Ultrasonic attenuation and absorption in liver tissue.

A large range of values for ultrasonic attenuation and absorption coefficients of tissues are reported in the literature. An important distinction both practically and theoretically is the magnitude of the true absorption, which characterizes the rate of conversion of ultrasonic to thermal energy, as compared with the total attenuation of the ultrasonic signal as it propagates through tissue. The magnitudes of these quantities were studied in bovine liver. Total attenuation was measured, in the range of 1-6 MHz, by both phase sensitive and phase insensitive insertion loss techniques. Ultrasonic absorption was determined by two thermal techniques. The standard "transient thermoelectric" or rate-of-heating method, and a new measurement technique based on the temperature decay following a short ultrasonic pulse were employed for the determination of the ultrasonic absorption coefficient. The results demonstrate that the ultrasonic amplitude attenuation and absorption coefficients at low megahertz frequencies are not significantly different in liver. The mean values cluster around 0.05 nepers/cm/MHz (0.4dB/cm/MHz). The sample-to-sample variation is indicated by the standard deviation in the measurements of 0.01 nepers/cm/MHz (0.09dB/cm/MHz) or less. The results show that in liver tissue, absorption is the dominant feature of attenuation over this frequency range.