The Effects of Energy on the Relationship between the Acoustic Focal Region and Biological Focal Region during Low-Power Cumulative HIFU Ablation

Abstract

The biological focal region (BFR) induced by a single high-intensity focused ultrasound (HIFU) exposure is considered to be the foundation of the ultrasound ablation of tumor lesions. The purpose of this study was to explore the relationship between the acoustic focal region (AFR) and the BFRs with different combinations of power and time in low-power cumulative HIFU treatment. The finite-difference time-domain (FDTD) method was used to simulate AFR and BFR during HIFU ablation. The acoustic fields, the temperature profiles, and the shapes of BFRs were calculated by the Westervelt equation, Pennes’ equation, and the equivalent thermal dose model. In order to verify the simulation rules, phantom and ex vivo bovine livers were exposed by HIFU with a different power and time. The results demonstrated that in the low-power cumulative HIFU treatment, when the lengths of BFRs and the length of AFR were approximately equal, the shape of the BFR induced by ‘high power × short time’ exposure was closer to that of AFR than the shape of the BFR induced by ‘low power × long time’ exposure, and the exposure energy required was significantly reduced. The analysis revealed the relationship between the BFR and the AFR with different acoustic power. This study provides a reference for doctors to determine power, time, and movement distance in clinical treatment.