Histotripsy bubble dynamics in tendon and anisotropic gel phantoms

Abstract

Collagenous, anisotropic tissues such as tendon have demonstrated resistance to liquefaction by histotripsy, despite the creation, oscillation, and collapse of bubbles verified using B-mode imaging. The objective of this work is to evaluate effects of anisotropy on bubble dynamics in tissue-mimicking hydrogels and compare to anisotropic tissues. Polyacrylamide, fibrin, and collagen hydrogels were fabricated; ex vivo bovine tendons were obtained. Sound speeds were measured in each axial direction to evaluate degree of anisotropy. Hydrogels and tendons were exposed to 1.5-MHz focused ultrasound with 10-ms pulses repeated at 1-Hz with p + =89 MPa, p− = 26 MPa. Cavitation activity was monitored with simultaneous high-speed photography and passive cavitation imaging using a Philips/ATL L7-4 transducer and Vantage® ultrasound system. Violent cavitation activity and fractionation was observed in polyacrylamide, collagen, and fibrin hydrogels with low degrees of anisotropy (<1.2); such behavior is unlike that of tendon. Dehydration of fibrin gels resulted in a 55% reduction in peak cavitation emission energy and a 260% increase in anisotropy compared to standard fibrin formulations. These gels demonstrated similar cavitation energy than tendon (within 4%) but 50% less anisotropy, indicating more hydrogel formulations should be explored to better mimic collagenous, anisotropic tissue. [Work supported by NIH R21EB027886.]