Effects of different hydrostatic pressure on lesions in ex vivo bovine livers induced by high intensity focused ultrasound

It is well-known that acoustic cavitation associated with the high intensity focused ultrasound (HIFU) treatment often would change the morphology and size of lesions in its treatment. In most studies reported in literature, high ambient hydrostatic pressure was used to suppress the cavitation completely. Investigation of the effects by varying the ambient hydrostatic pressure (Pstat) is still lacking. In this paper, the effects of HIFU on lesions in ex vivo bovine liver specimens under various Pstat are systematically investigated. A 1 MHz HIFU transducer, with an aperture diameter of 70 mm and a focal length of 55 mm, was used to generate two groups US exposure of different acoustic intensities and exposure time (6095 W/cm2 × 8 s and 9752 W/cm2 × 5 s), while keeping the same acoustic energies per unit area (48760 J/cm2). The peak acoustic negative pressures (p−) of the two groups were p1-=9.58MPa and p2-=10.82MPa, respectively, with the difference pd-=p2--p1-=1.24MPa. A passive cavitation detection (PCD) was used to monitor the ultrasonic cavitation signal during exposure of the two groups. The US exposures were done under the following ambient hydrostatic pressures, Pstat: atmospheric pressure, 0.5 MPa, 1.0 MPa, 1.5 MPa, 2.0 MPa, 2.5 MPa and3.0 MPa, respectively. The result of PCD showed that there was a statistically significant increase above background noise level in broadband emissions at dose of 9752 W/cm2 × 5 s, but not at dose of 6095 W/cm2 × 8 s under atmospheric pressure; i.e., the acoustic cavitation took place for p2- but not for p1- when under atmospheric pressure. The results also showed that there was no statistically difference of the morphology and size of lesions for 6095 W/cm2 × 8 s exposure under the aforementioned different ambient hydrostatic pressures. But the lesions generated at 9752 W/cm2 × 5 s exposure under Pstat = atmospheric pressure, 0.5 MPa, 1.0 MPa (all of them are less than pd-), were larger than those under 1.5 MPa, 2.0 MPa, 2.5 MPa and 3.0 MPa (all of them are over than pd-) which were consistence with 6095 W/cm2 × 8 s group. It was concluded that when Pstat>pd-, the acoustic cavitation was suppressed and prompted that there was no need to elevate Pstat higher than p− to suppress the acoustic cavitation in tissue, just need Pstat higher than pd-.