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

Evaluation of targeting accuracy of cone beam CT guided histotripsy in an in vivo porcine model

PURPOSE

The application of histotripsy, an emerging noninvasive, non-ionizing, and non-thermal tumor treatment, is currently limited by the inherent limitations of diagnostic ultrasound as the sole targeting modality. This study evaluates the feasibility and accuracy of cone beam computed tomography (CBCT) guidance for histotripsy treatments in an in vivo porcine model.

MATERIALS AND METHODS

Histotripsy treatments were performed in the liver of seven healthy swine under the guidance of a C-arm CBCT system that was calibrated to the robotic arm of the histotripsy system. For each treatment, pseudotumors (small histotripsy treatments of 15 mm) were created using conventional US guidance to serve as targets for subsequent CBCT guided treatments. A pretreatment CBCT with intravenous contrast was acquired for each swine and the center of the pseudotumor was selected as the target. The robotic arm automatically aligned the transducer to the selected target location. Ultrasound based aberration offset correction was performed when possible, and a 25 mm diameter treatment was performed. A post-treatment CBCT with intravenous contrast was then acquired to evaluate coverage, treatment size, and distance between the pseudotumor target and actual treatment zone center.

RESULTS

Treatments were technically successful and pseudotumors were completely covered in all seven treatments (7/7). The average treatment diameter was 39.3 ± 4.2 mm. The center-to-center distance between pseudotumor and actual treatments was 3.8 ± 1.3 mm.

CONCLUSION

CBCT provides accurate targeting for histotripsy treatment in vivo. While future work is required to assess safety and efficacy in the presence of obstructions, the proposed approach could supplement ultrasound imaging for targeting.

Enhancing boiling histotripsy efficacy with a tandem pulse sequence: Immediate hypoechoic Sonograhy and expanded lesion size

Boiling histotripsy (BH) has proven effective in noninvasively disintegrating various soft tissues through cavitation effects. Although liquefied tissue appears as hypoechoic in sonography, the transition of BH-induced bubbles from hyperechoic to hypoechoic in the focal region typically requires several minutes. To facilitate rapid clinical assessment, a tandem pulse sequence of high-power BH pulses (with an acoustic power of 1484 W, a pulse duration of 10 ms, and a pulse repetition frequency of 1 Hz) followed by low-power long ultrasound pulses (with an acoustic power of 240 W, a pulse duration of 100 ms, and a pulse repetition frequency of 1 Hz) was introduced to expedite bubble clearance, resulting in an immediate hypoechoic presentation in sonography. This method was evaluated through high-speed photography, red blood cell (RBC) phantom, and ex vivo tissue experiments. High-speed photography experiments captured the enhanced bubble clearance induced by the low-power long pulses, validating our hypothesis. In RBC phantom experiments, conventional BH sequences yielded hypoechic patterns after 4.39 ± 0.84 min, whereas the tandem pulse sequences achieved hypoechic appearance instantaneously post-treatment (p < 0.05). Moreover, the tandem pulse sequences increased the erosion area in the RBC layer by 7.8 folds, from 2.36 ± 0.88 mm2 to 18.43 ± 5.15 mm2 (p < 0.05), at the equivalent energy output. Ex vivo bovine liver experiments mirrored these findings, with hypoechoic appearance at > 10 min vs. 0 min (p < 0.05) and liquefied areas of 33.78 ± 3.28 mm2 vs. 66.52 ± 11.24 mm2 (p < 0.05), respectively. In summary, our results suggest that the strategic modulation of cavitation activities not only accelerates the immediate hypoechoic appearance in sonography but also enlarges the area of BH-induced disintegration. The tandem pulse sequence strategy presents a promising avenue for enhancing the efficacy and efficiency of BH treatment in clinical applications.

Study of Histotripsy With Subsequent Heating on In Vitro VX2 Cancer Cells

OBJECTIVE

Focused ultrasound has emerged as a precise and minimally invasive modality for effective cancer treatment. In this study, we propose a novel method that integrates the mechanical effects of focused ultrasound, known as histotripsy, with heating to enhance both the immediate and sustained cytotoxic effects on cancer cells.

METHODS

Our investigation focused on VX2 cancer cells in suspension, examining five experimental groups: blank control, negative control, heating alone, histotripsy alone, and histotripsy with subsequent heating. B-mode ultrasound imaging was utilized to visualize cavitation bubble cloud formation and its motion during histotripsy. The suspension was contained in individually sealed compartments obtained from bubble wrap (referred to as bubble wrap compartments) embedded within the agarose phantom. Residual living cells were examined immediately after treatment and cultured for 96 hours to analyze the growth patterns. Additionally, CFDA SE staining was employed to assess cell proliferation. Furthermore, both intracellular and extracellular heat shock protein 70 (HSP70) levels were measured to investigate the potential initiation of an immune response.

RESULTS

The combination of histotripsy and subsequent heating significantly reduced the normalized concentration of living cells immediately after treatment. It also decreased the proliferation rate of residual cells compared with the other experimental groups. Histotripsy with subsequent heating also increased the generation and release of HSP70, which might potentially enhance an innate anti-tumor immune response in vivo.

CONCLUSION

Histotripsy and subsequent heating improved the immediate lethal impact on VX2 cancer cells and curtailed the proliferation of residual cancer cells in suspension. This study presents a promising strategy for cancer therapy in the future.

Ultrasound Erosion of Rabbit Liver Induced by Locally Injected Phase-Shift Acoustic Droplets and With Lauromacrogol

OBJECTIVES

Our previous studies have found that low-frequency, low-pressure, weakly focused ultrasound (FUS) can induce acoustic droplet vaporization (ADV) of perfluoropentane (PFP) droplets and result in localized liver and prostate tissue controllable cavitation resonance and mechanical damage. To further investigate the mechanical erosion induced by ultrasound and locally injected phase-shift acoustic droplets in rabbit liver.

METHODS

The liver of each rabbit was treated with perfluoromethylcyclopentane (PFMCP) alone, FUS combined with PFMCP (FUS + PFMCP), and FUS combined with PFP (FUS + PFP).

RESULTS

Two-dimensional ultrasound images showed that immediately after the completion of FUS + PFP group treatments, a high echogenicity bubble cloud could be observed, while there were no significant differences in the PFMCP and FUS + PFMCP group before and after treatment. The liver necrotic area in the FUS + PFP group was 6.2 times that of the FUS + PFMCP group (P < .05), whereas no liver necrosis was observed in the PFMCP group. At the same time, the number of vacuoles in the liver in the FUS + PFP group was approximately 70 times that of the FUS + PFMCP group (P < .001), whereas no vacuoles were observed in the PFMCP group (P < .001).

CONCLUSIONS

Both FUS + PFMCP and PFMCP alone have poor mechanical erosion in liver tissue, and may even cause no damage. Only PFP droplets combined with FUS can cause significant mechanical destruction of liver tissue, leading to tissue necrosis in the droplet injection area.

MR-Cavitation Dynamics Encoded (MR-CaDE) imaging

PURPOSE

To develop methods for dynamic cavitation monitoring of a non-invasive ultrasound mechanical ablation technology (histotripsy) in the brain and test its feasibility for treatment monitoring in ex-vivo brain in a human MRI scanner.

METHODS

A Gradient Echo (GRE) pulse sequence was modified with a bipolar gradient to perform MR-Cavitation Dynamics Encoded (MR-CaDE) imaging. Cavitation generated by histotripsy sonication was monitored using MR-CaDE imaging in ex-vivo bovine brain tissues on a3 T $$ 3\mathrm{T} $$ human MRI scanner. Bipolar gradients with a b-value ofb = 50 s / mm 2 $$ \mathrm{b}=50\mathrm{s}/{\mathrm{mm}}^2 $$ and smaller were used while a trigger was sent from the MR scanner to the histotripsy driving electronics. MR acquisition was performed with TE/TR:19 ms / 100 ms $$ 19\kern.2em \mathrm{ms}/100\kern.2em \mathrm{ms} $$ with 1.5-cycle histotripsy sonications at 1 pulse/TR. Feasibility of treatment monitoring was also evaluated for histotripsy through an excised human skull.

RESULTS

The MR-CaDE imaging pulse sequence was used to perform treatment monitoring of cavitation generated by histotripsy with a temporal resolution of0.5 s $$ 0.5\kern.2em \mathrm{s} $$ with a spiral readout. A decrease in the image magnitude and an increase in the phase was observed with an increasing number of histotripsy sonications. The magnitude image exhibited a peak loss of 50%, and the phase image exhibited a maximum increase of 0.64rad compared to the baseline signal level in the brain. The peak signal magnitude change aligned well with the array's geometrical focus, and the post-histotripsy lesion visualized on a DWI (b = 1000 s/mm   2 $$ \mathrm{b}=1000\kern.2em \mathrm{s}/{\mathrm{mm}}^2 $$ ) scan with an alignment error of0.71 mm $$ 0.71\kern.2em \mathrm{mm} $$ and1.25 mm $$ 1.25\kern.2em \mathrm{mm} $$ in the transverse and longitudinal axes, respectively. The area of the histotripsy response using the spiral readout in the magnitude and phase images were3 . 38 mm × 5 . 62 mm $$ 3.38\kern0.3em \mathrm{mm}\times 5.62\kern0.3em \mathrm{mm} $$ and10 . 92 mm × 20 . 28 mm $$ 10.92\kern0.3em \mathrm{mm}\times 20.28\kern0.3em \mathrm{mm} $$ , respectively.

CONCLUSION

This work demonstrated the feasibility of the MR-CaDE pulse sequence, which can be used to monitor cavitation events in the brain generated by histotripsy.

Resonance-Induced Therapeutic Technique for Skin Cancer Cells

OBJECTIVE

This study aims to evaluate the viability of a hypothesis for selective targeting of skin cancer cells by exploiting the spectral gap with healthy cells using analytical and numerical simulation.

METHODS

The spectral gap was first identified using a viscoelastic dynamic model, with the physical and mechanical properties of healthy and cancerous skin cells deduced from previous experimental studies conducted on cell lines. The outcome of the analytical simulation was verified numerically using modal and harmonic analysis. Finally, transient analyses were conducted analytically and numerically to evaluate the difference in vibrational response of healthy and cancerous cells when their resonant frequencies were closely matched. For analysis, we used healthy nucleus diameters of 3 µm, 5 µm and 7 µm, whereas 34 kPa was taken as the stiffness of healthy skin epithelial cells. Based on established trends, the nucleus-to-cytoplasm ratio was utilised to predict physical and mechanical properties as cells undergo neoplastic transformation.

RESULTS

Analytical and numerical simulation revealed an approximate frequency difference of 50-100 KHz for the different nucleus diameters. The transient simulation revealed a significant difference in the growth rate of cancer cells' vibration amplitude, which was 10 times greater than that of healthy cells.

CONCLUSIONS

This study highlights that cancer cells are more prone to resonance with tuned ultrasound frequencies, emphasising the need for detailed dynamic models incorporating the basement membrane's influence and experimental validation.

Bubble cloud-mediated cavitation for tumor mechanical ablation and effector immune cell deployment

Histotripsy is a cavitation-based tumor ablation technology. To achieve precise cavitation-based ablation requires investigating the cavitation behavior of the bubble cloud and their impact on tumor tissue. This study explored the cavitation behavior of bubble clouds generated by perfluoropentane (PFP)-loaded nanodroplets and efficacy of bubble cloud cavitation in tumor ablation under varying ultrasound intensities. PFP-loaded nanodroplets (∼200 nm) were employed as exogenous cavitation nuclei to reducing the required ultrasound energy for activation of bubble cloud. We investigated the formation, vibration, and collapse of bubble clouds in solution and phantom models under varying ultrasound intensities. Results indicated distinct cavitation patterns: (1) Nanodroplets slowly vaporized and formed continuously vibrating bubble clouds; (2) Nanodroplets rapidly vaporized and resulted in quickly collapsing bubble clouds. At both the cellular and animal levels, cavitation ablation efficacy was examined, revealing that all bubble cloud cavitation patterns could induce immunogenic cell death (ICD), promoting the release of damage-associated molecular patterns (DAMPs) and triggering effector immune cell deployment of peripheral immune response and local tumor infiltration. During the treatment, the ultrasound intensity of 0.5 W/cm2 had the highest level of central tumor CD8+ T cell infiltration. The conclusion was that sustained bubble cloud oscillation, rather than rapid vaporization and rupture, proved more beneficial for antitumor therapy, particularly in enhancing the local infiltration of effector immune cells.

Histotripsy of Liver Tumors: Patient Selection, Ethical Discussions, and How We Do It

Liver malignancies, both primary and metastatic tumors, are a major cause of cancer-related mortality. Colorectal cancer alone results in liver metastases in nearly 50% of patients, with approximately 85% presenting with unresectable disease. Similarly, hepatocellular carcinoma and intrahepatic cholangiocarcinoma frequently present at advanced stages, limiting curative options. Systemic therapies provide modest survival benefits, underscoring the need for alternative treatments. Locoregional approaches, such as thermal ablation and chemoembolization, while effective, have notable limitations, including invasiveness, peri-procedural risks, and the requirement to interrupt systemic treatments. Histotripsy is a novel, non-invasive method that uses focused ultrasound-induced cavitation to enable precise tumor ablation without heat or radiation. Our institution utilizes a multidisciplinary tumor board approach to evaluate patients for histotripsy, particularly in cases involving unresectable disease, complex surgical candidacy, palliative intent related to disease control and symptom management, or as bridging therapy for transplantation. Early results, including preclinical data and the THERESA and #HOPE4LIVER trials, highlight its efficacy in treating liver tumors with minimal complications. This review outlines institutional protocols for histotripsy, covering pre- and post-procedural management, along with ethical considerations of current treatment paradigms. As a patient-centered approach, histotripsy offers a novel treatment option with a favorable safety profile and compatibility with systemic therapies.

The Emerging Role of Histotripsy in Liver Cancer Treatment: A Scoping Review

BACKGROUND/OBJECTIVE

Hepatocellular carcinoma (HCC) is an aggressive disease that is known to be resistant to conventional chemotherapy and radiotherapy. While surgical resection and transarterial therapy can improve overall survival, the biological aspects of HCC contribute to the complexity of its management and limit the effectiveness of current treatment options. The purpose of this scoping review is to identify the limitations of the currently available therapies for HCC and explore the emerging role that histotripsy could play in addressing these limitations, with the intent of informing the direction of future research and clinical management.

METHODS

The PRISMA checklist for scoping reviews was followed to structure this review, and a systematic search was conducted in the following online databases: PubMed/MEDLINE (National Library of Medicine), Embase (Elsevier), and Scopus (Elsevier).

RESULTS

The current evidence supports that histotripsy offers several key advantages that address the limitations of the current treatment strategies for HCC. Clinical trials have highlighted the ability of this technology to destroy solid tumors and induce remission with minimal side effects. In addition, current preclinical studies point to the potent immunostimulatory effects of histotripsy, including the induction of abscopal effects. This poses significant promise in treating tumor metastasis as well as improving clinical regimens by combining histotripsy with immunotherapy. Future research should aim to overcome the current limitations of histotripsy and enhance clinical outcomes for patients. This review examines existing treatments for HCC, emphasizing the promising potential of combining histotripsy with immunotherapy to target the metastatic and advanced stages of the disease.

Acoustic holography in biomedical applications

Acoustic holography can be used to construct an arbitrary wavefront at a desired 2D plane or 3D volume by beam shaping an emitted field and is a relatively new technique in the field of biomedical applications. Acoustic holography was first theorized in 1985 following Gabor's work in creating optical holograms in the 1940s. Recent developments in 3D printing have led to an easier and faster way to manufacture monolithic acoustic holographic lenses that can be attached to single-element transducers. As ultrasound passes through the lens material, a phase shift is applied to the waves, causing an interference pattern at the 2D image plane or 3D volume, which forms the desired pressure field. This technology has many applications already in use and has become of increasing interest for the biomedical community, particularly for treating regions that are notoriously difficult to operate on, such as the brain. Acoustic holograms could provide a non-invasive, precise, and patient specific way to deliver drugs, induce hyperthermia, or create tissue cell patterns. However, there are still limitations in acoustic holography, such as the difficulties in creating 3D holograms and the passivity of monolithic lenses. This review aims to outline the biomedical applications of acoustic holograms reported to date and discuss their current limitations and the future work that is needed for them to reach their full potential in the biomedical community.

Overview of Therapeutic Ultrasound Applications and Safety Considerations: 2024 Update

A 2012 review of therapeutic ultrasound was published to educate researchers and physicians on potential applications and concerns for unintended bioeffects (doi: 10.7863/jum.2012.31.4.623). This review serves as an update to the parent article, highlighting advances in therapeutic ultrasound over the past 12 years. In addition to general mechanisms for bioeffects produced by therapeutic ultrasound, current applications, and the pre-clinical and clinical stages are outlined. An overview is provided for image guidance methods to monitor and assess treatment progress. Finally, other topics relevant for the translation of therapeutic ultrasound are discussed, including computational modeling, tissue-mimicking phantoms, and quality assurance protocols.

Focused Ultrasound: Noninvasive Image-Guided Therapy

ABSTRACT

Invasive open surgery used to be compulsory to access tumor mass to perform excision or resection. Development of minimally invasive laparoscopic procedures followed, as well as catheter-based approaches, such as stenting, endovascular surgery, chemoembolization, brachytherapy, which minimize side effects and reduce the risks to patients. Completely noninvasive procedures bring further benefits in terms of reducing risk, procedure time, recovery time, potential of infection, or other side effects. Focusing ultrasound waves from the outside of the body specifically at the disease site has proven to be a safe noninvasive approach to localized ablative hyperthermia, mechanical ablation, and targeted drug delivery. Focused ultrasound as a medical intervention was proposed decades ago, but it only became feasible to plan, guide, monitor, and control the treatment procedures with advanced radiological imaging capabilities. The purpose of this review is to describe the imaging capabilities and approaches to perform these tasks, with the emphasis on magnetic resonance imaging and ultrasound. Some procedures already are in clinical practice, with more at the clinical trial stage. Imaging is fully integrated in the workflow and includes the following: (1) planning, with definition of the target regions and adjacent organs at risk; (2) real-time treatment monitoring via thermometry imaging, cavitation feedback, and motion control, to assure targeting and safety to adjacent normal tissues; and (3) evaluation of treatment efficacy, via assessment of ablation and physiological parameters, such as blood supply. This review also focuses on sonosensitive microparticles and nanoparticles, such as microbubbles injected in the bloodstream. They enable ultrasound energy deposition down to the microvascular level, induce vascular inflammation and shutdown, accelerate clot dissolution, and perform targeted drug delivery interventions, including focal gene delivery. Especially exciting is the ability to perform noninvasive drug delivery via opening of the blood-brain barrier at the desired areas within the brain. Overall, focused ultrasound under image guidance is rapidly developing, to become a choice noninvasive interventional radiology tool to treat disease and cure patients.

Histotripsy of the Proximal Ureter and Renal Pelvis: Evaluation of Urothelial Injury in a Porcine Survival Model

PURPOSE

To evaluate the response of the ureter and renal pelvis to direct targeting by histotripsy guided by cone-beam computed tomography (CT) in a human-scale porcine chronic-survival model.

MATERIALS AND METHODS

Bilateral ureteral histotripsy treatments were completed on 6 female swine (n = 12). Animals were divided into 2 groups: (a) acute (n = 2 animals, 4 treatments, sacrificed at Day 0) and (b) chronic (n = 4 animals, 8 treatments, sacrificed at Day 7 [n = 2] and Day 28 [n = 2]). For each treatment, a 2.5-cm sphere (ureter/renal pelvis and renal parenchyma) was targeted using cone-beam CT guidance. CT urography imaging was performed immediately after treatment for all animals, and on Days 7 and 28 for chronic animals, followed by sacrifice, necropsy, and histopathology. Serum chemistries were drawn before treatment and at Days 7 and 28.

RESULTS

All 12 treatments were successful in targeting the renal pelvis/ureter and renal parenchyma. CT urography findings at Day 0 included ureteral thickening (9/12), delayed parenchymal enhancement (3/12), and mild hydronephrosis (5/12), all resolving by Day 7. Histologic findings of low-grade damage resolved by Day 7. No urine leaks or ureteral strictures were observed. Renal function (creatinine and estimated glomerular filtration rate) remained within the normal range throughout the study.

CONCLUSIONS

Histotripsy treatment of the ureter and renal pelvis results in transient injuries, suggesting that treatment of central renal tumors adjacent to the ureter and renal pelvis is safe. The results of this study could help expand the range of renal tumors that can be treated with histotripsy.

Percutaneous Image-Guided Ablation of Renal Cancer: Traditional and Emerging Indications, Energy Sources, Techniques, and Future Developments

Percutaneous image-guided ablation (IGA) has emerged as an established alternative to surgical management for small renal masses. This comprehensive review examines traditional and emerging indications, energy sources, techniques, and future developments in IGA for renal cancer treatment. Traditionally, IGA has been indicated for frail or comorbid patients, those with solitary kidneys or chronic kidney disease, and those with histologically proven renal cell carcinomas less than 4 cm in size. Recent evidence supports expanding these indications to include T1b or T2 tumours and hereditary or recurrent renal cell carcinomas. The use of IGA combined with pre-ablation transarterial embolisation is discussed herein. This review then explores traditional energy sources including radiofrequency ablation, cryoablation, and microwave ablation, highlighting their respective advantages and limitations. Emerging technologies such as irreversible electroporation and histotripsy, as promising alternatives, are then presented, highlighting their advantage of being able to treat tumours near critical structures. Future research priorities highlight the need to establish high-quality evidence through innovative trial designs, as well as taking patient-reported outcome measures into account. Health economic considerations are key to ensuring that ablation therapies are cost-effective. The integration of artificial intelligence and radiomics shows vast potential for improving patient selection and treatment outcomes. Additionally, the immunomodulatory effects of ablative therapies suggest possible synergistic benefits when combined with immunotherapy which also require exploration in future research. Technological advancement and research developments will continue to broaden the role of IGA in clinical practice.

Unravelling the dynamics of coated nanobubbles and low frequency ultrasound using the Blake threshold and modified surface tension model

Objective.To develop a model that accurately describes the behavior of nanobubbles (NBs) under low-frequency ultrasound (US) insonation (<250 kHz), addressing the limitations of existing numerical models, such as the Marmottant model and Blake's Threshold model, in predicting NB behavior.Approach.A modified surface tension model, derived from empirical data, was introduced to capture the surface tension behavior of NBs as a function of bubble radius. This model was integrated into the Marmottant framework and combined with the Blake threshold to predict cavitation thresholds at low pressures, providing a comprehensive approach to understanding NB dynamics.Main results.Experimentally, inertial cavitation for NBs with a radius of 85 nm was observed at peak negative pressures of 200 kPa at 80 kHz and 1000 kPa at 250 kHz. The Marmottant model significantly overestimated these thresholds (1600 kPa). The modified surface tension model improved predictions at 250 kHz, while combining it with the Blake threshold accurately aligned cavitation thresholds at both frequencies (∼150 kPa at low pressures) with experimental results.Significance.This work bridges a critical gap in understanding the acoustic behavior of NBs at low US frequencies and offers a new theoretical framework for predicting cavitation thresholds of NBs at low US frequencies, advancing their application in biomedical US technologies.

Diagnostic ultrasound enhances, then reduces, exogenously induced brain activity of mice

Transcranially delivered diagnostic ultrasound (tDUS) applied to the human brain can modulate those brains such that they became more receptive to external stimulation relative to sham ultrasound exposure. Here, we sought to directly measure the effect of tDUS on mouse brain activity subjected to an external stimulation-a blinking light. Using electrocorticography, we observed a substantial increase in median brain activity due to tDUS plus a blinking light relative to baseline and relative to sham tDUS plus a blinking light. Subsequent brain activity decreased after cessation of tDUS but with continuation of the blinking light, though it remained above that demonstrated by mice exposed to only a blinking light. In a separate experiment, we showed that tDUS alone, without a blinking light, had no observable effect on median brain activity, but upon its cessation, brain activity decreased. These results demonstrate that simultaneous exposure to tDUS and blinking light can increase the receptivity of the visual cortex of mice exposed to that light, and that prior exposure to tDUS can reduce subsequent brain activity. In each case, these results are consistent with published data. Our results on mice echo published human results but do not directly explain them, since their test subjects received less intense diagnostic ultrasound than did our mice. Given the near ubiquity of diagnostic ultrasound systems, further progress along this line of research could one day lead to the widespread use of diagnostic ultrasound to intentionally modulate human brain function during exogenous stimulation.

Transcranial MRI-guided Histotripsy Targeting Using MR-thermometry and MR-ARFI

OBJECTIVE

Transcranial magnetic resonance imaging (MRI)-guided histotripsy has been demonstrated to treat various locations in in vivo swine brain through a human skull. To ensure that the histotripsy treatment is delivered to the intended target location, accurate pre-treatment targeting is necessary. In this work, we investigate the feasibility of MR-thermometry and MR-acoustic radiation force imaging (MR-ARFI) to perform pre-treatment targeting of histotripsy in ex vivo bovine brain through a human skull.

METHODS

A 700 kHz, 128-element MR-compatible histotripsy array was used to generate histotripsy and tone-burst sonications. The array's electronic drivers were modified to also generate low-amplitude tone-burst sonications to perform MR-thermometry and MR-ARFI-based targeting. Twelve ex vivo bovine brains were treated with histotripsy at 35 MPa, 75 MPa and through a skull at 36 MPa. Before treating the tissue, both MR-ARFI and MR-thermometry were used to estimate the lesion location. Finally, the location of the histotripsy lesion was compared with the focus estimated by MR-thermometry and MR-ARFI.

RESULTS

MR-thermometry and MR-ARFI were able to successfully perform pre-treatment targeting of histotripsy using the modified histotripsy array driver. Histotripsy focus was estimated with mean absolute errors along the transverse/longitudinal axis of 2.06/2.95 mm and 2.13/2.51 mm for MR-ARFI and MR-thermometry, respectively. The presence of the human skull reduced the pressure at the focal region, but it did not compromise the targeting accuracy of either of the two methods with a mean absolute error of 1.10/2.91 mm and 1.29/2.91 mm for MR-ARFI and MR-thermometry, respectively.

CONCLUSION

This study demonstrated that transcranial histotripsy pre-treatment targeting is feasible with MR-thermometry and MR-ARFI.

Assessment of Catheter-Directed Thrombolysis and Histotripsy Treatment for Deep Vein Thrombosis

PURPOSE

The hypothesis of this study was that histotripsy, an ultrasound therapy that disrupts tissue mechanically through the action of bubble clouds, increases the short-term rate of acute thrombus clearance for catheter-directed thrombolysis (CDT) in an animal model.

MATERIALS AND METHODS

Thrombi formed in the femoral vein of pigs were treated with CDT, histotripsy, or CDT and histotripsy (histotripsy+). Ultrasound (B-mode and color Doppler) and contrast fluoroscopy imaging data were scored by 4 observers for semiquantitative evaluation of each arm with ordinal regression models. Further, B-mode images were manually annotated by 3 observers to quantify the thrombus clearance rate.

RESULTS

A total of 27 thrombi (2.0 cm [SD ± 0.4] in length) in 27 animals were considered in this study (N = 8 for CDT, N = 9 for histotripsy, and N = 10 for histotripsy+). The mean treatment duration was 20.2 minutes (SD ± 1.3). The ordinal regression models indicated that the thrombus clearance rate increased for histotripsy+ relative to CDT based on B-mode and color Doppler but not fluoroscopy (P = .015, P = .001, and P = .900, respectively). Manual annotation of B-mode images denoted that histotripsy+ had an increased thrombus clearance rate relative to CDT and histotripsy (P = .001 and P = .022, respectively). Petechial hemorrhage was present in the perivascular soft tissue for 2 cases with histotripsy and 1 case with histotripsy+.

CONCLUSIONS

The clearance of acute thrombus was similar for treatment with CDT or histotripsy. Combining these individual approaches further increased the rate of thrombus clearance based on multiple imaging metrics.

Precision oncology: The role of minimally-invasive ablation therapy in the management of solid organ tumors

Solid organ tumors present a significant healthcare challenge, both economically and logistically, due to their high incidence and treatment complexity. In 2023, out of the 1.9 million new cancer cases in the United States, over 73% were solid organ tumors. Ablative therapies offer minimally invasive solutions for malignant tissue destruction in situ, often with reduced cost and morbidity compared to surgical resection. This review examines the current Food and Drug Administration-approved locoregional ablative therapies (radiofrequency, microwave, cryogenic, high-intensity focused ultrasound, histotripsy) and their evolving role in cancer care. Data were collected through a comprehensive survey of the PubMed-indexed literature on tumor ablation techniques, their clinical indications, and outcomes. Over time, emerging clinical data will help establish these therapies as the standard of care in solid organ tumor treatment, supported by improved long-term outcomes and progression-free survival.

JR, Xu Z, Wang S. Ultrasound-Guided Histotripsy Triggers the Release of Tumor-Associated Antigens from Breast Cancers

Background/Objectives: There is increasing evidence to indicate that histotripsy treatment can enhance the host anti-tumor immune responses both locally at the targeting tumor site as well as systemically from abscopal effects. Histotripsy is a non-invasive ultrasound ablation technology that mechanically disrupts target tissue via cavitation. A key factor contributing to histotripsy-induced abscopal effects is believed to be the release of tumor-specific antigens (TSAs) or tumor-associated antigens (TAAs) that induce a systemic immune response. In this study, we studied the effect of histotripsy treatment on the release of HER2, a well-defined TAA target for cancer immunotherapy. Methods: A range of doses of histotripsy administered to HER2-postive mammary tumor cells in an in vitro cell culture system and an ex vivo tumor were applied. In addition, a single dose of histotripsy was used for an in vivo murine tumor model. The released proteins, and specifically HER2, in both tumor cell-free supernatants and tumor cell pellets were analyzed by a BCA protein assay, an ultra-performance liquid chromatography (UPLC) assay, and Western blot. Results: Our results showed that histotripsy could significantly trigger the release of HER2 proteins in the current study. The level of HER2 proteins was actually higher in tumor cell-free supernatants than in tumor cell pellets, suggesting that HER2 was released from the intracellular domain into the extracellular compartment. Furthermore, proportionally more HER2 protein was released at higher histotripsy doses, indicating free HER2 was histotripsy-dose-dependent. Conclusions: In conclusion, we have qualitatively and quantitatively demonstrated that histotripsy treatment triggers the release of HER2 from the tumor cells into the extracellular compartment. The histotripsy-mediated release of HER2 antigens provides important insights into the mechanism underlying its immunostimulation and suggests the potential of TSA/TAA-based immunotherapies in numerous cancer types.

Focused Ultrasound in Cancer Immunotherapy: A Review of Mechanisms and Applications

Ultrasound is well-perceived for its diagnostic application. Meanwhile, ultrasound, especially focused ultrasound (FUS), has also demonstrated therapeutic capabilities, such as thermal tissue ablation, hyperthermia, and mechanical tissue ablation, making it a viable therapeutic approach for cancer treatment. Cancer immunotherapy is an emerging cancer treatment approach that boosts the immune system to fight cancer, and it has also exhibited enhanced effectiveness in treating previously considered untreatable conditions. Currently, cancer immunotherapy is regarded as one of the four pillars of cancer treatment because it has fewer adverse effects than radiation and chemotherapy. In recent years, the unique capabilities of FUS in ablating tumors, regulating the immune system, and enhancing anti-tumor responses have resulted in a new field of research known as FUS-induced/assisted cancer immunotherapy. In this work, we provide a comprehensive overview of this new research field by introducing the basics of focused ultrasound and cancer immunotherapy and providing the state-of-the-art applications of FUS in cancer immunotherapy: the mechanisms and preclinical and clinical studies. This review aims to offer the scientific community a reliable reference to the exciting field of FUS-induced/assisted cancer immunotherapy, hoping to foster the further development of related technology and expand its medical applications.

Self-Sensing Cavitation Detection for Pulsed Cavitational Ultrasound Therapy

OBJECTIVES

Monitoring cavitation during ultrasound therapy is crucial for assessing the procedure safety and efficacy. This work aims to develop a self-sensing and low-complexity approach for robust cavitation detection in moving organs such as the heart.

METHODS

An analog-to-digital converter was connected onto one channel of the therapeutic transducer from a clinical system dedicated to cardiac therapy, allowing to record signals on a computer. Acquisition of successive echoes backscattered by the cavitation cloud on the therapeutic transducer was performed at a high repetition rate. Temporal variations of the backscattered echoes were analyzed with a Singular-Value Decomposition filter to discriminate signals associated to cavitation, based on its stochastic nature. Metrics were derived to classify the filtered backscattered echoes. Classification of raw backscattered echoes was also performed with a machine learning approach. The performances were evaluated on 155 in vitro acquisitions and 110 signals acquired in vivo during transthoracic cardiac ultrasound therapy on 3 swine.

RESULTS

Cavitation detection was achieved successfully in moving tissues with high signal to noise ratio in vitro (cSNR = 25±5) and in vivo (cSNR = 20±6) and outperformed conventional methods (cSNR = 11±6). Classification methods were validated with spectral analysis of hydrophone measurements. High accuracy was obtained using either the clutter filter-based method (accuracy of 1) or the neural network-based method (accuracy of 0.99).

CONCLUSION

Robust self-sensing cavitation detection was demonstrated to be possible with a clutter filter-based method and a machine learning approach.

SIGNIFICANCE

The self-sensing cavitation detection method enables robust, reliable and low complexity cavitation activity monitoring during ultrasound therapy.

Gold removal from e-waste using high-intensity focused ultrasound

The demand for rare and precious metals (RPMs), e.g. gold, is increasing, as these are used in the ever-increasing amount of electronics needed for technological development and digitalization. Due to their rarity, virgin mining of RPMs is becoming more difficult and expensive. At the same time, over 62Mt of e-waste is created globally each year. The high concentration of gold and other RPMs in e-waste makes it an excellent source for recycling. Unfortunately, current recycling methods need to separate the different metals and the current pyrometallurgical and hydrometallurgical processes also create toxic pollutants, large amounts of wastewater and require highly corrosive substances. Here we present a new method for gold removal for the purpose of recycling, using only water and high-intensity focused ultrasound to induce material erosion through cavitation. An 11.8MHz ultrasonic transducer is used to first image the sample to locate gold-coated pads on discarded printed circuit boards (PCBs) and subsequently to remove only the gold layer. We demonstrate that the gold removal can be controlled by the number of transmitted ultrasonic bursts and that the energy efficiency is optimal when only minute amounts of the nickel layer beneath are also removed. Removing solely the gold layer also decreases the need for further processing steps. This greener gold removal method for e-waste is therefore well aligned with, and contributing to, the United Nations Sustainable Development Goal 12: Ensure sustainable consumption and production patterns.

Revolutionizing cardiovascular care: the power of histotripsy

Histotripsy, an innovative ultrasonic technique, is poised to transform the landscape of cardiovascular disease management. This review explores the multifaceted applications of histotripsy across various domains of cardiovascular medicine. In thrombolysis, histotripsy presents a non-invasive, drug-free, and precise method for recanalizing blood vessels obstructed by clots, minimizing the risk of vessel damage and embolism. Additionally, histotripsy showcases its potential in congenital heart defect management, offering a promising alternative to invasive procedures by creating intracardiac communications noninvasively. For patients with calcified aortic stenosis, histotripsy demonstrates its effectiveness in softening calcified bioprosthetic valves, potentially revolutionizing valve interventions. In the realm of arrhythmias, histotripsy could play an important role in scar-based ventricular tachycardia ablation, eliminating channel-like isthmuses of slowly conducting myocardium. Histotripsy`s potential applications also extend to structural heart interventions, enabling the safe sectioning of basal chordae and potentially addressing mitral regurgitation. Furthermore, it showcases its versatility by safely generating ventricular septal defects, providing a non-invasive means of creating intracardiac communications in neonates with congenital heart disease. Yet, most supporting studies are in-vitro or animal studies and there are possible challenges in translating experimental data on cardiac histotripsy to the clinical level. As histotripsy continues to evolve and mature, its remarkable potential in cardiovascular disease management holds promise for improving patient outcomes and reducing the burden of invasive procedures in the field of cardiology.

An update on the role of focused ultrasound in neuro-oncology

PURPOSE OF REVIEW

Brain tumor treatment presents challenges for patients and clinicians, with prognosis for many of the most common brain tumors being poor. Focused ultrasound (FUS) can be deployed in several ways to circumvent these challenges, including the need to penetrate the blood-brain barrier and spare healthy brain tissue. This article reviews current FUS applications within neuro-oncology, emphasizing ongoing or recently completed clinical trials.

RECENT FINDINGS

Most clinical interest in FUS for neuro-oncology remains focused on exploring BBB disruption to enhance the delivery of standard-of-care therapeutics. More recently, the application of FUS for radiosensitization, liquid biopsy, and sonodynamic therapy is garnering increased clinical attention to assist in tumor ablation, early detection, and phenotypic diagnosis. Preclinical studies show encouraging data for the immunomodulatory effects of FUS, but these findings have yet to be tested clinically.

SUMMARY

FUS is a burgeoning area of neuro-oncology research. Data from several forthcoming large clinical trials should help clarify its role in neuro-oncology care.

Development of an Injectable Hydrogel for Histotripsy Ablation Toward Future Glioblastoma Therapy Applications

Glioblastoma (GBM) is the most common and malignant type of primary brain tumor. Even after surgery and chemoradiotherapy, residual GBM cells can infiltrate the healthy brain parenchyma to form secondary tumors. To mitigate GBM recurrence, we recently developed an injectable hydrogel that can be crosslinked in the resection cavity to attract, collect, and ablate residual GBM cells. We previously optimized a thiol-Michael addition hydrogel for physical, chemical, and biological compatibility with the GBM microenvironment and demonstrated CXCL12-mediated chemotaxis can attract and entrap GBM cells into this hydrogel. In this study, we synthesize hydrogels under conditions mimicking GBM resection cavities and assess feasibility of histotripsy to ablate hydrogel-encapsulated cells. The results showed the hydrogel synthesis was bio-orthogonal, not shear-thinning, and can be scaled up for injection into GBM resection mimics in vitro. Experiments also demonstrated ultrasound imaging can distinguish the synthetic hydrogel from healthy porcine brain tissue. Finally, a 500 kHz transducer applied focused ultrasound treatment to the synthetic hydrogels, with results demonstrating precise histotripsy bubble clouds could be sustained in order to uniformly ablate red blood cells encapsulated by the hydrogel for homogeneous, mechanical fractionation of the entrapped cells. Overall, this hydrogel is a promising platform for biomaterials-based GBM treatment.

Combined drug delivery and treatment monitoring using a single high frequency ultrasound system

Ultrasound-mediated drug delivery is typically performed using transducers with center frequencies ≤ 1 MHz to promote acoustic cavitation. Such frequencies are not commonly used for diagnostic ultrasound due to limited spatial resolution. Therefore, delivery and monitoring of therapeutic ultrasound typically requires two transducers to enable both treatment and imaging. This study investigates the feasibility of using a single commercial ultrasound imaging transducer operating at 5 MHz for both drug delivery and real-time imaging. We compared a single-transducer system (STS) at 5 MHz with a conventional dual-transducer system (DTS) using a 1.1 MHz therapeutic transducer and an imaging probe. in vitro experiments demonstrated that the STS could achieve comparable extravasation depth and area as the DTS, with higher drug deposition observed at 5 MHz. Additionally, extravasation patterns were influenced by peak negative pressure (PNP) and duty cycle, with the narrower beam width at 5 MHz offering potential advantages for targeted drug delivery. in vivo experiments in a murine bladder cancer model confirmed the efficacy of the STS for real-time imaging and drug delivery, with cavitation dose correlating with drug deposition. The results suggest that a single-transducer approach may enhance the precision and efficiency of ultrasound-mediated drug delivery, potentially reducing system complexity and cost.

Histotripsy-A Novel and Intriguing Technique of Liver Ablation

This article discusses the use of histotripsy as a noninvasive method for tumor ablation that involves mechanical destruction of tissues without the generation of heat, resulting in accurate and precise treatment without surrounding tissue damage.

Neutrophil extracellular traps in tumor progression of gynecologic cancers

This article delves into the intricate interplay between tumors, particularly gynecologic malignancies, and neutrophil extracellular traps (NETs). The relationship between tumors, specifically gynecologic malignancies, and NETs is a multifaceted and pivotal area of study. Neutrophils, pivotal components of the immune system, are tasked with combating foreign invaders. NETs, intricate structures released by neutrophils, play a vital role in combating systemic infections but also play a role in non-infectious conditions such as inflammation, autoimmune diseases, and cancer. Cancer cells have the ability to attract neutrophils, creating tumor-associated neutrophils, which then stimulate the release of NETs into the tumor microenvironment. The impact of NETs within the tumor microenvironment is profound and intricate. They play a significant role in influencing cancer development and metastasis, as well as modulating tumor immune responses. Through the release of proteases and pro-inflammatory cytokines, NETs directly alter the behavior of tumor cells, increasing invasiveness and metastatic potential. Additionally, NETs can trigger epithelial-mesenchymal transition in tumor cells, a process associated with increased invasion and metastasis. The interaction between tumors and NETs is particularly critical in gynecologic malignancies such as ovarian, cervical, and endometrial cancer. Understanding the mechanisms through which NETs operate in these tumors can offer valuable insights for the development of targeted therapeutic interventions. Researchers are actively working towards harnessing this interaction to impede tumor progression and metastasis, opening up new avenues for future treatment modalities. As our understanding of the interplay between tumors and NETs deepens, it is anticipated that novel treatment strategies will emerge, potentially leading to improved outcomes for patients with gynecologic malignancies. This article provides a comprehensive overview of the latest research findings on the interaction between NETs and cancer, particularly in gynecologic tumors, serving as a valuable resource for future exploration in this field.

MR Imaging-Guided Focused Ultrasound for Breast Tumors

Breast tumors remain a complex and prevalent health burden impacting millions of individuals worldwide. Challenges in treatment arise from the invasive nature of traditional surgery and, in malignancies, the complexity of treating metastatic disease. The development of noninvasive treatment alternatives is critical for improving patient outcomes and quality of life. This review aims to explore the advancements and applications of focused ultrasound (FUS) technology over the past 2 decades. FUS offers a promising noninvasive, nonionizing intervention strategy in breast tumors including primary breast cancer, fibroadenomas, and metastatic breast cancer.

Optimizing high-intensity focused ultrasound-induced immunogenic cell-death using passive cavitation mapping as a monitoring tool

Over the past decade, ultrasound (US) has gathered significant attention and research focus in the realm of medical treatments, particularly within the domain of anti-cancer therapies. This growing interest can be attributed to its non-invasive nature, precision in delivery, availability, and safety. While the conventional objective of US-based treatments to treat breast, prostate, and liver cancer is the ablation of target tissues, the introduction of the concept of immunogenic cell death (ICD) has made clear that inducing cell death can take different non-binary pathways through the activation of the patient's anti-tumor immunity. Here, we investigate high-intensity focused ultrasound (HIFU) to induce ICD by unraveling the underlying physical phenomena and resulting biological effects associated with HIFU therapy using an automated and fully controlled experimental setup. Our in-vitro approach enables the treatment of adherent cancer cells (B16F10 and CT26), analysis for ICD hallmarks and allows to monitor and characterize in real time the US-induced cavitation activity through passive cavitation detection (PCD). We demonstrate HIFU-induced cell death, CRT exposure, HMGB1 secretion and antigen release. This approach holds great promise in advancing our understanding of the therapeutic potential of HIFU for anti-cancer strategies.

Mathematical 3D Liver Model for Surgical versus Ablative Therapy Treatment Planning for Colorectal Liver Metastases: Recommendations from the COLLISION and COLDFIRE Trial Expert Panels

Purpose To further define anatomic criteria for resection and ablation using an expert panel-based three-dimensional liver model to objectively predict local treatment recommendations for colorectal liver metastases (CRLM). Materials and Methods This study analyzed data from participants with small CRLM (≤3 cm) considered suitable for resection, thermal ablation, or irreversible electroporation (IRE), according to a multidisciplinary expert panel, who were included in two prospective multicenter trials (COLLISION [NCT03088150] and COLDFIRE-2 [NCT02082782]) between August 2017 and June 2022. Ten randomly selected participants were used to standardize the model's Couinaud segments. CRLM coordinates were measured and plotted in the model as color-coded lesions according to the treatment recommendations. Statistical validation was achieved through leave-one-out cross-validation. Results A total of 611 CRLM in 202 participants (mean age, 63 [range, 29-87] years; 138 male and 64 female) were included. Superficially located CRLM were considered suitable for resection, whereas more deep-seated CRLM were preferably ablated, with the transition zone at a subsurface depth of 3 cm. Ninety-three percent (25 of 27) of perihilar CRLM treated with IRE were at least partially located within 1 cm from the portal triad. Use of the model correctly predicted the preferred treatment in 313 of 424 CRLM (73.8%). Conclusion The results suggest that CRLM can be defined as superficial (preferably resected) and deep-seated (preferably ablated) if the tumor center is within versus beyond 3 cm from the liver surface, respectively, and as perihilar if the tumor margins extend to within 1 cm from the portal triad. Keywords: Ablation Techniques, CT, MRI, Liver, Abdomen/GI, Metastases, Oncology Supplemental material is available for this article. © RSNA, 2024.

Future Directions of MR-guided Focused Ultrasound

MR-guided focused ultrasound (MRgFUS) allows for the incisionless treatment of intracranial lesions in an outpatient setting. While this is currently approved for the surgical treatment of essential tremor and Parkinson's disease, advancements in imaging and ultrasound technology are allowing for the expansion of treatment indications to other intracranial diseases. In addition, these advancements are also making MRgFUS treatments easier, safer, and more efficacious.

Challenges in classifying cavitation: Correlating high-speed optical imaging and passive acoustic mapping of cavitation dynamics

Both the biological effects and acoustic emissions generated by cavitation are functions of bubble dynamics. Monitoring of acoustic emissions is therefore desirable to improve treatment safety and efficacy. The relationship between the emission spectra and bubble dynamics is, however, complex. The aim of this study was to characterise this relationship for single microbubbles using simultaneous ultra-high-speed optical imaging and passive acoustic mapping of cavitation emissions. As expected, both the number of discrete harmonics and broadband content in the emissions increased with increasing amplitude of bubble oscillation, but the spectral content was also dependent upon other variables, including the frequency of bubble collapse and receiving transducer characteristics. Moreover, phenomena, such as fragmentation and microjetting, could not be distinguished from spherical oscillations when using the full duration acoustic waveform to calculate the emission spectra. There was also no correlation between the detection of broadband noise and widely used thresholds for distinguishing bubble dynamics. It is therefore concluded that binary categorisations, such as stable and inertial cavitation, should be avoided, and different types of bubble behavior should not be inferred on the basis of frequency content alone. Treatment monitoring criteria should instead be defined according to the relevant bioeffect(s) for a particular application.

MR Imaging-Guided Focused Ultrasound-Clinical Applications in Managing Malignant Gliomas

Malignant gliomas (MGs) are the most common primary brain tumors in adults. Despite recent advances in understanding the biology and potential therapeutic vulnerabilities of MGs, treatment options remain limited as the delivery of drugs is often impeded by the blood-brain barrier (BBB), and safe, complete surgical resection may not always be possible, especially for deep-seated tumors. In this review, the authors highlight emerging applications for MR imaging-guided focused ultrasound (MRgFUS) as a noninvasive treatment modality for MGs. Specifically, the authors discuss MRgFUS's potential role in direct tumor cell killing, opening the BBB, and modulating antitumor immunity.

Rationally Designed Acoustic Holograms for Uniform Nanodroplet-Mediated Tissue Ablation

Nanodroplets (NDs) are phase-changing agents that have shown great potential for ultrasound (US) applications. When US is applied, NDs can undergo a phase transition into gas bubbles, enabling cavitation that can be used to reduce the pressure threshold required for mechanical ablation of tissues. Effective tissue fractionation depends on precise vaporization to achieve uniform and predictable bubble formation. This study aimed to optimize ND vaporization using acoustic holograms for improved ND-mediated histotripsy. Tissue ablation was conducted using a two-step approach, where a rotating imaging probe was used for ND vaporization followed by low-frequency US for detonation. We developed and validated three distinct acoustic hologram patterns targeting different regions within a circular area through simulations and experiments. Using custom-made gelatin phantoms designed for optimal ND vaporization imaging, the superpositioned patterns demonstrated significantly more uniform ND vaporization compared to standard single-focus steering, with ND coverage reaching % for the optimized vaporization approach versus % for the single focus steering. Ex vivo chicken liver experiments confirmed the enhanced efficiency of the optimized approach, resulting in significantly larger and more uniform lesion areas. Lesion areas generated by 120 s of treatment reached mm2 compared to mm2 for the standard approach, a 5.1-fold increase. These findings suggest that using acoustic holograms can improve ND vaporization uniformity and enhance the homogeneity of tissue fractionation, thereby potentially enhancing therapeutic outcomes.

Early-Stage Renal Cell Carcinoma Locoregional Therapies: Current Approaches and Future Directions

Renal cell carcinoma (RCC) is the most common primary renal malignancy. Prevalence of RCC in developed countries has slowly increased. Although partial or total nephrectomy has been the first-line treatment for early-stage RCC, improved or similar safety and treatment outcomes with locoregional therapies have challenged this paradigm. In this review, we explore locoregional techniques for early-stage RCC, including radiofrequency ablation, cryoablation, and microwave ablation with a focus on procedural technique, patient selection, and safety/treatment outcomes. Furthermore, we discuss future advances and novel techniques, including radiomics, combination therapy, high-intensity focused ultrasound, and catheter-directed techniques.

Ultrasound-Guided Mechanical High-Intensity Focused Ultrasound (Histotripsy) Through an Acoustically Permeable Polyolefin-Based Cranioplasty Device

Histotripsy is a non-thermal focused ultrasound therapy in development for the non-invasive ablation of cancerous tumors. Intracranial histotripsy has been limited by significant pressure attenuation through the skull, requiring large, complex array transducers to overcome this effect.

OBJECTIVE

Recently, a biocompatible, polyolefin-based cranioplasty device was developed to allow ultrasound (US) transmission into the intracranial space with minimal distortion. In this study, we investigated the in vitro feasibility of applying US-guided histotripsy procedures across the prosthesis.

METHODS

Pressure waveforms and beam profiles were collected for single- and multi-element histotripsy transducers. Then, high-speed optical images of the bubble cloud with and without the prosthesis were collected in water and tissue-mimicking agarose gel phantoms. Finally, red blood cell (RBC) tissue phantom and excised brain tissue experiments were completed to test the ablative efficacy across the prosthesis.

RESULTS

Single element tests revealed increased pressure loss with increasing transducer frequency and increasing transducer-to-prosthesis angle. Array transducer measurements at 1 MHz showed average pressure losses of >50% across the prosthesis. Aberration correction recovered up to 18% of the pressure lost, and high-speed optical imaging in water, agarose gels, and RBC phantoms demonstrated that histotripsy bubble clouds could be generated across the prosthesis at pulse repetition frequencies of 50-500 Hz. Histologic analysis revealed a complete breakdown of brain tissue treated across the prosthesis. Conclusion & Significance: Overall, the results of this study demonstrate that the cranial prosthesis may be used as an acoustic window through which intracranial histotripsy can be applied under US guidance without the need for large transcranial array transducers.

The Role of Surgery in "Oligometastatic" Pancreas Cancer

The majority of patients diagnosed with pancreatic cancer already have metastatic disease at the time of presentation, which results in a 5-year survival rate of only 13%. However, multiagent chemotherapy regimens can stabilize the disease in select patients with limited metastatic disease. For such patients, a combination of curative-intent therapy and systemic therapy may potentially enhance outcomes compared to using systemic therapy alone. Of note, the evidence supporting this approach is primarily derived from retrospective studies and may carry a significant selection bias. Looking ahead, ongoing prospective trials are exploring the efficacy of curative-intent therapy in managing oligometastatic pancreatic cancer and the implementation of treatment strategies based on specific biomarkers. The emergence of these trials, coupled with the development of less invasive therapeutic modalities, provides hope for patients with oligometastatic pancreatic cancer.

Oncologic Outcomes after Percutaneous Ablation for Colorectal Liver Metastases: An Updated Comprehensive Review

Colorectal cancer is a major cause of cancer-related mortality, with liver metastases occurring in over a third of patients, and is correlated with poor prognosis. Despite surgical resection being the primary treatment option, only about 20% of patients qualify for surgery. Current guidelines recommend thermal ablation either alone or combined with surgery to treat limited hepatic metastases, provided that all visible disease can be effectively eradicated. Several ablation modalities, including radiofrequency ablation, microwave ablation, cryoablation, irreversible electroporation and histotripsy, are part of the percutaneous ablation armamentarium. Thermal ablation, including radiofrequency, microwave ablation and cryoablation, can offer local tumor control rates comparable to limited resection for selected tumors that can be ablated with margins. This review aims to encapsulate the current clinical evidence regarding the efficacy and oncologic outcomes after percutaneous ablation for the treatment of colorectal liver metastatic disease.

Practical Targeting Errors During Optically Tracked Transcranial Focused Ultrasound Using MR-ARFI and Array- Based Steering

OBJECTIVE

Transcranial focused ultrasound (tFUS) is being explored for neuroscience research and clinical applications due to its ability to affect precise brain regions noninvasively. The ability to target specific brain regions and localize the beam during these procedures is important for these applications to avoid damage and minimize off-target effects. Here, we present a method to combine optical tracking with magnetic resonance (MR) acoustic radiation force imaging to achieve targeting and localizing of the tFUS beam. This combined method provides steering coordinates to target brain regions within a clinically practical time frame.

METHODS

Using an optically tracked hydrophone and bias correction with MR imaging we transformed the FUS focus coordinates into the MR space for targeting and error correction. We validated this method in vivo in 18 macaque FUS studies.

RESULTS

Across these in vivo studies a single localization scan allowed for the average targeting error to be reduced from 4.8 mm to 1.4 mm and for multiple brain regions to be targeted with one transducer position.

CONCLUSIONS

By reducing targeting error and providing the means to target multiple brain regions within a single session with high accuracy this method will allow further study of the effects of tFUS neuromodulation with more advanced approaches such as simultaneous dual or multi-site brain stimulation.

Mid-Infrared Photoacoustic Stimulation of Neurons through Vibrational Excitation in Polydimethylsiloxane

Photoacoustic (PA) emitters are emerging ultrasound sources offering high spatial resolution and ease of miniaturization. Thus far, PA emitters rely on electronic transitions of absorbers embedded in an expansion matrix such as polydimethylsiloxane (PDMS). Here, it is shown that mid-infrared vibrational excitation of C─H bonds in a transparent PDMS film can lead to efficient mid-infrared photoacoustic conversion (MIPA). MIPA shows 37.5 times more efficient than the commonly used PA emitters based on carbon nanotubes embedded in PDMS. Successful neural stimulation through MIPA both in a wide field with a size up to a 100 µm radius and in single-cell precision is achieved. Owing to the low heat conductivity of PDMS, less than a 0.5 °C temperature increase is found on the surface of a PDMS film during successful neural stimulation, suggesting a non-thermal mechanism. MIPA emitters allow repetitive wide-field neural stimulation, opening up opportunities for high-throughput screening of mechano-sensitive ion channels and regulators.

The #HOPE4LIVER Single-Arm Pivotal Trial for Histotripsy of Primary and Metastatic Liver Tumors

Background Histotripsy is a nonthermal, nonionizing, noninvasive, focused US technique that relies on cavitation for mechanical tissue breakdown at the focal point. Preclinical data have shown its safety and technical success in the ablation of liver tumors. Purpose To evaluate the safety and technical success of histotripsy in destroying primary or metastatic liver tumors. Materials and Methods The parallel United States and European Union and England #HOPE4LIVER trials were prospective, multicenter, single-arm studies. Eligible patients were recruited at 14 sites in Europe and the United States from January 2021 to July 2022. Up to three tumors smaller than 3 cm in size could be treated. CT or MRI and clinic visits were performed at 1 week or less preprocedure, at index-procedure, 36 hours or less postprocedure, and 30 days postprocedure. There were co-primary end points of technical success of tumor treatment and absence of procedure-related major complications within 30 days, with performance goals of greater than 70% and less than 25%, respectively. A two-sided 95% Wilson score CI was derived for each end point. Results Forty-four participants (21 from the United States, 23 from the European Union or England; 22 female participants, 22 male participants; mean age, 64 years ± 12 [SD]) with 49 tumors were enrolled and treated. Eighteen participants (41%) had hepatocellular carcinoma and 26 (59%) had non-hepatocellular carcinoma liver metastases. The maximum pretreatment tumor diameter was 1.5 cm ± 0.6 and the maximum post-histotripsy treatment zone diameter was 3.6 cm ± 1.4. Technical success was observed in 42 of 44 treated tumors (95%; 95% CI: 84, 100) and procedure-related major complications were reported in three of 44 participants (7%; 95% CI: 2, 18), both meeting the performance goal. Conclusion The #HOPE4LIVER trials met the co-primary end-point performance goals for technical success and the absence of procedure-related major complications, supporting early clinical adoption. Clinical trial registration nos. NCT04572633, NCT04573881 Published under a CC BY 4.0 license. Supplemental material is available for this article. See also the editorial by Nezami and Georgiades in this issue.

Therapeutic ultrasound transducer technology and monitoring techniques: a review with clinical examples

The exponential growth of therapeutic ultrasound applications demonstrates the power of the technology to leverage the combinations of transducer technology and treatment monitoring techniques to effectively control the preferred bioeffect to elicit the desired clinical effect.Objective: This review provides an overview of the most commonly used bioeffects in therapeutic ultrasound and describes existing transducer technologies and monitoring techniques to ensure treatment safety and efficacy.Methods and materials: Literature reviews were conducted to identify key choices that essential in terms of transducer design, treatment parameters and procedure monitoring for therapeutic ultrasound applications. Effective combinations of these options are illustrated through descriptions of several clinical indications, including uterine fibroids, prostate disease, liver cancer, and brain cancer, that have been successful in leveraging therapeutic ultrasound to provide effective patient treatments.Results: Despite technological constraints, there are multiple ways to achieve a desired bioeffect with therapeutic ultrasound in a target tissue. Visualizations of the interplay of monitoring modality, bioeffect, and applied acoustic parameters are presented that demonstrate the interconnectedness of the field of therapeutic ultrasound. While the clinical indications explored in this review are at different points in the clinical evaluation path, based on the ever expanding research being conducted in preclinical realms, it is clear that additional clinical applications of therapeutic ultrasound that utilize a myriad of bioeffects will continue to grow and improve in the coming years.Conclusions: Therapeutic ultrasound will continue to improve in the next decades as the combination of transducer technology and treatment monitoring techniques will continue to evolve and be translated in clinical settings, leading to more personalized and efficient therapeutic ultrasound mediated therapies.

Histotripsy and Catheter-Directed Lytic: Efficacy in Highly Retracted Porcine Clots In Vitro

OBJECTIVE

Standard treatment for deep vein thrombosis (DVT) involves catheter-directed anticoagulants or thrombolytics, but the chronic thrombi present in many DVT cases are often resistant to this therapy. Histotripsy has been found to be a promising adjuvant treatment, using the mechanical action of cavitating bubble clouds to enhance thrombolytic activity. The objective of this study was to determine if histotripsy enhanced recombinant tissue plasminogen activator (rt-PA) thrombolysis in highly retracted porcine clots in vitro in a flow model of occlusive DVT.

METHODS

Highly retracted porcine whole blood clots were treated for 1 h with either catheter-directed saline (negative control), rt-PA (lytic control), histotripsy, DEFINITY and histotripsy or the combination of rt-PA and histotripsy with or without DEFINITY. Five-cycle, 1.5 MHz histotripsy pulses with a peak negative pressure of 33.2 MPa and pulse repetition frequency of 40 Hz were applied along the clot. B-Mode and passive cavitation images were acquired during histotripsy insonation to monitor bubble activity.

RESULTS

Clots subjected to histotripsy with and without rt-PA exhibited greater thrombolytic efficacy than controls (7.0% flow recovery or lower), and histotripsy with rt-PA was more efficacious than histotripsy with saline (86.1 ± 10.2% compared with 61.7 ± 19.8% flow recovery). The addition of DEFINITY to histotripsy with or without rt-PA did not enhance either thrombolytic efficacy or cavitation dose. Cavitation dose generally did not correlate with thrombolytic efficacy.

CONCLUSION

Enhancement of thrombolytic efficacy was achieved using histotripsy, with and without catheter-directed rt-PA, in the presence of physiologic flow. This suggests these treatments may be effective as therapy for DVT.

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.

Histotripsy - hype or hope? Review of innovation and future implications

BACKGROUND

Histotripsy is a novel, ultrasound-based ablative technique that was recently approved by the Food and Drug Administration for hepatic targets. It has several promising additional theoretical applications that need to be further investigated. Its basis as a nonthermal cavitational technology presents a unique advantage over existing thermal ablation techniques in maximizing local effects while minimizing adjacent tissue destruction. This review discusses the technical basis and current preclinical and clinical data surrounding histotripsy.

METHODS

This was a comprehensive review of the literature surrounding histotripsy and the clinical landscape of existing ablative techniques using the PubMed database. A technical summary of histotripsy's physics and cellular effect was described. Moreover, data from recent clinical trials, including Hope4Liver, and future implications regarding its application in various benign and malignant conditions were discussed.

RESULTS

Preclinical data demonstrated the efficacy of histotripsy ablation in various organ systems with minimal tissue destruction when examined at the histologic level. The first prospective clinical trial involving histotripsy in hepatocellular carcinoma and liver metastases, Hope4Liver, demonstrated a primary efficacy of 95.5% with minimal complications (6.8%). This efficacy was replicated in similar trials involving the treatment of benign prostatic hypertrophy.

DISCUSSION

In addition to the noninvasive ability to ablate lesions in the liver, histotripsy offers additional therapeutic potential. Early data suggest a potential complementary therapeutic effect when combining histotripsy with existing immunologic therapies because of the technology's theoretical ability to sensitize tumors to adaptive immunity. As with most novel therapies, the effect of histotripsy on the oncologic therapeutic landscape remains uncertain.

In Vivo Cavitation-Based Aberration Correction of Histotripsy in Porcine Liver

Histotripsy is a noninvasive ablation technique that focuses ultrasound pulses into the body to destroy tissues via cavitation. Heterogeneous acoustic paths through tissue introduce phase errors that distort and weaken the focus, requiring additional power output from the histotripsy transducer to perform therapy. This effect, termed phase aberration, limits the safety and efficacy of histotripsy ablation. It has been shown in vitro that the phase errors from aberration can be corrected by receiving the acoustic signals emitted by cavitation. For transabdominal histotripsy in vivo, however, cavitation-based aberration correction (AC) is complicated by acoustic signal clutter and respiratory motion. This study develops a method that enables robust, effective cavitation-based AC in vivo and evaluates its efficacy in the swine liver. The method begins with a high-speed pulsing procedure to minimize the effects of respiratory motion. Then, an optimal phase correction is obtained in the presence of acoustic clutter by filtering with the singular value decomposition (SVD). This AC method reduced the power required to generate cavitation in the liver by 26% on average (range: 0%-52%) and required ~2 s for signal acquisition and processing per focus location. These results suggest that the cavitation-based method could enable fast and effective AC for transabdominal histotripsy.

Pilot Study on Boiling Histotripsy Treatment of Human Leiomyoma Ex Vivo

OBJECTIVE

As an alternative to surgical excision and magnetic resonance-guided thermal high-intensity focused ultrasound ablation of uterine leiomyoma, this work was aimed at pilot feasibility demonstration of use of ultrasound-guided boiling histotripsy for non-invasive non-thermal fractionation of human uterine leiomyoma ex vivo.

METHODS

A custom-made sector ultrasound transducer of 1.5-MHz operating frequency and nominal f-number F# = 0.75 was used to produce a volumetric lesion (two layers of 5 × 5 foci with a 1 mm step) in surgically resected human leiomyoma ex vivo. A sequence of 10 ms pulses (P+/P-/As = 157/-25/170 MPa in situ) with 1% duty cycle was delivered N = 30 times per focus under B-mode guidance. The treatment outcome was evaluated via B-mode imaging and histologically with hematoxylin and eosin and Masson's trichrome staining.

RESULTS

The treatment was successfully performed in less than 30 min and resulted in formation of a rectangular lesion visualized on B-mode images during the sonication as an echogenic region, which sustained for about 10 min post-treatment. Histology revealed loss of cellular structure, necrotic debris and globules of degenerated collagen in the target volume surrounded by injured smooth muscle cells.

CONCLUSION

The pilot experiment described here indicates that boiling histotripsy is feasible for non-invasive mechanical disintegration of human uterine leiomyoma ex vivo under B-mode guidance, encouraging further investigation and optimization of this potential clinical application of boiling histotripsy.