The effect of acoustic velocity on phantom measurements

Urethane rubber ultrasound (US) phantoms have a much lower acoustic velocity (1430-1450 m/s) than the accepted soft tissue average of 1540 m/s. Two important questions arise: can the rod positions in these rubber phantoms be adjusted so that they may be used to test equipment distance measurement accuracy for all types of multielement transducers, and can they be used to measure beam focus (using the spread of the rod blur patterns)? These questions were addressed for linear-, phased-, convex- and vector-array transducers. Theoretical predictions for the different transducers' distance measurement errors agreed with careful measurements obtained with a specially designed array of stainless-steel rods immersed in paraffin oil (1447 m/s). The conclusions of this study are that phantoms with acoustic velocities different from 1540 m/s cannot be used to check distance measurement accuracies of all the types of real-time transducers, nor to predict a transducer's focusing performance in clinical scans.