Second harmonic phase method for detecting sign of impedance change

A new method is proposed to detect whether the sign of acoustic impedance change in space is positive or negative. Ultrasonic signed echoes are obtained by transmitting a pulse containing both the fundamental and second harmonic components, calculating the reference second harmonic phase carrying over the fundamental phase of received echoes, and calculating the second harmonic phase of received echoes relative to the reference phase. A preliminary test showed the proposed method works at least with a simple reflector. Experiments with excised porcine liver lobes exposed to high intensity focal ultrasound (HIFU) showed the potential usefulness of the proposed method to detect the completion of thermally induced tissue coagulation.