Ultrasonic Cavitation-Enabled Treatment for Therapy of Hypertrophic Cardiomyopathy: Proof of Principle

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.

A unifying Rayleigh-Plesset-type equation for bubbles in viscoelastic media

Understanding the ultrasound pressure-driven dynamics of microbubbles confined in viscoelastic materials is relevant for multiple biomedical applications, ranging from contrast-enhanced ultrasound imaging to ultrasound-assisted drug delivery. The volumetric oscillations of spherical bubbles are analyzed using the Rayleigh-Plesset equation, which describes the conservation of mass and momentum in the surrounding medium. Several studies have considered an extension of the Rayleigh-Plesset equation for bubbles embedded into viscoelastic media, but these are restricted to a particular choice of constitutive model and/or to small deformations. Here, we derive a unifying equation applicable to bubbles in viscoelastic media with arbitrary complex moduli and that can account for large bubble deformations. To derive this equation, we borrow concepts from finite-strain theory. We validate our approach by comparing the result of our model to previously published results and extend it to show how microbubbles behave in arbitrary viscoelastic materials. In particular, we use our viscoelastic Rayleigh-Plesset model to compute the bubble dynamics in benchmarked viscoelastic liquids and solids.

Sono-activated materials for enhancing focused ultrasound ablation: Design and application in biomedicine

The ablation effect of focused ultrasound (FUS) has played an increasingly important role in the biomedical field over the past decades, and its non-invasive features have great advantages, especially for clinical diseases where surgical treatment is not available or appropriate. Recently, rapid advances in the adjustable morphology, enzyme-mimetic activity, and biostability of sono-activated materials have significantly promoted the medical application of FUS ablation. However, a systematic review of sono-activated materials based on FUS ablation is not yet available. This progress review focuses on the recent design, fundamental principles, and applications of sono-activated materials in the FUS ablation biomedical field. First, the different ablation mechanisms and the key factors affecting ablation are carefully determined. Then, the design of sono-activated materials with high FUS ablation efficiencies is comprehensively discussed. Subsequently, the representative biological applications are summarized in detail. Finally, the primary challenges and future perspectives are also outlined. We believe this timely review will provide key information and insights for further exploration of focused ultrasound ablation and new inspiration for designing future sono-activated materials. STATEMENT OF SIGNIFICANCE: The ablation effect of focused ultrasound (FUS) has played an increasingly important role in the biomedical field over the past decades. However, there are also some challenges of FUS ablation, such as skin burns, tumour recurrence after thermal ablation, and difficulty in controlling cavitation ablation. The rapid advance in adjustable morphology, enzyme-mimetic activity, and biostability of sono-activated materials has significantly promoted the medical application of FUS ablation. However, the systematic review of sono-activated materials based on FUS ablation is not yet available. This progress review focuses on the recent design, fundamental principles, and applications in the FUS ablation biomedical field of sono-activated materials. We believe this timely review will provide key information and insights for further exploration of FUS ablation.

Numerical Assessment of Combustion Behavior and Emission Formations in an Ultrasonic-Assisted Ignition Engine

By effective utilization of the dynamic mesh and coordinate transformation techniques, an ultrasonic horn is physically integrated in the chamber of an internal combustion engine. The consequences of multiple ultrasonic-fed strategies on the flow field, combustion process, and emission formation under the same working conditions are studied by numerical simulation. Based precisely on the bench test data, GT-Power and CONVERGE set up the original engine one-dimension (1d) and three-dimension (3d) simulation models. The chamber pressure and heat release rate of the 1d and 3d models under a full load condition of 3000 r·min-1 were validated, and the maximum relative error is less than 5%, proving the accuracy of the model. By reforming the 3d numerical model, ultrasonics is added to the gasoline engine's combustion chamber. Six different ultrasonic-fed schemes with 20 kHz amplitude of 30-300 μm are typically selected for in-depth research. The larger the amplitude, the stronger the turbulent kinetic energy (TKE), and the maximum TKE exceeds 46.6% at the ignition time. Stronger TKE can energetically encourage the generation of OH, O, and H radicals and improve the combustion reaction rate, and the peak pressure (PMAX) is increased by 1.9 MPa compared with scheme No. However, NOX and HC emissions gradually increase, reaching a maximum of 32.4 and 43.8%, respectively, while CO and soot emissions decrease, reaching a maximum of 11.4 and 11%, respectively. Four groups of ultrasonic-fed schemes with an amplitude of 100 μm and frequency of 20-50 kHz are scientifically studied. The findings indicated that the TKE level steadily increases as the frequency increases and the in-cylinder TKE increases by 16.4% at ignition time. The increase in ultrasonic frequency can promote the generation of active free radicals and meaningfully improve the combustion reaction rate to a certain extent. The PMAX can be increased up to 1 MPa compared with scheme No. At the same time, the NOX, HC, and soot also increased considerably, reaching 31.8, 17.9, and 21.9%, respectively. The CO showed a downward trend but gradually slowed, with a maximum decline of 6.5% at 20 kHz. The above simulation analysis is based on the full load condition of 3000 r·min-1, sufficiently proving that ultrasonics has a regulation effect on emissions and can achieve specific emissions through later optimization.

Effect of ultrasonic-fed time on combustion and emissions performance in a single-cylinder engine

In this study, a numerical simulation method for multi-field coupling is proposed in which the ultrasonic is physically fed in the combustion chamber of a gasoline engine. The fine-tuning regulation of activity and reaction paths of gas-liquid two-phase (GLP) fuel is studied by using ultrasonic under in-cylinder complex conditions. The three-dimensional (3D) computational fluid dynamics (CFD) model of the original engine is calibrated, based on the bench test data. The multi-field coupling model of the sound field and combustion field is established by embedding the feature of the sound source surface in the combustion chamber. The ultrasonic with 20 kHz frequency and 100 μm amplitude is fed into the combustion chamber by using the dynamic grid technology. By comparing the simulation results of four ultrasonic-fed schemes (S1∼S4) and ultrasonic-free scheme (No), it is concluded that compared with the No scheme, the average turbulent kinetic energy (TKE) of the schemes S1, S2, and S3 are all increased by 23.2% at the top dead center (TDC), the peak pressure of the schemes S1 and S2 are both increased by 0.58 MPa. The CO and soot formations of scheme S1 are the lowest at 6.5% and 6.1%, respectively, compared with the No scheme. The reasonable use of ultrasonic can promote the fuel oxidation and combustion process, and accelerate the formation of the OH radicals. The ultrasonic-fed has a significantly quantitative control effect on fuel activity and oxidation reaction paths within 10 ms, under the in-cylinder transient and complex combustion condition of the gasoline engine.

Tetralogy of Fallot: stent palliation or neonatal repair?

Surgical repair of Tetralogy of Fallot has excellent outcomes, with over 90% of patients alive at 30 years. The ideal time for surgical repair is between 3 and 11 months of age. However, the symptomatic neonate with Tetralogy of Fallot may require earlier intervention: either a palliative intervention (right ventricular outflow tract stent, ductal stent, balloon pulmonary valvuloplasty, or Blalock-Taussig shunt) followed by a surgical repair later on, or a complete surgical repair in the neonatal period. Indications for palliation include prematurity, complex anatomy, small pulmonary artery size, and comorbidities. Given that outcomes after right ventricular outflow tract stent palliation are particularly promising - there is low mortality and morbidity, and consistently increased oxygen saturations and increased pulmonary artery z-scores - it is now considered the first-line palliative option. Disadvantages of right ventricular outflow tract stenting include increased cardiopulmonary bypass time at later repair and the stent preventing pulmonary valve preservation. However, neonatal surgical repair is associated with increased short-term complications and hospital length of stay compared to staged repair. Both staged repair and primary repair appear to have similar long-term mortality and morbidity, but more evidence is needed assessing long-term outcomes for right ventricular outflow tract stent palliation patients.

Therapeutic oxygen delivery by perfluorocarbon-based colloids

After the protocol-related indecisive clinical trial of Oxygent, a perfluorooctylbromide/phospholipid nanoemulsion, in cardiac surgery, that often unduly assigned the observed untoward effects to the product, the development of perfluorocarbon (PFC)-based O₂ nanoemulsions ("blood substitutes") has come to a low. Yet, significant further demonstrations of PFC O₂-delivery efficacy have continuously been reported, such as relief of hypoxia after myocardial infarction or stroke; protection of vital organs during surgery; potentiation of O₂-dependent cancer therapies, including radio-, photodynamic-, chemo- and immunotherapies; regeneration of damaged nerve, bone or cartilage; preservation of organ grafts destined for transplantation; and control of gas supply in tissue engineering and biotechnological productions. PFC colloids capable of augmenting O₂ delivery include primarily injectable PFC nanoemulsions, microbubbles and phase-shift nanoemulsions. Careful selection of PFC and other colloid components is critical. The basics of O₂ delivery by PFC nanoemulsions will be briefly reminded. Improved knowledge of O₂ delivery mechanisms has been acquired. Advanced, size-adjustable O₂-delivering nanoemulsions have been designed that have extended room-temperature shelf-stability. Alternate O₂ delivery options are being investigated that rely on injectable PFC-stabilized microbubbles or phase-shift PFC nanoemulsions. The latter combine prolonged circulation in the vasculature, capacity for penetrating tumor tissues, and acute responsiveness to ultrasound and other external stimuli. Progress in microbubble and phase-shift emulsion engineering, control of phase-shift activation (vaporization), understanding and control of bubble/ultrasound/tissue interactions is discussed. Control of the phase-shift event and of microbubble size require utmost attention. Further PFC-based colloidal systems, including polymeric micelles, PFC-loaded organic or inorganic nanoparticles and scaffolds, have been devised that also carry substantial amounts of O₂. Local, on-demand O₂ delivery can be triggered by external stimuli, including focused ultrasound irradiation or tumor microenvironment. PFC colloid functionalization and targeting can help adjust their properties for specific indications, augment their efficacy, improve safety profiles, and expand the range of their indications. Many new medical and biotechnological applications involving fluorinated colloids are being assessed, including in the clinic. Further uses of PFC-based colloidal nanotherapeutics will be briefly mentioned that concern contrast diagnostic imaging, including molecular imaging and immune cell tracking; controlled delivery of therapeutic energy, as for noninvasive surgical ablation and sonothrombolysis; and delivery of drugs and genes, including across the blood-brain barrier. Even when the fluorinated colloids investigated are designed for other purposes than O₂ supply, they will inevitably also carry and deliver a certain amount of O₂, and may thus be considered for O₂ delivery or co-delivery applications. Conversely, O₂-carrying PFC nanoemulsions possess by nature a unique aptitude for ¹⁹F MR imaging, and hence, cell tracking, while PFC-stabilized microbubbles are ideal resonators for ultrasound contrast imaging and can undergo precise manipulation and on-demand destruction by ultrasound waves, thereby opening multiple theranostic opportunities.

Microbubble-Facilitated Ultrasound Catheter Ablation Causes Microvascular Damage and Fibrosis

High-intensity ultrasound (US) ablation produces deeper myocardial lesions than radiofrequency ablation. The presence of intravascular microbubble (MB) contrast agents enhances pulsed-wave US ablation via cavitation-related histotripsy, potentially facilitating ablation in persistently perfused/conducting myocardium. US ablation catheters were developed and tested in the presence of MBs using ex vivo and in vivo models. High-frame-rate videomicroscopy and US imaging of gel phantom models confirmed MB destruction by inertial cavitation. MB-facilitated US ablation in an ex vivo perfused myocardium model generated shallow (2 mm) lesions and, in an in vivo murine hindlimb model, reduced perfusion by 42% with perivascular hemorrhage and inflammation, but no myonecrosis.

Hypertrophic cardiomyopathy: the future of treatment

Hypertrophic cardiomyopathy (HCM) is a heterogeneous genetic disorder most often caused by sarcomeric mutations resulting in left ventricular hypertrophy, fibrosis, hypercontractility, and reduced compliance. It is the most common inherited monogenic cardiac condition, affecting 0.2% of the population. Whereas currently available therapies for HCM have been effective in reducing morbidity, there remain important unmet needs in the treatment of both the obstructive and non-obstructive phenotypes. Novel pharmacotherapies directly target the molecular underpinnings of HCM, while innovative procedural techniques may soon offer minimally-invasive alternatives to current septal reduction therapy. With the advent of embryonic gene editing, there now exists the potential to correct underlying genetic mutations that may result in disease. This article details the recent developments in the treatment of HCM including pharmacotherapy, septal reduction procedures, mitral valve manipulation, and gene-based therapies.

Colloids, nanoparticles, and materials for imaging, delivery, ablation, and theranostics by focused ultrasound (FUS)

This review focuses on different materials and contrast agents that sensitize imaging and therapy with Focused Ultrasound (FUS). At high intensities, FUS is capable of selectively ablating tissue with focus on the millimeter scale, presenting an alternative to surgical intervention or management of malignant growth. At low intensities, FUS can be also used for other medical applications such as local delivery of drugs and blood brain barrier opening (BBBO). Contrast agents offer an opportunity to increase selective acoustic absorption or facilitate destructive cavitation processes by converting incident acoustic energy into thermal and mechanical energy. First, we review the history of FUS and its effects on living tissue. Next, we present different colloidal or nanoparticulate approaches to sensitizing FUS, for example using microbubbles, phase-shift emulsions, hollow-shelled nanoparticles, or hydrophobic silica surfaces. Exploring the science behind these interactions, we also discuss ways to make stimulus-responsive, or "turn-on" contrast agents for improved selectivity. Finally, we discuss acoustically-active hydrogels and membranes. This review will be of interest to those working in materials who wish to explore new applications in acoustics and those in acoustics who are seeking new agents to improve the efficacy of their approaches.

Multiple ultrasound cavitation-enabled treatments for myocardial reduction

Background

Ultrasound myocardial cavitation enabled treatment (MCET) is an image-guided method for tissue reduction. In this study, a strategy of fractionated (multiple) treatments was tested for efficacy.

Methods

Dahl SS rats were anesthetized and prepared for treatment with a focused ultrasound transducer in a warm water bath. Aiming at the anterior left ventricular wall was facilitated by imaging with a 10 MHz phased array (10S, GE Vivid 7, GE Vingmed Ultrasound, Horten, Norway). MCET was accomplished at 1.5 MHz by pulse bursts of 4 MPa peak rarefactional pressure amplitude, which were intermittently triggered 1:8 from the ECG during infusion of a microbubble suspension for cavitation nucleation. Test groups were sham, a 200 s treatment, three 200 s treatments a week apart, and a 600 s treatment. Treatment outcome was observed by plasma troponin after 4 h, echocardiographic monitoring and histology at 6 wk.

Results

The impacts of the fractionated treatments summed to approximately the same as the long treatment; e. g. the troponin result was 10.5 ± 3.2 for 200 s, 22.7 ± 5.4 (p < 0.001) for the summed fractionated treatments and 29.9 ± 6.4 for 600 s (p = 0.06 relative to the summed fractionated). While wall thickness was not reduced for the fractionated treatment, tissue strain was reduced by 35% in the target area relative sham (p < 0.001).

Conclusion

The ability to fractionate treatment may be advantageous for optimizing patient outcome relative to all-or nothing therapy by surgical myectomy or alcohol ablation.