Absorption in liver at the focus of an ultrasonic shock wave field

This experimental study is an extension of a previous investigation of finite amplitude ultrasound absorption at 1 MHz fundamental frequency in freshly excised rat liver at 37 degrees C (Fry et al. 1989). The work reported here includes measurements of the absorption as a function of intensity under a variety of conditions (temperature, pressure, and tissue state). The maximum intensity employed in the investigation (700 W/cm-2) corresponds to a shock parameter of approximately 1.7 based on previous characterization of the acoustic field which established that shock (sigma = 1) occurred in the range 225 W/cm-2 to 275 W/cm-2 (Fry et al. 1989). Of the three temperatures chosen for the thermal study (30 degrees C, 37 degrees C, and 41 degrees C), the only statistically significant differences (p less than 0.05) in absorption were between the 30 degrees C and 41 degrees C data in the intensity range just above sigma = 1 (300-500 W/cm-2). The intensity-dependent absorption coefficient was also determined for excised liver under hyperbaric conditions and for liver in situ at 37 degrees C. At a pressure of 350 psi, the absorption was generally less than at atmospheric pressure. In situ liver at 37 degrees C had a lower absorption above 200 W/cm-2 than freshly excised liver, but the difference was only significant between 200 and 300 W/cm-2 and between 600 and 700 W/cm-2.