Effects of sub-atmospheric pressure and dissolved oxygen concentration on lesions generated in ex vivo tissues by high intensity focused ultrasound.

Background: Acoustic cavitation plays an important role in the medical treatment using high-intensity focused ultrasound (HIFU), but unnecessarily strong cavitation also could deform the morphology and enlarge the size of lesions. It is known that the increase of ambient hydrostatic pressure (P _stat ) can control the acoustic cavitation. But the question of how the decrease of P _stat and dissolved oxygen concentration (DOC) influence the strength of cavitation has not been thoroughly answered. In this study, we aimed to investigate the relationship among the P _stat , DOC and the strength of cavitation.
Methods: Ex vivo bovine liver tissues were immersed in degassed water with different DOC of 1.0 mg/L, 1.5 mg/L and 2.0 mg/L. Ultrasound (US) of 1 MHz and the spatial and temporal average intensity (I _sata ) of 6500 W/cm ² was used to expose two groups of in vitro bovine livers for 2 s; one group was under atmospheric pressure (P _stat  = 1 bar) and the other was under sub-atmospheric pressure (P _stat  = 0.1 bar). Acoustic cavitation was detected by a passive cavitation detector (PCD) during the exposure process. Echo signals at the focal zone of HIFU were monitored by B-mode ultrasound imaging before and after exposure. The difference between two pressure groups was tested using paired sample t-test. The difference among different DOC groups was evaluated by one-way analysis of variance (ANOVA).
Results: The results demonstrated a significant difference of broadband acoustic emissions from the cavitation bubbles, echo signals on B-mode image, morphology of lesions under various conditions of ambient pressure and DOC. The lesion volume in tissue was increased with the increase of ambient pressure and DOC.
Conclusion: Cavitation could be suppressed through sub-atmospheric pressure and low DOC level in liver tissue, which could provide a method of controlling cavitation in HIFU treatment to avoid unpredictable lesions.
(© 2021. The Author(s).)