Acoustic measurements in a tissue mimicking liquid.

A liquid has been developed that mimics soft tissue in terms of propagation speed, attenuation, and nonlinearity parameter B/A. Since the slope of the attenuation coefficient is nearly constant up to at least 18 MHz and the value can be anywhere in the range 0.1 through 0.7 dB/(cm MHz), the material is ideal for assessing the effectiveness of attenuation derating of output acoustic intensities and pressures measured in water. A clinical ultrasound system was used to transmit into the tissue mimicking liquid. The pulse intensity integral and rarefactional pressure at various field locations in the tissue mimicking liquid were computed and compared to the corresponding values measured in water with attenuation derating applied. The slope of the attenuation coefficient in the tissue mimicking liquid was used for the derating. From this data it is seen that the present practice of modeling in vivo exposure using linearly derated measurements made in water is not always conservative.