Dependence of the ultrasonic nonlinearity parameter B/A on cellular-level structure

The nonlinearity parameter B/A of biological media is considered useful as a tissue characterization parameter and as an accurate model of finite amplitude wave propagation in such applications as ultrasonic hyperthermia and lithotripsy. The dependence of B/A on cellular-level structure has been investigated by the thermodynamic method using liposomes as a cell model. B/A measurements were made using a pressure jump scheme to create an adiabatic pressure change and employing a cross-correlation method to determine the time delay between a pulse transmitted through a sample and a reference pulse and, thereby, the speed of wave propagation. It was found that B/A increases considerably near the transition temperature of the liposomes, suggesting that the nonlinearity parameter reflects the state of the liposome membrane