Measurements of the effect of polypropylene vials on ultrasound propagation.

Abstract

Polypropylene vials are commonly used in in vitro experiments to hold cell preparations that are exposed to ultrasound. The vial has an acoustic impedance very close to water, in which case there should be little transmission loss as sound propagates into the vial. Measurements of the acoustic field within polypropylene vials have been carried out using both pulsed medical ultrasound and lithotripter shock waves. It has been found that for certain orientations of the acoustic field and vial there is significant loss of pressure amplitude within the vial. In particular, sound that enters the vial through the round, hemispherical end is attenuated and distorted. Hot and cold spots within the vial are also measured. When the rounded end is replaced with a flat surface very little distortion and attenuation of the waveform occurs. The distortion induced by the round end is apparently due to refraction introduced by the vial—the speed of sound in polypropylene is about 1.7 times that of water. A simple ray analysis indicates the presence of hot and cold spots within the vial in qualitative agreement with observations. These results indicate that cell injury experiments may be dependent on the vial orientation. [Work supported by NIH and the ASA Hunt Fellowship.]