Nanoparticle-mediated histotripsy (NMH) using perfluorohexane ‘nanocones’

Nanoparticle-Mediated Histotripsy Using Dual-Frequency Pulsing Methods

OBJECTIVE

Nanoparticle-mediated histotripsy (NMH) is a novel ablation method that combines nanoparticles as artificial cavitation nuclei with focused ultrasound pulsing to achieve targeted, non-invasive, and cell-selective tumor ablation. The study described here examined the effect of dual-frequency histotripsy pulsing on the cavitation threshold, bubble cloud characteristics, and ablative efficiency in NMH. High-speed optical imaging was used to analyze bubble cloud characteristics and to measure ablation efficiency for NMH inside agarose tissue phantoms containing perfluorohexane-filled nanocone clusters, which were previously developed to reduce the histotripsy cavitation threshold for NMH.

METHODS

Dual-frequency histotripsy pulsing was applied at a 1:1 pressure ratio using a modular 500 kHz and 3 MHz dual-frequency array transducer. Optical imaging results revealed predictable, well-defined bubble clouds generated for all tested cases with similar reductions in the cavitation thresholds observed for single-frequency and dual-frequency pulsing.

RESULTS

Dual-frequency pulsing was seen to nucleate small, dense clouds in agarose phantoms, intermediate in size of their component frequencies but closer in area to that of the higher component frequency. Red blood cell experiments revealed complete ablations were generated by dual-frequency NMH in all phantoms in <1500 pulses. This result was a significant increase in ablation efficiency compared with the ∼4000 pulses required in prior single-frequency NMH studies.

CONCLUSION

Overall, this study indicates the potential for using dual-frequency histotripsy methods to increase the ablation efficacy of NMH.

Investigation of Optimum Production Conditions and the Stability of β-Cyclodextrin-Perfluorocarbon Nanocone Clusters for Histotripsy Applications

Nanocone clusters (NCCs) have been developed as clusters with inclusion complexes of FDA-approved β-cyclodextrin (βCD) and perfluorocarbons (PFC) (i.e., perfluoropentane (PFP) and perfluorohexane (PFH)) and have shown promise in nanoparticle-mediated histotripsy (NMH) applications owing to their lowered cavitation threshold, ease of production, and fluorocarbon quantification. However, there is still a lack of information on the best conditions of the synthesis of NCCs as a product that can have a maximum determinable fluorocarbon content and maintain the stability of the NCC during synthesis and when used as histotripsy agents or exposed to physiological conditions. These concerns about the stability of the clusters and the best possible formulation are investigated in the current work. The cluster formation potential was tested taking into consideration the nature of both PFCs and βCD by employing different synthesis conditions in terms of solution and environmental parameters such as concentration of solvent, stoichiometry between βCD and PFCs, temperature, pH, solvent type, etc. The best route of synthesis was then translated into various batch sizes and investigated in terms of the PFC loading and yield. These studies revealed that preparing NCCs in double-distilled water in an ice bath at the optimized solution concentration gave the highest yields and optimal PFC loading, as determined from gas chromatography. Furthermore, the stability of the clusters with different stoichiometries was scrutinized in varying concentrations, mechanical disruption times, pH levels, and temperature conditions, showing effects on each cluster's particle size in dynamic light scattering, visualized in transmission electron microscopy, and cavitation behavior in agarose gel tissue phantoms. These studies revealed stable clusters for all formulations, with PFH-containing NCCs emerging to be the most stable in terms of their cluster size and bubble formation potential in histotripsy. Finally, the shelf life of these clusters was investigated using DLS, which revealed a stable cluster. In conclusion, NCCs have shown high stability in terms of both synthesis, which can be replicated in gram-level production, and the cluster itself, which can be exposed to harsher conditions and still form stable bubbles in histotripsy.

Probability of Cavitation in a Custom Iron-Based Coupling Medium for Transcranial Magnetic Resonance-Guided Focused Ultrasound Procedures

OBJECTIVE

A coupling bath of circulating, chilled, degassed water is essential to safe and precise acoustic transmittance during transcranial magnetic resonance-guided focused ultrasound (tMRgFUS) procedures, but the circulating water impairs the critical real-time magnetic resonance imaging (MRI). An iron-based coupling medium (IBCM) using iron oxide nanoparticles previously developed by our group increased the relaxivity of the coupling bath such that it appears to be invisible on MRI compared with degassed water. However, the nanoparticles also reduced the pressure threshold for cavitation. To address this concern for prefocal cavitation, our group recently developed an IBCM of electrosterically stabilized and aggregation-resistant poly(methacrylic acid)-coated iron oxide nanoparticles (PMAA-FeOX) with a similar capability to reduce the MR signal of degassed water. This study examines the effect of the PMAA-FeOX IBCM on the cavitation threshold.

METHODS

Increasing concentrations of PMAA-FeOX nanoparticles in degassed, deionized water were placed at the focus of two different transducers to assess low and high duty-cycle pulsing parameters which are representative of two modes of focused ultrasound being investigated for tMRgFUS. Passive cavitation detection and high-speed optical imaging were used to measure cavitation threshold pressures.

RESULTS

The mean cavitation threshold was determined in both cases to be indistinguishable from the degassed water control, between 6-8 MPa for high duty-cycle pulsing (CW) and between 25.5-26.5 MPa for very low duty-cycle pulsing.

CONCLUSION

The findings of this study indicate that an IBCM of PMAA-FeOX nanoparticles is a possible solution to reducing MRI interference from the coupling bath without increasing the risk of prefocal cavitation.

Locoregional Therapies and Remodeling of Tumor Microenvironment in Pancreatic Cancer

Despite the advances made in treatment, the prognosis of pancreatic ductal adenocarcinoma (PDAC) remains dismal, even in the locoregional and locally advanced stages, with high relapse rates after surgery. PDAC exhibits a chemoresistant and immunosuppressive phenotype, and the tumor microenvironment (TME) surrounding cancer cells actively participates in creating a stromal barrier to chemotherapy and an immunosuppressive environment. Recently, there has been an increasing use of interventional radiology techniques for the treatment of PDAC, although they do not represent a standard of care and are not included in clinical guidelines. Local approaches such as radiation therapy, hyperthermia, microwave or radiofrequency ablation, irreversible electroporation and high-intensity focused ultrasound exert their action on the tumor tissue, altering the composition and structure of TME and potentially enhancing the action of chemotherapy. Moreover, their action can increase antigen release and presentation with T-cell activation and reduction tumor-induced immune suppression. This review summarizes the current evidence on locoregional therapies in PDAC and their effect on remodeling TME to make it more susceptible to the action of antitumor agents.

Catheter-Based Medical Device Biofilm Ablation Using Histotripsy: A Parameter Study

OBJECTIVE

Biofilm formation in medical catheters is a major source of hospital-acquired infections which can produce increased morbidity and mortality for patients. Histotripsy is a non-invasive, non-thermal focused ultrasound therapy and recently has been found to be effective at removal of biofilm from medical catheters. Previously established histotripsy methods for biofilm removal, however, would require several hours of use to effectively treat a full-length medical catheter. Here, we investigate the potential to increase the speed and efficiency with which biofilms can be ablated from catheters using histotripsy.

METHODS

Pseudomonas aeruginosa (PA14) biofilms were cultured in in vitro Tygon catheter mimics and treated with histotripsy using a 1 MHz histotripsy transducer and a variety of histotripsy pulsing rates and scanning methods. The improved parameters identified in these studies were then used to explore the bactericidal effect of histotripsy on planktonic PA14 suspended in a catheter mimic.

RESULTS

Histotripsy can be used to remove biofilm and kill bacteria at substantially increased speeds compared with previously established methods. Near-complete biofilm removal was achieved at treatment speeds up to 1 cm/s, while a 4.241 log reduction in planktonic bacteria was achieved with 2.4 cm/min treatment.

CONCLUSION

These results represent a 500-fold increase in biofilm removal speeds and a 6.2-fold increase in bacterial killing speeds compared with previously published methods. These findings indicate that histotripsy shows promise for the treatment of catheter-associated biofilms and planktonic bacteria in a clinically relevant time frame.

The histotripsy spectrum: differences and similarities in techniques and instrumentation

Since its inception about two decades ago, histotripsy - a non-thermal mechanical tissue ablation technique - has evolved into a spectrum of methods, each with distinct potentiating physical mechanisms: intrinsic threshold histotripsy, shock-scattering histotripsy, hybrid histotripsy, and boiling histotripsy. All methods utilize short, high-amplitude pulses of focused ultrasound delivered at a low duty cycle, and all involve excitation of violent bubble activity and acoustic streaming at the focus to fractionate tissue down to the subcellular level. The main differences are in pulse duration, which spans microseconds to milliseconds, and ultrasound waveform shape and corresponding peak acoustic pressures required to achieve the desired type of bubble activity. In addition, most types of histotripsy rely on the presence of high-amplitude shocks that develop in the pressure profile at the focus due to nonlinear propagation effects. Those requirements, in turn, dictate aspects of the instrument design, both in terms of driving electronics, transducer dimensions and intensity limitations at surface, shape (primarily, the F-number) and frequency. The combination of the optimized instrumentation and the bio-effects from bubble activity and streaming on different tissues, lead to target clinical applications for each histotripsy method. Here, the differences and similarities in the physical mechanisms and resulting bioeffects of each method are reviewed and tied to optimal instrumentation and clinical applications.

Focused ultrasound-mediated small-molecule delivery to potentiate immune checkpoint blockade in solid tumors

In the last decade, immune checkpoint blockade (ICB) has revolutionized the standard of treatment for solid tumors. Despite success in several immunogenic tumor types evidenced by improved survival, ICB remains largely unresponsive, especially in "cold tumors" with poor lymphocyte infiltration. In addition, side effects such as immune-related adverse events (irAEs) are also obstacles for the clinical translation of ICB. Recent studies have shown that focused ultrasound (FUS), a non-invasive technology proven to be effective and safe for tumor treatment in clinical settings, could boost the therapeutic effect of ICB while alleviating the potential side effects. Most importantly, the application of FUS to ultrasound-sensitive small particles, such as microbubbles (MBs) or nanoparticles (NPs), allows for precise delivery and release of genetic materials, catalysts and chemotherapeutic agents to tumor sites, thus enhancing the anti-tumor effects of ICB while minimizing toxicity. In this review, we provide an updated overview of the progress made in recent years concerning ICB therapy assisted by FUS-controlled small-molecule delivery systems. We highlight the value of different FUS-augmented small-molecules delivery systems to ICB and describe the synergetic effects and underlying mechanisms of these combination strategies. Furthermore, we discuss the limitations of the current strategies and the possible ways that FUS-mediated small-molecule delivery systems could boost novel personalized ICB treatments for solid tumors.

Cellulose Nanofiber-Coated Perfluoropentane Droplets: Fabrication and Biocompatibility Study

Purpose

To study the effect of cellulose nanofiber (CNF)-shelled perfluoropentane (PFP) droplets on the cell viability of 4T1 breast cancer cells with or without the addition of non-encapsulated paclitaxel.

Methods

The CNF-shelled PFP droplets were produced by mixing a CNF suspension and PFP using a homogenizer. The volume size distribution and concentration of CNF-shelled PFP droplets were estimated from images taken with an optical microscope and analyzed using Fiji software and an in-house Matlab script. The thermal stability was qualitatively assessed by comparing the size distribution and concentration of CNF-shelled PFP droplets at room temperature (~22°) and 37°C. The cell viability of 4T1 cells was measured using a 3-[4,5-dimethylthiazol-2yl]-2,5-diphenyltetrazolium bromide (MTT) assay. Additionally, a hemolysis assay was performed to assess blood compatibility of CNF-shelled PFP droplets.

Results

The droplet diameter and concentration of CNF-shelled PFP droplets decreased after 48 hours at both room temperature and 37°C. In addition, the decrease in concentration was more significant at 37°C, from 3.50 ± 0.64×10⁶ droplets/mL to 1.94 ± 0.10×10⁶ droplets/mL, than at room temperature, from 3.65 ± 0.29×10⁶ droplets/mL to 2.56 ± 0.22×10⁶ droplets/mL. The 4T1 cell viability decreased with increased exposure time and concentration of paclitaxel, but it was not affected by the presence of CNF-shelled PFP droplets. No hemolysis was observed at any concentration of CNF-shelled PFP droplets.

Conclusion

CNF-shelled PFP droplets have the potential to be applied as drug carriers in ultrasound-mediated therapy.

Ultrasound-guided noninvasive pancreas ablation using histotripsy: feasibility study in an in vivo porcine model

Pancreatic cancer is a malignant disease associated with poor survival and nearly 80% present with unresectable tumors. Treatments such as chemotherapy and radiation therapy have shown overall improved survival benefits, albeit limited. Histotripsy is a noninvasive, non-ionizing, and non-thermal focused ultrasound ablation modality that has shown efficacy in treating hepatic tumors and other malignancies. In this novel study, we investigate histotripsy for noninvasive pancreas ablation in a pig model. In two studies, histotripsy was applied to the healthy pancreas in 11 pigs using a custom 32-element, 500 kHz histotripsy transducer attached to a clinical histotripsy system, with treatments guided by real-time ultrasound imaging. A pilot study was conducted in 3 fasted pigs with histotripsy applied at a pulse repetition frequency (PRF) of 500 Hz. Results showed no pancreas visualization on coaxial ultrasound imaging due to overlying intestinal gas, resulting in off-target injury and no pancreas damage. To minimize gas, a second group of pigs (n = 8) were fed a custard diet containing simethicone and bisacodyl. Pigs were euthanized immediately (n = 4) or survived for 1 week (n = 4) post-treatment. Damage to the pancreas and surrounding tissue was characterized using gross morphology, histological analysis, and CT imaging. Results showed histotripsy bubble clouds were generated inside pancreases that were visually maintained on coaxial ultrasound (n = 4), with 2 pigs exhibiting off-target damage. For chronic animals, results showed the treatments were well-tolerated with no complication signs or changes in blood markers. This study provides initial evidence suggesting histotripsy's potential for noninvasive pancreas ablation and warrants further evaluation in more comprehensive studies.

Bioconjugated β-Cyclodextrin-Perfluorohexane Nanocone Clusters as Functional Nanoparticles for Nanoparticle-Mediated Histotripsy

Nanocone clusters (NCCs) are new-generation agents of nanoparticle-mediated histotripsy (NMH) recently developed to address the limitations of previously designed nanodroplets (NDs). NCCs can be obtained by simply mixing FDA-approved cyclodextrins (CD) and suitable perfluorocarbons (PFCs), which result in smaller size aggregates, detectable PFC amount, and more stable long-term storage since the obtained powder can be stored and redispersed as needed. Previous experimental and computational studies showed that NCCs consist of an organization of inclusion complexes of CD and PFC around free PFC droplets, and their aggregate behavior depends on the localization of PFC in the cavity and the water solubility of CD derivatives. It has been shown that β-cyclodextrin (βCD) and perfluorohexane (PFH) are ideal candidates for NCCs that can be isolated as a powder with high PFC content among various CD and PFC derivatives. This study focuses on the further development of the selected NCC composition to enhance the potential of NMH therapy while also enabling more detailed future experiments in vitro and in vivo. It is aimed to show the bioconjugation potential of NCCs through the example of the most commonly used functionalization methods such as targeting, PEGylation, and fluorescent labeling. For this purpose, βCD as a building block was monofunctionalized with groups such as azide, alkyne, and amine groups that allow for effective coupling reactions such as the "click" reaction and N-(3-dimethylaminopropyl)-N'-ethylcarbodiimide hydrochloride/N-hydroxysuccinimide (EDC/NHS) coupling. These monofunctional βCDs were used as building blocks of NCCs in the presence of PFH to obtain functional NCCs as precursors of bioconjugation. EPPT1 as a synthetic peptide specific to uMUC1 and folic acid (FA) as the most commonly used targeting agent along with PEGylation were successfully shown as bioconjugation examples. Lastly, fluorescently labeled NCCs were obtained via fluorescein isothiocyanate (FITC) and alkyne functional NCC reaction through propargyl amine and isothiocyanate group reaction. The obtained bioconjugates were tested in vitro to validate the conjugation, and the ability to lower the histotripsy cavitation threshold, which is necessary for NMH, was demonstrated for all bioconjugates. Overall, the results showed that all obtained bioconjugates successfully lowered the cavitation threshold pressure while also fulfilling the desired bioconjugation metrics to serve as improved tools to enhance NMH as a targeted noninvasive ablation method.

Experimental and Computational Investigation of Clustering Behavior of Cyclodextrin-Perfluorocarbon Inclusion Complexes as Effective Histotripsy Agents

Recently developed nanocones (NCs), which are inclusion complexes that are made up of cyclodextrins (CDs) and perfluorocarbons (PFCs), have shown promising results in nanoparticle-mediated histotripsy (NMH) applications due to stable inclusion complexation, PFC quantification, simple synthesis, and processing. FDA-approved βCD and its modified versions such as low-degree methylated βCD have been previously demonstrated as prime examples of structures capable of accommodating PFC molecules. However, the complex formation potential of different CDs with various cavity sizes in the presence of PFC molecules, and their consequent aggregation, needs to be explored. In the present study, the complexation and aggregation potential of some natural CDs and their respective derivatives either exposed to perfluoropentane (PFP) or perfluorohexane (PFH) were studied in the wet lab. Computational studies were also performed to account for the limitations faced in PFC quantification because of the low optical density of PFCs within the CD complex and to discover the best candidate for NMH applications. All results revealed that only βCD and γCD (except HMγCD) derivatives form an inclusion complex with PFCs and only LMβCD, βCD, and γCD form nanocone clusters (NCCs), which precipitate and can be collected for use. Furthermore, the data collectively show that βCD and PFCs have the best complexation due to stable complex formation, ease of production, and product recovery, especially with PFH as a more suitable candidate due to its high boiling point, which allows workability during synthesis. Although simulations suggest that highly stable inclusion complexes exist, such as HPβCD, the cluster formation resulting in precipitation is hindered due to the high solubility of CDs in water, resulting in intangible yields to work with even after employing general laboratory recovery methods. Conclusively, histotripsy cavitation experiments successfully showed a decreased cavitation threshold among optimal NCC candidates that were identified, supporting their use in NMH.

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.

Histotripsy Ablation in Preclinical Animal Models of Cancer and Spontaneous Tumors in Veterinary Patients: A Review

New therapeutic strategies are direly needed in the fight against cancer. Over the last decade, several tumor ablation strategies have emerged as stand-alone or combination therapies. Histotripsy is the first completely noninvasive, nonthermal, and nonionizing tumor ablation method. Histotripsy can produce consistent and rapid ablations, even near critical structures. Additional benefits include real-time image guidance, high precision, and the ability to treat tumors of any predetermined size and shape. Unfortunately, the lack of clinically and physiologically relevant preclinical cancer models is often a significant limitation with all focal tumor ablation strategies. The majority of studies testing histotripsy for cancer treatment have focused on small animal models, which have been critical in moving this field forward and will continue to be essential for providing mechanistic insight. While these small animal models have notable translational value, there are significant limitations in terms of scale and anatomical relevance. To address these limitations, a diverse range of large animal models and spontaneous tumor studies in veterinary patients have emerged to complement existing rodent models. These models and veterinary patients are excellent at providing realistic avenues for developing and testing histotripsy devices and techniques designed for future use in human patients. Here, we provide a review of animal models used in preclinical histotripsy studies and compare histotripsy ablation in these models using a series of original case reports across a broad spectrum of preclinical animal models and spontaneous tumors in veterinary patients.

Focused Ultrasound Biofilm Ablation: Investigation of Histotripsy for the Treatment of Catheter-Associated Urinary Tract Infections (CAUTIs)

Urinary catheters often become contaminated with biofilms, resulting in catheter-associated urinary tract infections (CAUTIs) that adversely affect patient outcomes. Histotripsy is a noninvasive focused ultrasound therapy previously developed for the noninvasive ablation of cancerous tumors and soft tissues. Histotripsy has also previously shown the ability to treat biofilms on glass slides and surgical meshes. Here, we investigate the potential of histotripsy for the treatment of CAUTIs for the first time in vitro. Clinically relevant catheter materials (Tygon, Silicone, and latex catheter mimics) and commonly used clinical catheters were tested to determine the feasibility of producing luminal histotripsy bubble clouds. A Pseudomonas aeruginosa (strain PA14) biofilm model was developed and tested to produce luminal biofilms in an in vitro Tygon catheter mimic. This model was treated with histotripsy to determine the ability to remove a luminal biofilm. Finally, the bactericidal effects of histotripsy were tested by treating PA14 suspended inside the Tygon catheter mimic. Results showed that histotripsy produced precise luminal cavitation within all tested catheter mimics and clinical catheters. Histotripsy treatment of a PA14 biofilm with histotripsy reduced luminal biofilm OD590 signal down to background levels. Further, the treatment of suspended PA14 in Luria-Bertani (LB) showed a 3.45 ± 0.11 log₁₀ reduction in CFU/mL after six histotripsy scans across the catheter mimics. Overall, the results of this study demonstrate the potential of histotripsy to provide a new modality for removing bacterial biofilms from catheter-based medical devices and suggest that additional work is warranted to investigate histotripsy for the treatment of CAUTIs and other biomaterial-associated infections.

High-Intensity Focused Ultrasound: A Review of Mechanisms and Clinical Applications

High Intensity Focused Ultrasound (HIFU) is an emerging and increasingly useful modality in the treatment of cancer and other diseases. Although traditional use of ultrasound at lower frequencies has primarily been for diagnostic imaging purposes, the development of HIFU has allowed this particular modality to expand into therapeutic use. This non-invasive and acoustic method involves the use of a piezoelectric transducer to deliver high-energy pulses in a spatially coordinated manner, while minimizing damage to tissue outside the target area. This review describes the history of the development of diagnostic and therapeutic ultrasound and explores the biomedical applications utilizing HIFU technology including thermally ablative treatment, therapeutic delivery mechanisms, and neuromodulatory phenomena. The application of HIFU across various tumor types in multiple organ systems is explored in depth, with particular attention to successful models of HIFU in the treatment of various medical conditions. Basic mechanisms, preclinical models, previous clinical use, and ongoing clinical trials are comparatively discussed. Recent advances in HIFU across multiple medical fields reveal the growing importance of this biomedical technology for the care of patients and for the development of possible pathways for the future use of HIFU as a commonplace treatment modality.

Investigation of the Acoustic Vaporization Threshold of Lipid-Coated Perfluorobutane Nanodroplets Using Both High-Speed Optical Imaging and Acoustic Methods

A combination of ultrahigh-speed optical imaging (5 × 10⁶ frames/s), B-mode ultrasound and passive cavitation detection was used to study the vaporization process and determine both the acoustic droplet vaporization (ADV) and inertial cavitation (IC) thresholds of phospholipid-coated perfluorobutane nanodroplets (PFB NDs, diameter = 237 ± 16 nm). PFB NDs have not previously been studied with ultrahigh-speed imaging and were observed to form individual microbubbles (1-10 μm) within two to three cycles and subsequently larger bubble clusters (10-50 μm). The ADV and IC thresholds did not statistically significantly differ and decreased with increasing pulse length (20-20,000 cycles), pulse repetition frequency (1-100 Hz), concentration (10⁸-10¹⁰ NDs/mL), temperature (20°C-45°C) and decreasing frequency (1.5-0.5 MHz). Overall, the results indicate that at frequencies of 0.5, 1.0 and 1.5 MHz, PFB NDs can be vaporized at moderate peak negative pressures (<2.0 MPa), pulse lengths and pulse repetition frequencies. This finding is encouraging for the use of PFB NDs as cavitation agents, as these conditions are comparable to those required to achieve therapeutic effects with microbubbles, unlike those reported for higher-boiling-point NDs. The differences between the optically and acoustically determined ADV thresholds, however, suggest that application-specific thresholds should be defined according to the biological/therapeutic effect of interest.

Focused ultrasound tumour ablation in small animal oncology

The cancer incidence rates for humans and animals remain high, and efforts to improve cancer treatment are crucial. Cancer treatment for solid tumours includes both treatment of the primary tumour and of metastasis. Surgery is commonly employed to resect primary and metastatic tumours, but is invasive, and is not always the optimal treatment modality. Prevention and treatment of metastatic disease often utilizes a multimodal approach, but metastasis remains a major cause of death for both human and veterinary cancer patients. Focused ultrasound (FUS) tumour ablation techniques represent a novel non-invasive approach to treating cancer. FUS ablation is precise, thus sparing adjacent critical structures while ablating the tumour. FUS ablation can occur in a thermal or non-thermal fashion. Thermal FUS ablation, also known as high intensity focused ultrasound (HIFU) ablation, destroys tumour cells via heat, whereas non-thermal FUS, known as histotripsy, ablates tumour cells via mechanical disintegration of tissue. Not only can HIFU and histotripsy ablate tumours, they also demonstrate potential to upregulate the host immune system towards an anti-tumour response. The aim of this report is provide a description of HIFU and histotripsy tumour ablation, with a focus on the basic principles of their ablation mechanisms and their clinical applicability in the field of veterinary oncology.

Bubble Cloud Behavior and Ablation Capacity for Histotripsy Generated from Intrinsic or Artificial Cavitation Nuclei

The study described here examined the effects of cavitation nuclei characteristics on histotripsy. High-speed optical imaging was used to compare bubble cloud behavior and ablation capacity for histotripsy generated from intrinsic and artificial cavitation nuclei (gas-filled microbubbles, fluid-filled nanocones). Results showed a significant decrease in the cavitation threshold for microbubbles and nanocones compared with intrinsic-nuclei controls, with predictable and well-defined bubble clouds generated in all cases. Red blood cell experiments showed complete ablations for intrinsic and nanocone phantoms, but only partial ablation in microbubble phantoms. Results also revealed a lower rate of ablation in artificial-nuclei phantoms because of reduced bubble expansion (and corresponding decreases in stress and strain). Overall, this study demonstrates the potential of using artificial nuclei to reduce the histotripsy cavitation threshold while highlighting differences in the bubble cloud behavior and ablation capacity that need to be considered in the future development of these approaches.

Review on Acoustic Droplet Vaporization in Ultrasound Diagnostics and Therapeutics

Acoustic droplet vaporization (ADV) is the physical process in which liquid undergoes phase transition to gas after exposure to a pressure amplitude above a certain threshold. In recent years, new techniques in ultrasound diagnostics and therapeutics have been developed which utilize microformulations with various physical and chemical properties. The purpose of this review is to give the reader a general idea on how ADV can be implemented for the existing biomedical applications of droplet vaporization. In this regard, the recent developments in ultrasound therapy which shed light on the ADV are considered. Modern designs of capsules and nanodroplets (NDs) are shown, and the material choices and their implications for function are discussed. The influence of the physical properties of the induced acoustic field, the surrounding medium, and thermophysical effects on the vaporization are presented. Lastly, current challenges and potential future applications towards the implementation of the therapeutic droplets are discussed.