Effect of focus splitting on ultrasound propagation through the rib cage in focused ultrasound surgery

The presence of the rib cage is a significant hindrance to the clinical applicability of focused ultrasound surgery for various internal organs, including liver, kidney, and pancreas. The main problems associated with the rib cage are considered to be: the strong reflections that occur at the soft tissue/rib interfaces; the highly attenuating effect of the ribs; and the scattering and diffraction that occur at the ribs boundaries. In this study, a 2D high speed ultrasound scanning system was used to directly measure the acoustic field in the focal plane after propagation of focused ultrasound through an acoustic absorber rib-mimicking material and an epoxy rib-mimicking material. Numerical simulations were also performed with finite element analysis software (PZFlex, Weidlinger Associates, USA) to investigate the changes in the acoustic field with different anatomical configurations. Briefly, due to the defocusing nature of the rib-mimics, the acoustic pressure level at the focus was substantially reduced. This study quantitatively analyzed the effect of focus splitting associated with the ribs to provide reference information for further clinical applications.