Respiratory motion effects and mitigation strategies on boiling histotripsy in porcine liver and kidney

Boiling histotripsy (BH) is a pulsed high-intensity focused ultrasound (HIFU)-based method of extracorporeal non-thermal tissue disintegration under real-time ultrasound (US) guidance. Respiratory motion in abdominal targets can affect BH precision and completeness. This study compares two motion mitigation strategies based on pulse/echo US motion tracking: robotic arm-based unidirectional motion compensation by HIFU transducer manipulation and BH pulse gating during expiratory pause. BH ablations were generated in liver and kidney of anesthetized pigs with 2-10ms pulses using 256-element 1.5-MHz HIFU array. A coaxial US imaging probe was used for targeting, tracking skin surface and monitoring real-time bubble activity. The axial (anterior-posterior, AP) displacement of the skin surface was found to be synchronous with liver and kidney motion in both cranio-caudal (CC) and AP directions. BH lesions were produced either with no motion mitigation, or with pulse gating, or with 1D motion compensation. Dimensions of completely fractionated and affected tissue areas were measured histologically. In liver, gating and motion compensation improved fractionation completeness within targeted volumes and reduced off-target tissue damage in AP direction vs no motion mitigation; only gating reduced off-target damage in CC direction. In kidney, gating improved BH completeness in both directions vs no mitigation, but did not affect off-target damage due to lower displacement amplitudes in kidney comparable with gating tolerance limits. In both liver and kidney, gating increased treatment time by 24%. These results suggest that BH pulse gating using US-based AP skin surface tracking is an adequate approach for treating organs with pronounced 3D respiratory motion.