Robot-assisted histotripsy ablation noninvasively creates ablation zones

Robotically assisted sonic therapy (RAST) has proven to be effective in producing clinically relevant hepatic ablation zones without being invasive to the subject, according to a new study published in Radiology.  

To determine the usefulness of the clinically relevant ablation technique using RAST, researchers based their methods of ultrasound therapy on histotripsy.  

"Histotripsy is a nonthermal, noninvasive ultrasonic ablation method that controllably fractionates soft tissues into an acellular homogenate through the precise control of acoustic cavitation generated by high-pressure, short-duration ultrasound pulses at low duty cycles," according to lead author of the study Amanda Smolock, MD, PhD, a radiology fellow from the University of Wisconsin.   

Ten female pigs were handled by RAST with a three-centimeter spherical treatment region and then underwent abdominal MR imaging. According to study methods, three pigs belonging to the acute group were immediately killed following MR imaging, while the remaining seven in the chronic group survived for four weeks and were again imaged before being put down. Researchers also harvested the livers of the pigs and performed RAST ablations on the porcine liver dome, near the gallbladder, in roughly 25 minutes.   

"Student t tests were performed to compare prescribed versus achieved ablation diameter, difference of sphericity from one, and change in ablation zone volume from acute to chronic imaging," according to Smolock.  

Technical success rate of the ablation treatment and survival was 70 percent overall, according to the study. The ablation zones all were found to have the following measurements and characteristics:  

• An average sphericity index of 0.99.
• Anteroposterior and transverse dimensions were not significantly different from prescribed. 
• The craniocaudal dimension was significantly larger than prescribed, likely because of respiratory motion.  
• The central ablation zone demonstrated complete cell destruction and a zone of partial necrosis. 
• A fibrous capsule surrounded the ablation zone by four weeks; on four-week follow-up images, ablation zone volumes decreased by 64 percent.  

Overall, the study was successful in demonstrating how robotically assisted sonic therapy has potential for future use in hepatic ablations.  

"The results of our study demonstrate that RAST can rapidly produce clinically useful ablation zones that adhere closely to the prescribed size and shape and are not associated with any substantial complications," Smolock concluded. "The targeted tissue was completely destroyed into an acellular homogenate surrounded by only a narrow zone of partial necrosis, demonstrating the precision of RAST. In addition, the ablation zone rapidly involutes over time."