A method of measuring the nonlinearity parameter B/A of soft tissue in vitro was developed for thermodynamic phenomenon. Using a gated ultrasonic rf pulse, the propagation time through the tissue was measured at two different thickness values. The B/A of the tissue could be estimated by the thickness difference, and the propagation time difference between atmosphere and a pressurized (3 atms) condition. A fundamental experiment was carried out to confirm the validity of the method. The phase (time) difference was measured by the interference method with a two-channel synthesizer. The B/A of water was proportional to its temperature from 10 to 40 °C and was 5.1 at 20 °C. A B/A of 9.2 for a chicken liver in vitro was also obtained.
The nonlinearity parameters B/A of liquid and liquid-like media such as soft tissue were measured by the thermodynamic method. The temperature dependences of B/A were obtained from 20 to 40°C. We also got B/A of water mixed with methanol, and varied the methanol mixture weight ratio from 10 to 75%. The B/A of water, methanol mixture and soft tissues are approximately proportional to the temperature. However, that of the methanol mixture did not change linearly with the methanol mixture ratio; it appeared to be constant at the ratio from 0 to 25% and from 75 to 100%, but changed rapidly at the ratio of about 50%.
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