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 nonthermal 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 the liver and kidney of anesthetized pigs with 2-10-ms pulses using a 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 1-D 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 versus no motion mitigation; only gating reduced off-target damage in CC direction. In kidney, gating improved BH completeness in both directions versus no mitigation, but did not affect off-target damage due to lower displacement amplitudes in the 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 3-D respiratory motion.