Acoustic properties of butadiene and silicone elastomers at megahertz frequencies

Abstract

A tissue mimic material with a sound velocity of 1540 m s^–1 and an acoustic attenuation of 0·5 dB MHz^–1 cm^–1 is required for calibrating medical Doppler ultrasound transducers. These properties can be obtained with a solid material comprised of a gel containing a high water content. If solid elastomers are to be used to provide a more stable tissue mimic, modelling suggests that the materials would require low viscoelastic damping, with a low glass transition temperature, T_g. Castable polyurethanes have been made using a polybutadiene soft segment and by adding process oil, the acoustic loss has been reduced to 0·9 dB MHz^–1 cm^–1 with a sound velocity of 1459 m s^–1. Unfilled butadiene rubber (with a high-cis microstructure) had an acoustic loss around 1 dB MHz^–1 cm^–1, the low loss being associated with its low T_g, with a sound velocity of 1567 m s^–1. Silicone elastomers made from unfilled poly dimethyl siloxane had an acoustic loss of 0·71 dB MHz^–1 cm^–1, but the sound velocity was too low at 1033 m s^–1. Filler particles were added to raise the sound velocity, but they seemed to act as scattering centres and the composite had high acoustic attenuation. None of the elastomers were able to reproduce the acoustic attenuation characteristics of the tissue mimic gel.