Effect of Tissue in HIFU Beam Pathway on Ultrasound Propagation

In HIFU therapy, it is very important to determine the ultrasound attenuation of different tissue from skin to target in HIFU beampath for predicting in advance the dose. So the reduction in acoustic radiation force in the presence of different tissue in HIFU beampath has been studied by a radiation force method. At a room temperature 20°C, the acoustic radiation force in the presence and absence of the prepared different tissue such as bovine liver of various thickness, skin, intercostal tissue closed to liver (including skin, fat, intercostal muscle, ribs and parietal pleura) and ribs in front of absorber in degassed water was measured at same acoustic power at transducer surface. Then the ratio of radiation force in the presence of different tissue relative to degassed water in HIFU beampath and the ultrasound attenuation were calculated. The ratio of radiation force in the presence of skin, intercostal tissue and ribs of pig relative to degassed water in HIFU beampath was 10%, 85.5% and 57.5% respectively. The ultrasound attenuation coefficient α in bovine liver was shown to be frequency f dependent, fitting the equation as follows α=0.43f^1.05 [dB.cm^-1.s^1.05]. The results showed that the ultrasound energy was attenuated markedly when HIFU traveled through intercostal tissue, especially ribs in HIFU beampath. This work offered experimental data that should be useful for future HIFU dosage studies.