Focus shift and phase correction in soft tissues during focused ultrasound surgery.

During the treatment of soft tissue tumors using focused ultrasound surgery (FUS), the focus can shift away from the desired point due to tissue inhomogeneity. In this paper, a numerical method to calculate the focus shift in multiple-layered tissues and a faster phase-correction method to restore the focus were developed. Data from the simulations showed that the focus shifted about 2 mm along the transducer axis in multiple-layered soft tissues. After phase correction, the focus was restored at the desired point. The ex vivo experiments were conducted to verify the simulations, and the results agreed well with those of the simulations. An empirical formula was obtained to estimate the focus shift in a two-layered water-tissue model and was verified by numerical calculations. Moreover, the focus shift in multiple-layered tissues can be summed by the shifts in the component of each layer of tissues. The factors affecting the focus shift were studied. The focus shift varied linearly with the tissue acoustic speed and tissue thickness, whereas it slightly changed with transducer F number (radius of curvature/diameter). Overall, the findings of this study can help in the development of a better treatment plan for FUS in soft tissues.