In vivotranscostal histotripsy therapy without aberration correction

Ultrasonic Nakagami imaging for automatically positioning and identifying the treated lesion induced by histotripsy

Histotripsy has been proposed as a non-invasive surgical procedure for clinical use that liquefies the tissue into acellular debris by utilizing the mechanical mechanism of bubbles. Accurate and reliable imaging guidance is essential for successful clinical histotripsy implementation. Nakagami imaging is a promising method to evaluate the microstructural change induced by high intensity focused ultrasound. However, practically, it is difficult for the Nakagami imaging to distinguish the treated lesion induced by histotripsy from the surrounding normal biological tissues. In this study, we introduce the use of noise-assisted correlation algorithm (NCA) in Nakagami images as a solution to suppress the background normal tissue and identify the treated lesion induced by histotripsy. Experiments are conducted on fresh porcine liver ex vivo by cavitation-cloud histotripsy. Results show that the contrast-to-noise ratio between the treated lesion and surrounding tissue corresponding to the Nakagami image after NCA and original Nakagami image is 3.434 and 0.505, respectively. The optimal artificial noise level is 1-fold of the background normal tissue amplitude, and the corresponding optimal threshold of correlation coefficient should be between 0.6 and 0.8 in the application of NCA. Therefore, the use of NCA in Nakagami image can suppress the background normal tissues without affecting the information of treated lesion for an appropriate artificial noise level and threshold used in the NCA. Moreover, the Nakagami images after the application of the NCA can also be used for automatically distinguishing and measuring the tissue fractionation accurately using binarization. The proposed Nakagami images overlaid on the B-mode images can provide a promising method for positioning and visualizing the treated lesion to achieve precise histotripsy treatment.

Insights from in vivo preclinical cancer studies with histotripsy

Histotripsy is the first noninvasive, non-ionizing, and non-thermal ablation technique that mechanically fractionates target tissue into acellular homogenate via controlled acoustic cavitation. Histotripsy has been evaluated for various preclinical applications requiring noninvasive tissue removal including cancer, brain surgery, blood clot and hematoma liquefaction, and correction of neonatal congenital heart defects. Promising preclinical results including local tumor suppression, improved survival outcomes, local and systemic anti-tumor immune responses, and histotripsy-induced abscopal effects have been reported in various animal tumor models. Histotripsy is also being investigated in veterinary patients with spontaneously arising tumors. Research is underway to combine histotripsy with immunotherapy and chemotherapy to improve therapeutic outcomes. In addition to preclinical cancer research, human clinical trials are ongoing for the treatment of liver tumors and renal tumors. Histotripsy has been recently approved by the FDA for noninvasive treatment of liver tumors. This review highlights key learnings from in vivo shock-scattering histotripsy, intrinsic threshold histotripsy, and boiling histotripsy cancer studies treating cancers of different anatomic locations and discusses the major considerations in planning in vivo histotripsy studies regarding instrumentation, tumor model, study design, treatment dose, and post-treatment tumor monitoring.

Non-Thermal Liver Ablation: Existing and New Technology

Cancer has and continues to be a complex health crisis plaguing millions around the world. Alcohol ablation was one of the initial methods used for the treatment of liver lesions. It was surpassed by thermal ablation which has played a big role in the therapeutic arsenal for primary and metastatic liver tumors. However, thermal ablation has several shortcomings and limitations that prompted the development of alternative technologies including electroporation and histotripsy. Percutaneous alcohol injection in the liver lesion leads to dehydration and coagulative necrosis. This technology is limited to the lesion with relative sparing of the surrounding tissue, making it safe to use adjacent to sensitive structures. Electroporation utilizes short high-voltage pulses to permeabilize the cell membrane and can result in cell death dependent on the threshold reached. It can effectively target the tumor margins and has lower damage rates to surrounding structures due to the short pulse duration. Histotripsy is a novel technology, and although the first human trial was just completed, its results are encouraging, given the sharp demarcation of the targeted tissue, lack of thermal damage, and potential for immunomodulation of the tumor microenvironment. Herein, we discuss these techniques, their uses, and overall clinical benefit.

Clinical translation of abdominal histotripsy: a review of preclinical studies in large animal models

Histotripsy is an emerging noninvasive, non-thermal, and non-ionizing focused ultrasound (US) therapy that can be used to destroy targeted tissue. Histotripsy has evolved from early laboratory prototypes to clinical systems which have been comprehensively evaluated in the preclinical environment to ensure safe translation to human use. This review summarizes the observations and results from preclinical histotripsy studies in the liver, kidney, and pancreas. Key findings from these studies include the ability to make a clinically relevant treatment zone in each organ with maintained collagenous architecture, potentially allowing treatments in areas not currently amenable to thermal ablation. Treatments across organ capsules have proven safe, including in anticoagulated models which may expand patients eligible for treatment or eliminate the risk associated with taking patients off anti-coagulation. Treatment zones are well-defined with imaging and rapidly resorb, which may allow improved evaluation of treatment zones for residual or recurrent tumor. Understanding the effects of histotripsy in animal models will help inform physicians adopting histotripsy for human clinical use.

Aberration correction in abdominal histotripsy

In transabdominal histotripsy, ultrasound pulses are focused on the body to noninvasively destroy soft tissues via cavitation. However, the ability to focus is limited by phase aberration, or decorrelation of the ultrasound pulses due to spatial variation in the speed of sound throughout heterogeneous tissue. Phase aberration shifts, broadens, and weakens the focus, thereby reducing the safety and efficacy of histotripsy therapy. This paper reviews and discusses aberration effects in histotripsy and in related therapeutic ultrasound techniques (e.g., high intensity focused ultrasound), with an emphasis on aberration by soft tissues. Methods for aberration correction are reviewed and can be classified into two groups: model-based methods, which use segmented images of the tissue as input to an acoustic propagation model to predict and compensate phase differences, and signal-based methods, which use a receive-capable therapy array to detect phase differences by sensing acoustic signals backpropagating from the focus. The relative advantages and disadvantages of both groups of methods are discussed. Importantly, model-based methods can correct focal shift, while signal-based methods can restore substantial focal pressure, suggesting that both methods should be combined in a 2-step approach. Aberration correction will be critical to improving histotripsy treatments and expanding the histotripsy treatment envelope to enable non-invasive, non-thermal histotripsy therapy for more patients.

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.

Histotripsy ablation for the treatment of feline injection site sarcomas: a first-in-cat in vivo feasibility study

PURPOSE

Feline soft tissue sarcoma (STS) and injection site sarcoma (fISS) are rapidly growing tumors with low metastatic potential, but locally aggressive behavior. Histotripsy is a non-invasive focused ultrasound therapy using controlled acoustic cavitation to mechanically disintegrate tissue. In this study, we investigated the in vivo safety and feasibility of histotripsy to treat fISS using a custom 1 MHz transducer.

MATERIALS AND METHODS

Three cats with naturally-occurring STS were treated with histotripsy before surgical removal of the tumor 3 to 6 days later. Gross and histological analyses were used to characterize the ablation efficacy of the treatment, and routine immunohistochemistry and batched cytokine analysis were used to investigate the acute immunological effects of histotripsy.

RESULTS

Results showed that histotripsy ablation was achievable and well-tolerated in all three cats. Precise cavitation bubble clouds were generated in all patients, and hematoxylin & eosin stained tissues revealed ablative damage in targeted regions. Immunohistochemical results identified an increase in IBA-1 positive cells in treated tissues, and no significant changes in cytokine concentrations were identified post-treatment.

CONCLUSIONS

Overall, the results of this study demonstrate the safety and feasibility of histotripsy to target and ablate superficial feline STS and fISS tumors and guide the clinical development of histotripsy devices for this application.

A Multi-centre, Single Arm, Non-randomized, Prospective European Trial to Evaluate the Safety and Efficacy of the HistoSonics System in the Treatment of Primary and Metastatic Liver Cancers (#HOPE4LIVER)

PURPOSE

Image-guided thermal ablation are established treatment options for non-surgical patients with primary and metastatic liver cancers. However, there are limitations with nonuniformity of cancer tissue destruction, heat sink effect and the risk of thermal ablative injury. The current non-thermal ablative techniques have high risk of local recurrence and are not widely adopted. Histotripsy is a treatment technology that destroys targeted tissue under ultrasound visualization via mechanical destruction through the precise application of acoustic cavitation and can offer the potential of non-invasive, non-thermal and non-ionizing radiation cancer treatment. The aim of this multi-centre non-randomized phase I/II trial is to assess the initial safety and efficacy of the prototype investigational 'System' in the treatment of primary and metastatic liver cancers.

METHODS/DESIGN

All non-surgical patients with primary/metastatic liver cancers having had previous liver directed therapy, radiation therapy or image-guided ablation may be offered image-guided Histotripsy as per trial protocol. The co-primary endpoints are technical success and procedural safety. Technical success is determined, at ≤ 36 h post procedure, by evaluating the histotripsy treatment size and coverage. The procedural safety is defined by procedure related major complications, defined as Common Terminology Criteria for Adverse Events (CTCAE version 5) grade 3 or higher toxicities, up to 30 days post procedure. This phase I/II trial has intended to recruit up to 45 patients to show safety and efficacy of image-guided histotripsy in liver cancers.

TRAIL REGISTRATION

Clinicaltrials.gov identifier-NCT04573881; NIHR CRN CPMS-ID 47572.

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.

The role of anti-tumor immunity of focused ultrasound for the malignancies: depended on the different ablation categories

Improving anti-tumor immunity has promising outcomes in eradicating malignant tumors. Tumor cells can escape from immune surveillance and killing; therefore, various strategies are continuously developing to inhibit immune escape. Focused ultrasound (FUS) has recently emerged to play an important role in immune modulation. After FUS therapy, various tumor antigens and related signals are released. The non-thermal effect of FUS strengthens the blood and lymph circulation, increases cell permeability, and helps in crossing the physical barrier like the blood-brain barrier and blood-tumor barrier. However, the different ablation of FUS is proposed to have a different anti-tumor immune effect. Therefore, we categorized the FUS ablation into thermal and non-thermal ablation and summarized possible anti-tumor immunity mechanisms.

Effects of phase aberration on transabdominal focusing for a large aperture, lowf-number histotripsy transducer

Objective. Soft tissue phase aberration may be particularly severe for histotripsy due to large aperture and lowf-number transducer geometries. This study investigated how phase aberration from human abdominal tissue affects focusing of a large, strongly curved histotripsy transducer.Approach.A computational model (k-Wave) was experimentally validated withex vivoporcine abdominal tissue and used to simulate focusing a histotripsy transducer (radius: 14.2 cm,f-number: 0.62, central frequencyfc: 750 kHz) through the human abdomen. Abdominal computed tomography images from 10 human subjects were segmented to create three-dimensional acoustic property maps. Simulations were performed focusing at 3 target locations in the liver of each subject with ideal phase correction, without phase correction, and after separately matching the sound speed of water and fat to non-fat soft tissue.Main results.Experimental validation in porcine abdominal tissue showed that simulated and measured arrival time differences agreed well (average error, ∼0.10 acoustic cycles atfc). In simulations with human tissue, aberration created arrival time differences of 0.65μs (∼0.5 cycles) at the target and shifted the focus from the target by 6.8 mm (6.4 mm pre-focally along depth direction), on average. Ideal phase correction increased maximum pressure amplitude by 95%, on average. Matching the sound speed of water and fat to non-fat soft tissue decreased the average pre-focal shift by 3.6 and 0.5 mm and increased pressure amplitude by 2% and 69%, respectively.Significance.Soft tissue phase aberration of large aperture, lowf-number histotripsy transducers is substantial despite low therapeutic frequencies. Phase correction could potentially recover substantial pressure amplitude for transabdominal histotripsy. Additionally, different heterogeneity sources distinctly affect focusing quality. The water path strongly affects the focal shift, while irregular tissue boundaries (e.g. fat) dominate pressure loss.

Histotripsy Ablation of Spontaneously Occurring Canine Bone Tumors In Vivo

OBJECTIVE

Osteosarcoma (OS) is a devastating primary bone tumor in dogs and humans with limited non-surgical treatment options. As the first completely non-invasive and non-thermal ablation technique, histotripsy has the potential to significantly improve the standard of care for patients with primary bone tumors.

INTRODUCTION

Standard of care treatment for primary appendicular OS involves surgical resection via either limb amputation or limb-salvage surgery for suitable candidates. Biological similarities between canine and human OS make the dog an informative comparative oncology research model to advance treatment options for primary OS. Evaluating histotripsy for ablating spontaneous canine primary OS will build a foundation upon which histotripsy can be translated clinically into a standard of care therapy for canine and human OS.

METHODS

Five dogs with suspected spontaneous OS were treated with a 500 kHz histotripsy system guided by real-time ultrasound image guidance. Spherical ablation volumes within each tumor (1.25-3 cm in diameter) were treated with single cycle histotripsy pulses applied at a pulse repetition frequency of 500 Hz and a dose of 500 pulses/point.

RESULTS

Tumor ablation was successfully identified grossly and histologically within the targeted treatment regions of all subjects. Histotripsy treatments were well-tolerated amongst all patients with no significant clinical adverse effects. Conclusion & Significance: Histotripsy safely and effectively ablated the targeted treatment volumes in all subjects, demonstrating its potential to serve as a non-invasive treatment modality for primary bone tumors.

Nanodroplet-Mediated Low-Energy Mechanical Ultrasound Surgery

Mechanical ultrasound surgery methods use short, high-intensity pulses to fractionate tissues. This study reports the development of a two-step technology for low-energy mechanical ultrasound surgery of tissues using nanodroplets to reduce the pressure threshold. Step 1 consists of vaporizing the nanodroplets into gaseous microbubbles via megahertz ultrasound excitation. Then, low-frequency ultrasound is applied to the microbubbles, which turns them into therapeutic warheads that trigger potent mechanical effects in the surrounding tissue. The use of nanoscale nanodroplets coupled with low-frequency ultrasound reduces the pressure threshold required for mechanical ultrasound surgery by an order of magnitude. In addition, their average diameter of 300 nm can overcome challenges associated with the size of microbubbles. Optimization experiments were performed to determine the ultrasound parameters for nanodroplet vaporization and the subsequent microbubble implosion processes. Optimal vaporization was obtained when transmitting a 2-cycle excitation pulse at a center frequency of 5 MHz and a peak negative pressure of 4.1 MPa (mechanical index = 1.8). Low-frequency insonation of the generated microbubbles at a center frequency of 850, 250 or 80 kHz caused enhanced contrast reduction at a center frequency of 80 kHz, compared with the other frequencies, while operating at the same mechanical index of 0.9. Nanodroplet-mediated insonation of ex vivo chicken liver samples generated mechanical damage. Low-frequency treatment at a mechanical index of 0.9 and a center frequency of 80 kHz induced the largest lesion area (average of 0.59 mm²) compared with 250- and 850-kHz treatments with the same mechanical index (average lesions areas of 0.29 and 0.19 mm², respectively, p < 0.001). The two-step approach makes it possible to conduct both the vaporization and implosion stages at mechanical indices below 1.9, thus avoiding undesired mechanical damage. The findings indicate that coupled with low-frequency ultrasound, nanodroplets can be used for low-energy mechanical ultrasound surgery.

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.

Histotripsy Ablation of Bone Tumors: Feasibility Study in Excised Canine Osteosarcoma Tumors

Osteosarcoma (OS) is a primary bone tumor affecting both dogs and humans. Histotripsy is a non-thermal, non-invasive focused ultrasound method using controlled acoustic cavitation to mechanically disintegrate tissue. In this study, we investigated the feasibility of treating primary OS tumors with histotripsy using a 500-kHz transducer on excised canine OS samples harvested after surgery at the Veterinary Teaching Hospital at Virginia Tech. Samples were embedded in gelatin tissue phantoms and treated with the 500-kHz histotripsy system using one- or two-cycle pulses at a pulse repetition frequency of 250 Hz and a dosage of 4000 pulses/point. Separate experiments also assessed histotripsy effects on normal canine bone and nerve using the same pulsing parameters. After treatment, histopathological evaluation of the samples was completed. To determine the feasibility of treating OS through intact skin/soft tissue, additional histotripsy experiments assessed OS with overlying tissues. Generation of bubble clouds was achieved at the focus in all tumor samples at peak negative pressures of 26.2 ± 4.5 MPa. Histopathology revealed effective cell ablation in treated areas for OS tumors, with no evidence of cell death or tissue damage in normal tissues. Treatment through tissue/skin resulted in generation of well-confined bubble clouds and ablation zones inside OS tumors. Results illustrate the feasibility of treating OS tumors with histotripsy.

Partial Respiratory Motion Compensation for Abdominal Extracorporeal Boiling Histotripsy Treatments With a Robotic Arm

Extracorporeal boiling histotripsy (BH), a noninvasive method for mechanical tissue disintegration, is getting closer to clinical applications. However, the motion of the targeted organs, mostly resulting from the respiratory motion, reduces the efficiency of the treatment. Here, a practical and affordable unidirectional respiratory motion compensation method for BH is proposed and evaluated in ex vivo tissues. The BH transducer is fixed on a robotic arm following the motion of the skin, which is tracked using an inline ultrasound imaging probe. In order to compensate for system lags and obtain a more accurate compensation, an autoregressive motion prediction model is implemented. BH pulse gating is also implemented to ensure targeting accuracy. The system is then evaluated with ex vivo BH treatments of tissue samples undergoing motion simulating breathing with the movement of amplitudes between 5 and 10 mm, the frequency between 16 and 18 breaths/min, and a maximum speed of 14.2 mm/s. Results show a reduction of at least 89% of the value of the targeting error during treatment while only increasing the treatment time by no more than 1%. The lesions obtained by treating with the motion compensation were close in size and affected area to the no-motion case, whereas lesions obtained without the compensation were often incomplete and had larger affected areas. This approach to motion compensation could benefit extracorporeal BH and other histotripsy methods in clinical translation.

Histotripsy for the Treatment of Cholangiocarcinoma Liver Tumors: In Vivo Feasibility and Ex Vivo Dosimetry Study

Histotripsy is a noninvasive, nonionizing, and nonthermal focused ultrasound ablation method that is currently being developed for the treatment of liver cancer. Promisingly, histotripsy has been shown for ablating primary [hepatocellular carcinoma (HCC)] and metastatic [colorectal liver metastasis (CLM)] liver tumors in preclinical and early clinical studies. The feasibility of treating cholangiocarcinoma (CC), a less common primary liver tumor that arises from the bile ducts, has not been explored previously. Given that prior work has established that histotripsy susceptibility is based on tissue mechanical properties, there is a need to explore histotripsy as a treatment for CC due to its dense fibrotic stromal components. In this work, we first investigated the feasibility of histotripsy for ablating CC tumors in vivo in a patient-derived xenograft mouse model. The results showed that histotripsy could generate CC tumor ablation using a 1-MHz small animal histotripsy system with treatment doses of 250, 500, and 1000 pulses/point. The second set of experiments compared the histotripsy doses required to ablate CC tumors to HCC and CLM tumors ex vivo. For this, human tumor samples were harvested after surgery and treated ex vivo with a 700-kHz clinical histotripsy transducer. Results demonstrated that significantly higher treatment doses were required to ablate CC and CLM tumors compared to HCC, with the highest treatment dose required for CC tumors. Overall, the results of this study suggest that histotripsy has the potential to be used for the ablation of CC tumors while also highlighting the need for tumor-specific treatment strategies.

Transcostal Histotripsy Ablation in an In Vivo Acute Hepatic Porcine Model

PURPOSE

To determine whether histotripsy can create human-scale transcostal ablations in porcine liver without causing severe thermal wall injuries along the beam path.

MATERIALS AND METHODS

Histotripsy was applied to the liver using a preclinical prototype robotic system through a transcostal window in six female swine. A 3.0 cm spherical ablation zone was prescribed. Duration of treatment (75 min) was longer than a prior subcostal treatment study (24 min, 15 s) to minimize beam path heating. Animals then underwent contrast-enhanced MRI, necropsy, and histopathology. Images and tissue were analyzed for ablation zone size, shape, completeness of necrosis, and off-target effects.

RESULTS

Ablation zones demonstrated complete necrosis with no viable tissue remaining in 6/6 animals by histopathology. Ablation zone volume was close to prescribed (13.8 ± 1.8 cm³ vs. prescribed 14.1 cm³). Edema was noted in the body wall overlying the ablation on T2 MRI in 5/5 (one animal did not receive MRI), though there was no gross or histologic evidence of injury to the chest wall at necropsy. At gross inspection, lung discoloration in the right lower lobe was present in 5/6 animals (mean size: 1 × 2 × 4 cm) with alveolar hemorrhage, preservation of blood vessels and bronchioles, and minor injuries to pneumocytes noted at histology.

CONCLUSION

Transcostal hepatic histotripsy ablation appears feasible, effective, and no severe injuries were identified in an acute porcine model when prolonged cooling time is added to minimize body wall heating.

Noninvasive thyroid histotripsy treatment: proof of concept study in a porcine model

INTRODUCTION

This study was performed to determine the feasibility and safety of creating superficial histotripsy treatment in a live porcine thyroid model.

METHODS

The porcine thymus comparable in size, shape and location to the human thyroid was used for this study. This model has been used for thyroid surgery studies due to the diminutive size of the porcine thyroid. Four female swine underwent a total of eight histotripsy treatments performed with a prototype therapy system (HistoSonics, Inc., Ann Arbor, MI). Two treatments were performed in each animal: a spherical 1.0 × 1.0 × 1.0 cm and ovoid 1.0 × 1.0 × 2.0 cm treatment zones. MRI immediately post-procedure was evaluated for histotripsy treatment zone size and imaging appearance, followed immediately by sacrifice. Tissue was then reviewed for percent cellular destruction and precision.

RESULTS

Treatment zones measured on post treatment MRI were similar to prescribed volumes (spherical = 0.60 (+/- 0.11) cm³, ovoid = 1.23 (+/- 0.40) cm³, p > 0.05 vs. prescribed). MRI demonstrated well demarcated treatment zones and imaging findings consistent with cellular destruction. Histology demonstrated sharp transitions to normal tissue (mean 0.33 (+/- 0.13) cm), and high degrees of cellular destruction (mean 76% (+/- 12.5), range of 50-100%) in the treated tissue. Edema within the overlying muscle was seen in 2/8 treatments.

CONCLUSION

Histotripsy is capable of safely creating precise histotripsy treatments within the superficial neck of a porcine thyroid model without evidence of considerable complications.

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.

Histotripsy: the first noninvasive, non-ionizing, non-thermal ablation technique based on ultrasound

Histotripsy is the first noninvasive, non-ionizing, and non-thermal ablation technology guided by real-time imaging. Using focused ultrasound delivered from outside the body, histotripsy mechanically destroys tissue through cavitation, rendering the target into acellular debris. The material in the histotripsy ablation zone is absorbed by the body within 1-2 months, leaving a minimal remnant scar. Histotripsy has also been shown to stimulate an immune response and induce abscopal effects in animal models, which may have positive implications for future cancer treatment. Histotripsy has been investigated for a wide range of applications in preclinical studies, including the treatment of cancer, neurological diseases, and cardiovascular diseases. Three human clinical trials have been undertaken using histotripsy for the treatment of benign prostatic hyperplasia, liver cancer, and calcified valve stenosis. This review provides a comprehensive overview of histotripsy covering the origin, mechanism, bioeffects, parameters, instruments, and the latest results on preclinical and human studies.

Feasibility and safety of non-invasive ultrasound therapy (NIUT) on an porcine aortic valve

Calcific aortic stenosis (CAS) is associated with advanced age and comorbidities, therefore a non-invasive therapy for it would be beneficial. We previously demonstrated that ultrasound therapy improved calcified bioprosthetic valve function in an open chest model. For translational applications, we tested non-invasive ultrasound therapy (NIUT) transthoracically on swine aortic valves and investigated the need for antithrombotic treatment as a follow-up. Primary objective: feasibility and safety of NIUT. Secondary objectives: occurrence, severity and evolution of side effects during therapy and at 1 month follow-up. The device (Valvosoft, Cardiawave) consisted of an electronically steered multi-element transducer and a 2D echocardiographic probe. Three groups of swine received treatment on aortic valves: NIUT (group 1; n = 10); NIUT and 1 month antithrombotic treatment (group 2; n = 5); sham group (group 3; n = 4). Feasibility was successfully reached in all treated swine (n = 15) and no life-threatening arrhythmia were detected. Non-sustained ventricular tachycardia occurred during the procedure in seven swine. Decrease or interruption of NIUT ended arrhythmia. Histopathology revealed no valve or surrounding tissue damage and echocardiography revealed no valvular dysfunction. Only one animal had side effects [right ventricle (RV) dilatation], but the RV normalized after therapy cessation with no sequelae at follow-up. No disturbance in biological markers nor valve thrombosis were observed at follow-up. Antithrombotic treatment did not demonstrate any advantage. Survival at 30 d was 100%. We demonstrated, in vivo, the feasibility and safety of transthoracic NIUT on aortic valves in a swine model without serious adverse events. We expect this first-time transthoracic delivery of NIUT to pave the way towards a new non-invasive approach to valve softening in human CAS to restore valve function.

Assessment of histotripsy-induced liquefaction with diagnostic ultrasound and magnetic resonance imaging in vitro and ex vivo

Histotripsy is a therapeutic ultrasound modality under development to liquefy tissue mechanically via bubble clouds. Image guidance of histotripsy requires both quantification of the bubble cloud activity and accurate delineation of the treatment zone. In this study, magnetic resonance (MR) and diagnostic ultrasound imaging were combined to assess histotripsy treatment in vitro and ex vivo. Mechanically ablative histotripsy pulses were applied to agarose phantoms or porcine livers. Bubble cloud emissions were monitored with passive cavitation imaging (PCI), and hyperechogenicity via plane wave imaging. Changes in the medium structure due to bubble activity were assessed with diagnostic ultrasound using conventional B-mode imaging and T ₁-, T ₂-, and diffusion-weighted MR images acquired at 3 Tesla. Liquefaction zones were correlated with diagnostic ultrasound and MR imaging via receiver operating characteristic (ROC) analysis and Dice similarity coefficient (DSC) analysis. Diagnostic ultrasound indicated strong bubble activity for all samples. Histotripsy-induced changes in sample structure were evident on conventional B-mode and T ₂-weighted images for all samples, and were dependent on the sample type for T ₁- and diffusion-weighted imaging. The greatest changes observed on conventional B-mode or MR imaging relative to baseline in the samples did not necessarily indicate the regions of strongest bubble activity. Areas under the ROC curve for predicting phantom or liver liquefaction were significantly greater than 0.5 for PCI power, plane wave and conventional B-mode grayscale, T ₁, T ₂, and ADC. The acoustic power mapped via PCI provided a better prediction of liquefaction than assessment of the liquefaction zone via conventional B-mode or MR imaging for all samples. The DSC values for T ₂-weighted images were greater than those derived from conventional B-mode images. These results indicate diagnostic ultrasound and MR imaging provide complimentary sets of information, demonstrating that multimodal imaging is useful for assessment of histotripsy liquefaction.

Soft-Tissue Aberration Correction for Histotripsy

Acoustic aberrations caused by natural heterogeneities of biological soft tissue are a substantial problem for histotripsy, a therapeutic ultrasound technique that uses acoustic cavitation to mechanically fractionate and destroy unwanted target tissue without damaging surrounding tissue. These aberrations, primarily caused by sound speed variations, result in severe defocusing of histotripsy pulses, thereby decreasing treatment efficacy. The gold standard for aberration correction (AC) is to place a hydrophone at the desired focal location to directly measure phase aberrations, which is a method that is infeasible in vivo. We hypothesized that the acoustic cavitation emission (ACE) shockwaves from the initial expansion of inertially cavitating microbubbles generated by histotripsy can be used as a point source for AC. In this study, a 500-kHz, 112-element histotripsy phased array capable of transmitting and receiving ultrasound on all channels was used to acquire ACE shockwaves. These shockwaves were first characterized optically and acoustically. It was found that the shockwave pressure increases significantly as the source changes from a single bubble to a dense cavitation cloud. The first arrival of the shockwave received by the histotripsy array was from the outer-most cavitation bubbles located closest to the histotripsy array. Hydrophone and ACE AC methods were then tested on ex vivo porcine abdominal tissue samples. Without AC, the focal pressure is reduced by 49.7% through the abdominal tissue. The hydrophone AC approach recovered 55.5% of the lost pressure. Using the ACE AC method, over 20% of the lost pressure was recovered, and the array power required to induce cavitation was reduced by approximately 31.5% compared to without AC. These results supported our hypothesis that the ACE shockwaves coupled with a histotripsy array with transmit and receive capability can be used for AC for histotripsy through soft tissue.

Focused ultrasound: tumour ablation and its potential to enhance immunological therapy to cancer

Various kinds of image-guided techniques have been successfully applied in the last years for the treatment of tumours, as alternative to surgical resection. High intensity focused ultrasound (HIFU) is a novel, totally non-invasive, image-guided technique that allows for achieving tissue destruction with the application of focused ultrasound at high intensity. This technique has been successfully applied for the treatment of a large variety of diseases, including oncological and non-oncological diseases. One of the most fascinating aspects of image-guided ablations, and particularly of HIFU, is the reported possibility of determining a sort of stimulation of the immune system, with an unexpected "systemic" response to treatments designed to be "local". In the present article the mechanisms of action of HIFU are described, and the main clinical applications of this technique are reported, with a particular focus on the immune-stimulation process that might originate from tumour ablations.

Non-invasive, Rapid Ablation of Tissue Volume Using Histotripsy

Histotripsy is a non-invasive, non-thermal ablation technique that uses high-amplitude, focused ultrasound pulses to fractionate tissue via acoustic cavitation. The goal of this study was to illustrate the potential of histotripsy with electronic focal steering to achieve rapid ablation of a tissue volume at a rate matching or exceeding those of current clinical techniques (∼1-2 mL/min). Treatment parameters were established in tissue-mimicking phantoms and applied to ex vivo tissue. Six-microsecond pulses were delivered by a 250-kHz array. The focus was electrically steered to 1000 locations at a pulse repetition frequency of 200 Hz (0.12% duty cycle). Magnetic resonance imaging and histology of the treated tissue revealed a distinct region of necrosis in all samples. Mean lesion volume was 35.6 ± 4.3 mL, generated at 0.9-3.3 mL/min, a speed faster than that of any current ablation method for a large volume. These results suggest that histotripsy has the potential to achieve non-invasive, rapid, homogeneous ablation of a tissue volume.

The response of MRI contrast parameters in in vitro tissues and tissue mimicking phantoms to fractionation by histotripsy

Histotripsy is a non-invasive, focused ultrasound lesioning technique that can ablate precise volumes of soft tissue using a novel mechanical fractionation mechanism. Previous research suggests that magnetic resonance imaging (MRI) may be a sensitive image-based feedback mechanism for histotripsy. However, there are insufficient data to form some unified understanding of the response of the MR contrast mechanisms in tissues to histotripsy. In this paper, we investigate the response of the MR contrast parameters R1, R2, and the apparent diffusion coefficient (ADC) to various treatment levels of histotripsy in in vitro porcine liver, kidney, muscle, and blood clot as well in formulations of bovine red blood cells suspended in agar gel. We also make a histological analysis of histotripsy lesions in porcine liver. We find that R2 and the ADC are both sensitive to ablation in all materials tested here, and the degree of response varies with tissue type. Correspondingly, under histologic analysis, the porcine liver exhibited various levels of mechanical disruption and necrotic debris that are characteristic of histotripsy. While the area of intact red blood cells and nuclei found within these lesions both decreased with increasing amounts of treatment, the area of red blood cells decreased much more rapidly than the area of intact nuclei. Additionally, the decrease in area of intact red blood cells saturated at the same treatment levels at which the response of the R2 saturated while the area of intact nuclei appeared to vary linearly with the response of the ADC.

Pulsed Cavitational Ultrasound Softening: a new non-invasive therapeutic approach of calcified bioprosthetic valve stenosis

BACKGROUND

The majority of prosthetic heart valves currently implanted are tissue valves that can be expected to calcify with time and eventually fail. Surgical or percutaneous redux valve replacement is associated with higher rate of complications. We propose a novel non-invasive therapeutic approach based on the use of pulsed cavitational ultrasound (PCU) to improve the valvular function of degenerative calcified bioprosthesis.

OBJECTIVES

Our study aims to demonstrate in vitro and in vivo on an ovine model that PCU can significantly improve the bioprosthesis opening by softening remotely the calcified stiff cusps.

METHODS

All the experiments were performed on calcified bioprosthetic valves explanted from human patients. PCU was performed in vitro on calcified bioprosthesis mounted on a hydraulic bench with pulsatile flow (n=8) and in vivo on an ovine model with implanted calcified bioprosthesis (n=7). We used 3D echocardiography, pressure and flow sensors, quantitative stiffness evaluation using shear wave elastography, micro-CT imaging and histology to evaluate in vitro and in vivo the effect of PCU.

RESULTS

The transvalvular gradient was found to decrease by a mean of 50% after PCU in both in vitro (from 21.1±3.9 to 9.6±1.7 mmHg, p<0.001) and in vivo setup (from 16.2±3.2 to 8.2±1.3 mmHg, p<0.001), with a decrease of valve stiffness (in vitro: from 105.8±9 to 46.6±4 kPa, p<0.001; in vivo: from 82.6±10 to 41.7±7 kPa, p<0.001) and an increase of valve area (from 1.10±0.1 to 1.58±0.1 cm2, p<0.001). Histology and micro-CT imaging showed modifications of calcification structure without loss of calcification volume or alteration of the leaflet superficial structures.

CONCLUSIONS

We have demonstrated in vitro and in vivo that PCU can decrease a calcified bioprosthesis stenosis by softening the leaflets remotely. This new non-invasive approach has the potential to improve the outcome of patients with severe bioprosthesis stenosis.

Non-Invasive Thrombolysis Using Microtripsy in a Porcine Deep Vein Thrombosis Model

Histotripsy is a non-invasive therapeutic technique that uses ultrasound generated from outside the body to create controlled cavitation in targeted tissue, and fractionates it into acellular debris. We have developed a new histotripsy approach, termed microtripsy, to improve targeting accuracy and to avoid collateral tissue damage. This in vivo study evaluates the safety and efficacy of microtripsy for non-invasive thrombolysis in a porcine deep vein thrombosis model. Acute thrombi were formed in left femoral veins of pigs (∼35 kg) by occluding the vessel using two balloon catheters and infusing with thrombin. Guided by real-time ultrasound imaging, microtripsy thrombolysis treatment was conducted in 14 pigs; 10 pigs were euthanized on the same day (acute) and 4 at 2 wk (subacute). To evaluate vessel damage, 30-min free-flow treatment in the right femoral vein (no thrombus) was also conducted in 8 acute pigs. Blood flow was successfully restored or significantly increased after treatment in 13 of the 14 pigs. The flow channels re-opened by microtripsy had a diameter up to 64% of the vessel diameter (∼6 mm). The average treatment time was 16 min per centimeter-long thrombus. Only mild intravascular hemolysis was induced during microtripsy thrombolysis. No damage was observed on vessel walls after 2 wk of recovery, venous valves were preserved, and there was no sign of pulmonary embolism. The results of this study indicate that microtripsy has the potential to be a safe and effective treatment for deep vein thrombosis in a porcine model.

Non-Invasive Liver Ablation Using Histotripsy: Preclinical Safety Study in an In Vivo Porcine Model

This study investigates the safety profile for use of histotripsy, a non-invasive ultrasonic ablation method currently being developed for the treatment of liver cancer, for liver ablation in an in vivo porcine model. Histotripsy treatments were applied to the liver and hepatic veins of 22 porcine subjects, with half of the subjects receiving systemic heparinization. Vital signs (heart rate, blood pressure, temperature, electrocardiogram and SpO₂) were monitored throughout the procedure and for 1 h post-treatment. Blood was drawn at six points during the experiment to analyze blood gases, liver function and free hemoglobin levels. All treatments were guided and monitored by real-time ultrasound imaging. After treatment, the tissue was harvested for histological analysis. Results indicated that histotripsy generated well-defined lesions inside the liver and around the treated hepatic veins of all subjects in both treatment groups. Vital signs and blood analysis revealed that animals responded well to histotripsy, with all animals surviving the treatment. One animal in the non-heparinized group had a transient increase in pH and decreases in blood pressure, heart rate and PCO₂ during the 15-min vessel treatment, with these changes returning to baseline levels soon after the treatment. Overall, the results indicate that histotripsy can safely be performed on the liver without the need for systemic heparinization, even in regions containing large hepatic vessels, supporting its future use for the treatment of liver cancer.

Non-Invasive Ultrasound Liver Ablation Using Histotripsy: Chronic Study in an In Vivo Rodent Model

Hepatocellular carcinoma, or liver cancer, has the fastest growing incidence among cancers in the United States. Current liver ablation methods are thermal-based and share limitations due to the heat sink effect from the blood flow through the highly vascular liver. Recently, our group has investigated histotripsy as a non-invasive liver cancer ablation method. Histotripsy is a non-thermal ultrasonic ablation method that fractionates tissue through the control of acoustic cavitation. Previous experiments in an in vivo porcine model show that histotripsy can create well-confined lesions in the liver through ribcage obstruction without damaging the overlying ribs and other tissues. Histotripsy can also completely fractionate liver tissue surrounding major vessels while preserving the vessels. In this study, we investigate the long-term effects of histotripsy liver ablation in a rodent model. We hypothesize that the fractionated histotripsy lesion will be resorbed by the liver, resulting in effective tissue healing. To test this hypothesis, the livers of 20 healthy rats were treated with histotripsy using an 8-element 1-MHz histotripsy transducer. Rats were euthanized after 0, 3, 7, 14 and 28 days (n = 4). In vivo and post mortem results showed histotripsy lesions were successfully generated through the intact abdomen in all 20 rats. Magnetic resonance imaging found primarily negative contrast on day 0, positive contrast on day 3 and rapid normalization of signal intensity thereafter (i.e., signal amplitude returned to baseline levels seen in healthy liver tissue). Histologically, lesions were completely fractionated into an acellular homogenate. The lesions had a maximum cross-sectional area of 17.2 ± 1.9 mm(2) and sharp boundaries between the lesion and the healthy surrounding tissue after treatment. As the animals recovered after treatment, the histotripsy tissue homogenate was almost completely replaced by regenerated liver parenchyma, resulting in a small fibrous lesion (<1 mm(2) maximum cross-section) remaining after 28 d. The results of this study suggest that histotripsy has potential as a non-invasive liver ablation method for effective tissue removal.

Histotripsy methods in mechanical disintegration of tissue: towards clinical applications

In high intensity focused ultrasound (HIFU) therapy, an ultrasound beam is focused within the body to locally affect the targeted site without damaging intervening tissues. The most common HIFU regime is thermal ablation. Recently there has been increasing interest in generating purely mechanical lesions in tissue (histotripsy). This paper provides an overview of several studies on the development of histotripsy methods toward clinical applications. Two histotripsy approaches and examples of their applications are presented. In one approach, sequences of high-amplitude, short (microsecond-long), focused ultrasound pulses periodically produce dense, energetic bubble clouds that mechanically disintegrate tissue. In an alternative approach, longer (millisecond-long) pulses with shock fronts generate boiling bubbles and the interaction of shock fronts with the resulting vapour cavity causes tissue disintegration. Recent preclinical studies on histotripsy are reviewed for treating benign prostatic hyperplasia (BPH), liver and kidney tumours, kidney stone fragmentation, enhancing anti-tumour immune response, and tissue decellularisation for regenerative medicine applications. Potential clinical advantages of the histotripsy methods are discussed. Histotripsy methods can be used to mechanically ablate a wide variety of tissues, whilst selectivity sparing structures such as large vessels. Both ultrasound and MR imaging can be used for targeting and monitoring the treatment in real time. Although the two approaches utilise different mechanisms for tissue disintegration, both have many of the same advantages and offer a promising alternative method of non-invasive surgery.