental procedure adopted for demonstrating in vitro the invariance of the Doppler spectral bandwidth are described.
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