Losses in tissue associated with finite amplitude ultrasound transmission

The ultrasonic absorption coefficient of freshly-excised rat liver is measured as a function of intensity in a 1 MHz focused field. The 7 cm diameter source has a geometric focus of 20 cm and a half-intensity linear focal beamwidth of 4.65 mm. Free-field harmonic content and radiation force measurements within the focal region indicate that quasilinear shock (sigma = 1) is attained in the intensity range of 225 W cm-2 to 275 W cm-2 SPTP intensity. Radiometric measurements of focal beamwidth and intensity response (focal intensity versus transducer input voltage squared) demonstrated beam narrowing up to 700 W cm-2 and focal intensities greater than would be expected from linear theory. This nonlinear acoustic field is employed to measure the absorption coefficient of liver at the beam focus. The pressure absorption coefficient, measured via the transient thermoelectric technique, was 0.038 Np cm-1 at low intensities (below 100 W cm-2 SPTP) and rose to approximately 0.117 Np cm-1 at 750 W cm-2 SPTP intensity. The absorption predicted using spherically converging beam theory is in agreement with the experimental data at the lower (below 200 W cm-2) and the higher (above 650 W cm-2) intensities. Gaussian theory provided a poor fit to the experimental data even after deficiencies in the model were considered.