Histotripsy bubble dynamics in anisotropic hydrogels

Abstract

Collagenous, anisotropic tissues such as tendon have demonstrated resistance to fractionation by histotripsy. B-mode imaging verifies the creation of bubbles, but oscillation and collapse of these bubbles does not cause fractionation. The objective of this work is to evaluate effects of anisotropy on bubble dynamics in tissue-mimicking hydrogels. Polyacrylamide, fibrin, and collagen hydrogels were fabricated. Axial sound speeds were measured in each direction to evaluate degree of anisotropy. Hydrogels 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 high-speed photography and passive cavitation imaging with a Philips/ATL L7-4 transducer and Vantage® ultrasound system. Preliminary results show violent cavitation activity and rapid fractionation in polyacrylamide, collagen, and fibrin hydrogels which have low degrees of anisotropy (<1.2); such behavior is unlike what is observed in collagenous tissues. To make a gel more similar to collagenous tissues, fibrin gels were dehydrated and a >90% reduction of water content resulted in a 55% reduction in peak cavitation emission energy and a 260% increase in anisotropy. This suggests an influence from anisotropy, and therefore varying axial elastic moduli, on cavitation activity that will be explored further. [Work supported by NIH R21EB027886.]