Effects of Histotripsy on Local Tumor Progression in an in vivo Orthotopic Rodent Liver Tumor Model

Objective and Impact Statement.This is the first longitudinal study investigating the effects of histotripsy on local tumor progression in an in vivo orthotopic,immunocompetent rat hepatocellular carcinoma(HCC)model.Introduction.Histotripsy is the first noninvasive,nonionizing,nonthermal,mechanical ablation technique using ultrasound to generate acoustic cavitation to liquefy the target tissue into acellular debris with millimeter accuracy.Previously,histotripsy has demonstrated in vivo ablation of noncancerous liver tissue.Methods.N1-S1 HCC tumors were generated in the livers of immunocompetent rats(n=6,control;n=15,treatment).Real-time ultrasound-guided histotripsy was applied to ablate either 100%tumor volume+up to 2 mm margin(n=9,complete treatment)or 50-75%tumor volume(n=6,partial treatment)by delivering 1-2 cycle histotripsy pulses at 100 Hz PRF(pulse repetition frequency)with p−≥30 MPa using a custom 1 MHz transducer.Rats were monitored weekly using MRI(magnetic resonance imaging)for 3 months or until tumors reached~25 mm.Results.MRI revealed effective post-histotripsy reduction of tumor burden with near-complete resorption of the ablated tumor in 14/15(93.3%)treated rats.Histopathology showed<5 mm shrunken,non-tumoral,fibrous tissue at the treatment site at 3 months.Rats with increased tumor burden(3/6 control and 1 partial treatment)were euthanized early by 2-4 weeks.In 3 other controls,histology revealed fibrous tissue at original tumor site at 3 months.There was no evidence of histotripsy-induced off-target tissue injury.Conclusion.Complete and partial histotripsy ablation resulted in effective tumor removal for 14/15 rats,with no evidence of local tumor progression or recurrence.

Noninvasive Low-Intensity Focused Ultrasound Mediates Tissue Protection following Ischemic Stroke

Objective and Impact Statement.This study examined the efficacy and safety of pulsed,low-intensity focused ultrasound(LIFU)and determined its ability to provide neuroprotection in a murine permanent middle cerebral artery occlusion(pMCAO)model.Introduction.Focused ultrasound(FUS)has emerged as a new therapeutic strategy for the treatment of ischemic stroke;however,its nonthrombolytic properties remain ill-defined.Therefore,we examined how LIFU influenced neuroprotection and vascular changes following stroke.Due to the critical role of leptomeningeal anastomoses or pial collateral vessels,in cerebral blood flow restoration and tissue protection following ischemic stroke,we also investigated their growth and remodeling.Methods.Mice were exposed to transcranial LIFU(fundamental frequency:1.1 MHz,sonication duration:300 ms,interstimulus interval:3 s,pulse repetition frequency:1 kHz,duty cycle per pulse:50%,and peak negative pressure:-2.0 MPa)for 30 minutes following induction of pMCAO and then evaluated for infarct volume,blood-brain barrier(BBB)disruption,and pial collateral remodeling at 24 hrs post-pMCAO.Results.We found significant neuroprotection in mice exposed to LIFU compared to mock treatment.These findings correlated with a reduced area of IgG deposition in the cerebral cortex,suggesting attenuation of BBB breakdown under LIFU conditions.We also observed increased diameter of CD31-postive microvessels in the ischemic cortex.We observed no significant difference in pial collateral vessel size between FUS and mock treatment at 24 hrs post-pMCAO.Conclusion.Our data suggests that therapeutic use of LIFU may induce protection through microvascular remodeling that is not related to its thrombolytic activity.

Endoscopic Coregistered Ultrasound Imaging and Precision Histotripsy:Initial In Vivo Evaluation

Objective.Initial performance evaluation of a system for simultaneous high-resolution ultrasound imaging and focused mechanical submillimeter histotripsy ablation in rat brains.Impact Statement.This study used a novel combination of high-resolution imaging and histotripsy in an endoscopic form.This would provide neurosurgeons with unprecedented accuracy in targeting and executing nonthermal ablations in minimally invasive surgeries.Introduction.Histotripsy is a safe and effective nonthermal focused ablation technique.However,neurosurgical applications,such as brain tumor ablation,are difficult due to the presence of the skull.Current devices are too large to use in the minimally invasive approaches surgeons prefer.We have developed a combined imaging and histotripsy endoscope to provide neurosurgeons with a new tool for this application.Methods.The histotripsy component had a 10 mm diameter,operating at 6.3 MHz.Affixed within a cutout hole in its center was a 30 MHz ultrasound imaging array.This coregistered pair was used to ablate brain tissue of anesthetized rats while imaging.Histological sections were examined,and qualitative descriptions of ablations and basic shape descriptive statistics were generated.Results.Complete ablations with submillimeter area were produced in seconds,including with a moving device.Ablation progress could be monitored in real time using power Doppler imaging,and B-mode was effective for monitoring post-ablation bleeding.Collateral damage was minimal,with a 100μm maximum distance of cellular damage from the ablation margin.Conclusion.The results demonstrate a promising hardware suite to enable precision ablations in endoscopic procedures or fundamental preclinical research in histotripsy,neuroscience,and cancer.

Particle-Mediated Histotripsy for the Targeted Treatment of Intraluminal Biofilms in Catheter-Based Medical Devices

Objective.This paper is an initial work towards developing particle-mediated histotripsy(PMH)as a novel method of treating catheter-based medical device(CBMD)intraluminal biofilms.Impact Statement.CBMDs commonly become infected with bacterial biofilms leading to medical device failure,infection,and adverse patient outcomes.Introduction.Histotripsy is a noninvasive focused ultrasound ablation method that was recently proposed as a novel method to remove intraluminal biofilms.Here,we explore the potential of combining histotripsy with acoustically active particles to develop a PMH approach that can noninvasively remove biofilms without the need for high acoustic pressures or real-time image guidance for targeting.Methods.Histotripsy cavitation thresholds in catheters containing either gas-filled microbubbles(MBs)or fluid-filled nanocones(NCs)were determined.The ability of these particles to sustain cavitation over multiple ultrasound pulses was tested after a series of histotripsy exposures.Next,the ability of PMH to generate selective intraluminal cavitation without generating extraluminal cavitation was tested.Finally,the biofilm ablation and bactericidal capabilities of PMH were tested using both MBs and NCs.Results.PMH significantly reduced the histotripsy cavitation threshold,allowing for selective luminal cavitation for both MBs and NCs.Results further showed PMH successfully removed intraluminal biofilms in Tygon catheters.Finally,results from bactericidal experiments showed minimal reduction in bacteria viability.Conclusion.The results of this study demonstrate the potential for PMH to provide a new modality for removing bacterial biofilms from CBMDs and suggest that additional work is warranted to develop histotripsy and PMH for treatment of CBMD intraluminal biofilms.