Ultrasonic wave speed measurement using the time-delay profile of rf-backscattered signals

Conventional methods determine the ultrasonic wave speed by measuring the medium path length propagated by a pulsed wave and the corresponding time-of-flight. In this study, the wave speed is determined without the need of the path length. A transmitting transducer sends a pulsed wave into the medium (constant wave speed along the beam axis) and the backscattered signal is collected by a hydrophone placed at two distinct positions near the transmitted beam. The time-delay profile, between gated windows of the two rf-signals received by the hydrophone, is determined using a cross-correlation method. Also, a theoretical time-delay profile is determined considering the wave speed as a parameter. The measured wave speed is obtained upon minimization of the RMS error between theoretical and experimental time-delay profiles. A PZT conically focused transmitting transducer with center frequency of 3.3 MHz, focal depth of 20 mm and beam width (-6 dB) of 2 mm at the focus was used together with a PZT hydrophone, 0.8 mm in aperture. The method was applied to three phantoms (wave speed of 1220, 1501 and 1715 m/s) and, in vitro, to fresh bovine liver sample, immersed in a temperature-controlled water bath. The results vary within 3% of those obtained with a conventional method.