Generation of uniform lesions in high intensity focused ultrasound ablation

High intensity focused ultrasound (HIFU) is emerging as an effective oncology treatment modality according to the clinical experience in the last decade. The temperature at the focus can reach over 65°C within seconds, denaturing cellular proteins and resulting in coagulative necrosis. HIFU parameters are usually kept the same for each treatment spot in tumor ablation. Because of the thermal diffusion from nearby spots, the lesion size will gradually increase as the HIFU therapy progresses, which leads to insufficient treatment of initial spots and over exposure of later ones. From the viewpoint of the physician, uniform lesions with the least energy exposure and the least energy are preferred in tumor ablation. In this study, an algorithm was developed to determine the number of HIFU pulses delivered to each spot in order to generate uniform lesions that fill the region-of-interest completely. The exposure energies required using different scanning pathways (raster scanning, spiral scanning from the center to the outside, and spiral scanning from the outside to the center), spot spacing (1mm, 2mm, 4mm, and 6mm) and motion time (from 0s to 400s) were compared with each other. It is found that spiral scanning from the outside to the center with spot spacing of 2mm and motion time less than 10s needs the least numbers of pulses or HIFU energy in uniform lesion production with the minimal temperature elevation. In addition, the effects of thermal properties of tissue (i.e., specific heat capacity, convective heat transfer coefficient, and thermal conductivity) on HIFU ablation were investigated in order to determine the HIFU treatment planning for various targets. Uniform lesion production in the transparent gel phantom and ex vivo bovine liver samples using the proposed algorithm proved effective and accord with the simulation for different scanning pathways by an extracorporeal clinical HIFU system. Therefore, dynamically adjusting ultrasound exposure energy can improve the efficacy and safety of HIFU ablation, and the treatment planning depends on the scanning protocol and thermal properties of the target.