ULTRASOUND PROPERTIES OF LIVER-TISSUE DURING HEATING

The objective of this work was to determine changes in the ultrasound properties of heated tissues, with potential application to monitoring of minimally invasive thermal therapy (MITT). Changes in backscatter coefficients and frequency-dependent attenuation coefficients were mea sured over the frequency range 2.5 MHz to 5 MHz from heated samples of store-bought fresh bovine liver, which was used as a tissue model. In dividual liver samples were heated from 37 degrees C to either 50 degr ees C, 55 degrees C, 60 degrees C, 65 degrees C or 70 degrees C by war m water. The backscatter coefficient increased during the first 3 min by a factor of 1.09 and 1.11 before the tissue reached 50 degrees C an d 55 degrees C, respectively. A decrease in backscatter coefficient fo llowed at 50 degrees C by a factor of 1.12 below the initial level and , at 55 degrees C, the backscatter coefficient dropped below the initi al level by a factor of 1.19. The backscatter coefficient decreased wi thin the first 2 min by a factor of 1.22 before the tissue reached 60 degrees C, then increased gradually to a factor of 1.05 below the init ial level. At 65 degrees C and 70 degrees C, the changes in backscatte r coefficient were highly variable, which may have been due to product ion of gas microbubbles in the heated tissues. The ultrasound attenuat ion coefficient increased by as much as 1.48 dB cm(-1) over a 30-min p eriod at 70 degrees C. First-order rate parameters derived from the at tenuation results revealed one rate process at 50 degrees C and 55 deg rees C and two rate processes at 60 degrees C, 65 degrees C and 70 deg rees C. An activation energy of 1.00 x 10(4) cal mol(-1) was derived f rom the second rate constants at 60 degrees C, 65 degrees C and 70 deg rees C, which indicates that changes in attenuation may be due to prot ein denaturation. In conclusion, ultrasound image monitoring of therma l therapy treatment in liver may be feasible; however, the backscatter coefficient changes during heating are small and are of the same orde r as the variation in these changes from point to point in the tissue. (C) 1997 World Federation for Ultrasound in Medicine & Biology.