Monitoring cavitation dynamics evolution in tissue mimicking hydrogels for repeated exposures via acoustic cavitation emissions

Acoustic cavitation emission (ACE)/shockwave signals generated by cavitation during histotripsy have been demonstrated to undergo characteristic changes correlated with the level of damage generated in select target media following repeated cavitation exposures, and may allow assessment of therapy-induced tissue damage during histotripsy. Here we investigate the impacts of nucleation medium on the evolutions of cavitation dynamics and ACE signals during repeated cavitation exposures. Repeated cavitation is generated in agarose, gelatin, and polyacrylamide hydrogels using a 700-kHz histotripsy array. High-speed imaging, broadband hydrophones, and histotripsy array elements are used to measure ACE signals and bubble dynamics. Bubble lifespans and collapse shockwave amplitudes were observed to be equivalent in optical and acoustic measurements. However, the evolutions of these properties, as well as bubble maximum radii, varied significantly across materials. Maximum radii initially increased and then decreased in agarose, but remained constant in gelatin and polyacrylamide. Lifespans increased monotonically in agarose and gelatin, but decreased in polyacrylamide. Collapse shockwave amplitudes evolved differently in all materials. The observed evolutions of the maximum radii, lifespans, and collapse shockwave amplitudes between media suggest that these features are dependent on the structure and stiffness of the nucleation medium and must be further investigated in tissues.