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A new technique to measure phase distribution of ultrasound beams is presented. This method is based on the phase measurement of the Doppler signal backscattered by a moving target traveling at constant velocity, which represents a scanning element exploring the held under examination. It is produced by mechanical equipment consisting of a modified string phantom in which the commonly used cotton thread was substituted by a thin nylon thread with some tight knots over it. The thread diameter was chosen small enough to produce knots smaller than the acoustic field wavelength in order to have "pointlike" moving targets. This paper describes how the Doppler signal phase is related to the phase field and the numerical method to extract a phase field distribution from the backscattered signal. Experimental results obtained for two circular transducers, strongly and weakly focused, respectively, are finally shown and commented.

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