Acoustical Methods for Cavitation Control in Shockwave Lithotripsy and Histotripsy

The overall goal of the work presented in this dissertation is to develop acoustic mechanisms to modulate, or manipulate, cavitation events in histotripsy and lithotripsy therapies in order to achieve efficient and fast histotripsy, high shock rate lithotripsy, and active tissue protection. We investigated the effects of applying properly tuned low pressure acoustic pulses before and during therapy in order to control the cavitation threshold, the shape of the resulting bubble cloud, and the behavior and interactions of residual microbubbles. Histotripsy is a tissue ablation method that utilizes focused high amplitude ultrasound to generate a cavitation bubble cloud that mechanically fractionates tissue. Effective histotripsy depends on initiation, control, and maintenance of cavitation bubble clouds in the targeted area. The work in this dissertation seeks to develop active tissue protection techniques by modulating the pressure threshold of bubble cloud initiation and focal sharpening using bubble suppressing pulses. We demonstrated that by applying a properly tuned low pressure pulse sequence before and/or during shock scattering histotripsy therapy, both the cavitation initiation pressure threshold and the growth of the cavitation bubble can be modified. This mechanism can be used to produce well defined lesions with minimal collateral damage. It can also be a way to actively protect soft tissue from cavitation damage during both lithotripsy and histotripsy by increasing the pressure threshold for bubble cloud initiation in the periphery zone. Cavitation also plays a significant role in the efficacy of stone comminution during shockwave lithotripsy (SWL). Although cavitation on the surface of urinary stones helps to improve stone fragmentation, cavitation bubbles along the propagation path may shield or block subsequent shockwaves and potentially induce collateral tissue damage. At low firing rates, there is sufficient time for the majority of the bubbles to passively dissolve, while at high firing rates the per shockwave efficacy is significantly reduced due to pre-focal persisting bubbles. We investigated acoustic methods for removing residual bubble nuclei in order to avoid shielding effects. Previous in vitro work has shown that applying low amplitude acoustic waves after each shockwave can force bubbles to consolidate and enhance SWL efficacy. In this work, the feasibility of applying acoustic bubble coalescence (ABC) in vivo was examined. We further optimized the parameters of bubble coalescing pulses, and conducted a feasibility investigation of bubble dispersion by forcing the residual bubble nuclei to disperse from the propagation path away or toward the targeted area before the arrival of the next therapy pulse. These results suggest that manipulation of residual bubbles after each shockwave can be further optimized by acoustic bubble coalescence and dispersion, which can reduce the shielding effect of residual bubble nuclei more efficiently than relying only on immediate coalescence of residual bubbles, resulting in a more efficient SWL treatment.PHDBiomedical EngineeringUniversity of Michigan, Horace H. Rackham School of Graduate Studieshttps://deepblue.lib.umich.edu/bitstream/2027.42/149949/1/alavi_1.pd

Acoustic Bubble Removal to Enhance SWL Efficacy at High Shock Rate: An In Vitro Study

Rate-dependent efficacy has been extensively documented in shock wave lithotripsy (SWL) stone comminution, with shock waves (SWs) delivered at a low rate producing more efficient fragmentation in comparison to those delivered at high rates. Cavitation is postulated to be the primary source underlying this rate phenomenon. Residual bubble nuclei that persist along the axis of SW propagation can drastically attenuate the waveform's negative phase, decreasing the energy which is ultimately delivered to the stone and compromising comminution. The effect is more pronounced at high rates, as residual nuclei have less time to passively dissolve between successive shocks. In this study, we investigate a means of actively removing such nuclei from the field using a low-amplitude acoustic pulse designed to stimulate their aggregation and subsequent coalescence. To test the efficacy of this bubble removal scheme, model kidney stones were treated in vitro using a research electrohydraulic lithotripter. SWL was applied at rates of 120, 60, or 30?SW/min with or without the incorporation of bubble removal pulses. Optical images displaying the extent of cavitation in the vicinity of the stone were also collected for each treatment. Results show that bubble removal pulses drastically enhance the efficacy of stone comminution at the higher rates tested (120 and 60?SW/min), while optical images show a corresponding reduction in bubble excitation along the SW axis when bubble removal pulses are incorporated. At the lower rate of 30?SW/min, no difference in stone comminution or bubble excitation was detected with the addition of bubble removal pulses, suggesting that remnant nuclei had sufficient time for more complete dissolution. These results corroborate previous work regarding the role of cavitation in rate-dependent SWL efficacy, and suggest that the effect can be mitigated via appropriate control of the cavitation environment surrounding the stone.Peer Reviewedhttps://deepblue.lib.umich.edu/bitstream/2027.42/140375/1/end.2013.0313.pd

Enhanced High-Rate Shockwave Lithotripsy Stone Comminution in an In Vivo Porcine Model Using Acoustic Bubble Coalescence

Cavitation plays a significant role in the efficacy of stone comminution during shockwave lithotripsy (SWL). Although cavitation on the surface of urinary stones helps to improve fragmentation, cavitation bubbles along the propagation path may shield or block subsequent shockwaves (SWs) and potentially induce collateral tissue damage. Previous in vitro work has shown that applying low-amplitude acoustic waves after each SW can force bubbles to consolidate and enhance SWL efficacy. In this study, the feasibility of applying acoustic bubble coalescence (ABC) in vivo was tested. Model stones were percutaneously implanted and treated with 2500 lithotripsy SWs at 120 SW/minute with or without ABC. Comparing the results of stone comminution, a significant improvement was observed in the stone fragmentation process when ABC was used. Without ABC, only 25% of the mass of the stone was fragmented to particles <2?mm in size. With ABC, 75% of the mass was fragmented to particles <2?mm in size. These results suggest that ABC can reduce the shielding effect of residual bubble nuclei, resulting in a more efficient SWL treatment.Peer Reviewedhttps://deepblue.lib.umich.edu/bitstream/2027.42/140087/1/end.2016.0407.pd