Impedance changes in liver tissue exposed in vitro to high-energy ultrasound.

The present study reports the impedance changes observed in bovine liver samples exposed in vitro to high-intensity ultrasound. The measurement frequency ranged from 80 kHz to 2 MHz. The treatment resulted in the average increase of 20% in impedance magnitude at low frequency and the average decrease of 30% at high frequency. The phase angle increased significantly by more than 15 degrees at all measurement frequencies. The slope of the log-modulus of impedance against log-frequency increased in treated tissue at frequencies above 500 kHz. This change was attributed to the alteration of the capacitive response of the tissue. The experimental observations are consistent with the known changes induced by high-energy ultrasound in liver tissue. This study confirmed that ultrasound energy produces measurable changes in a tissue's impedance and that indices can be derived to distinguish between original and treated tissues. The results obtained in liver tissue need confirmation in organs treatable with therapeutic ultrasound, such as breast and prostate.