Feasibility of Targeting Atherosclerotic Plaques by High-Intensity–focused Ultrasound: An In Vivo Study

Investigating the potential of catheter-assisted pulsed focused ultrasound ablation for atherosclerotic plaques

BACKGROUND

Atherosclerosis is a condition in which an adhesive substance called plaque accumulates over time inside the arteries. Plaque buildup results in the constriction of arteries, causing a shortage of blood supply to tissues and organs. Removing atherosclerotic plaques controls the development of acute ischemic stroke and heart diseases. It remains imperative for positive patient outcomes.

PURPOSE

This study sought to develop a minimally invasive technique for removing arterial plaques by applying focused ultrasound (FUS) energy on the metal surface of a nitinol catheter wire to induce inertial cavitation. The induced cavitation can deplete plaque mechanically inside the arteries, leading towards improved recanalization of blood vessels.

METHODS

The enhanced cavitation effect induced by combining FUS with a metal catheter was first verified by exposing agar phantom gels with or without a 0.9-mm diameter nitinol wire to an acoustic field produced by a 0.5-MHz FUS transducer. The phenomenon was further confirmed in pork belly fat samples with or without a 3-mm diameter nitinol catheter wire. Cavitation was monitored by detecting the peaks of emitted ultrasound signals from the samples using a passive cavitation detector (PCD). Cavitation threshold values were determined by observing the jump in the peak amplitude of signals received by the PCD when the applied FUS peak negative pressure (PNP) increased. To simulate arterial plaque removal, FUS with or without a catheter was used to remove tissues from pork belly fat samples and the lipid cores of human atherosclerotic plaque samples using 2500-cycle FUS bursts at 10% duty cycle and a burst repetition rate of 20 Hz. Treatment outcomes were quantified by subtracting the weight of samples before treatment from the weight of samples after treatment. All measurements were repeated 5 times (n = 5) unless otherwise indicated, and paired t-tests were used to compare the means of two groups. A p-value of <0.05 will be considered significant.

RESULTS

Our results showed that with a nitinol wire, the cavitation threshold in agar phantoms was reduced to 2.6 MPa from 4.3 MPa PNP when there was no nitinol wire in the focal region of FUS. For pork belly fat samples, cavitation threshold values were 1.0 and 2.0 MPa PNP, with and without a catheter wire, respectively. Pork belly fat tissues and lipid cores of atherosclerotic plaques were depleted at the interface between a catheter and the samples at 2 and 4 MPa FUS PNP, respectively. The results showed that with a catheter wire in the focal region of a 3-min FUS treatment session, 24.7 and 25.6 mg of lipid tissues were removed from pork belly fat and human atherosclerotic samples, respectively. In contrast, the FUS-only group showed no reduction in sample weight. The differences between FUS-only and FUS-plus-catheter groups were statistically significant (p < 0.001 for the treatment on pork belly samples, and p < 0.01 for the treatment on human atherosclerotic samples).

CONCLUSION

This study demonstrated the feasibility of catheter-assisted FUS therapy for removing atherosclerotic plaques.

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.

Focused Ultrasound, an Emerging Tool for Atherosclerosis Treatment: A Comprehensive Review

Focused ultrasound (FUS) has emerged as a promising noninvasive therapeutic modality for treating atherosclerotic arterial disease. High-intensity focused ultrasound (HIFU), a noninvasive and precise modality that generates high temperatures at specific target sites within tissues, has shown promising results in reducing plaque burden and improving vascular function. While low-intensity focused ultrasound (LIFU) operates at lower energy levels, promoting mild hyperthermia and stimulating tissue repair processes. This review article provides an overview of the current state of HIFU and LIFU in treating atherosclerosis. It focuses primarily on the therapeutic potential of HIFU due to its higher penetration and ability to achieve atheroma disruption. The review summarizes findings from animal models and human trials, covering the effects of FUS on arterial plaque and arterial wall thrombolysis in carotid, coronary and peripheral arteries. This review also highlights the potential benefits of focused ultrasound, including its noninvasiveness, precise targeting, and real-time monitoring capabilities, making it an attractive approach for the treatment of atherosclerosis and emphasizes the need for further investigations to optimize FUS parameters and advance its clinical application in managing atherosclerotic arterial disease.

Immunomodulation and targeted drug delivery with high intensity focused ultrasound (HIFU): Principles and mechanisms

High intensity focused ultrasound (HIFU) is a non-invasive and non-ionizing sonic energy-based therapeutic technology for inducing thermal and non-thermal effects in tissues. Depending on the parameters, HIFU can ablate tissues by heating them to >55 °C to induce denaturation and coagulative necrosis, improve radio- and chemo-sensitizations and local drug delivery from nanoparticles at moderate hyperthermia (~41-43 °C), and mechanically fragment cells using acoustic cavitation (also known as histotripsy). HIFU has already emerged as an attractive modality for treating human prostate cancer, veterinary cancers, and neuromodulation. Herein, we comprehensively review the role of HIFU in enhancing drug delivery and immunotherapy in soft and calcified tissues. Specifically, the ability of HIFU to improve adjuvant treatments from various classes of drugs is described. These crucial insights highlight the opportunities and challenges of HIFU technology and its potential to support new clinical trials and translation to patients.

Image-Guided Measurement of Radiation Force Induced by Focused Ultrasound Beams

The radiation force balance (RFB) is a widely used method for measuring acoustic power output of ultrasonic transducers. The reflecting cone target is attractive due to its simplicity and long-term stability, at a reasonable cost. However, accurate measurements using this method depend on the alignment between the ultrasound beam and cone axes, especially for highly focused beams utilized in therapeutic applications. With the advent of dual-mode ultrasound arrays (DMUAs) for imaging and therapy, image-guided measurements of acoustic output using the RFB method can be used to improve measurement accuracy. In this article, we describe an image-guided RFB measurement of focused DMUA beams using a widely used commercial instrument. DMUA imaging is used to optimize the alignment between the acoustic beam and reflecting cone axes. In addition to image-guided alignment, DMUA echo data is used to track the displacement of the cone, which provides an auxiliary measurement of acoustic power. Experimental results using a DMUA prototype with [Formula: see text] shows that 1-2 mm of misalignment can result in 5%-14% error in the measured acoustic power. In addition to the use of B-mode image guidance for improving measurement accuracy, we present preliminary results demonstrating the benefit of displacement tracking using real-time DMUA imaging during the application of (sub)therapeutic focused beams. Displacement tracking provides a direct measurement of the radiation force with high sensitivity and follows the expected dependence on changes in amplitude and duty cycle (DC) of the focused ultrasound (FUS) beam. This could lead to simpler, more reliable methods for measuring acoustic power based on the radiation force principle. Combined with appropriate computational modeling, the direct measurement of acoustic radiation force could lead to reliable dosimetry in situ in emerging applications such as transcranial FUS (tFUS) therapies.

A narrative review on the application of high-intensity focused ultrasound for the treatment of occlusive and thrombotic arterial disease

Objectives

High-intensity focused ultrasound (HIFU) is a noninvasive therapeutic modality with a variety of applications. It is approved for the treatment of essential tremors, ablation of prostate, hepatic, breast, and uterine tumors. Although not approved for use in the treatment of atherosclerotic arterial disease, there is a growing body of evidence investigating applications of HIFU. Currently, percutaneous endovascular techniques are predominant for the treatment of arterial pathology; however, there are no endovascular techniques of HIFU available. This study aims to review the state of current evidence for the application of HIFU in atherosclerotic arterial disease.

Methods

All English-language articles evaluating the effect of HIFU on arterial occlusive and thrombotic disease until 2021 were reviewed. Both preclinical and human clinical studies were included. Study parameters such as animal or clinical model and outcomes were reviewed. In addition, details pertaining to settings on the HIFU device used were also reviewed.

Results

In preclinical models, atherosclerotic plaque progression was inhibited by HIFU, through decreases in oxidized low-density lipoprotein cholesterol and increases in macrophage apoptosis. Additionally, HIFU promotes angiogenesis in hindlimb ischemic models by the upregulation of angiogenic and antiapoptotic factors, with increased angiogenesis at higher line densities of HIFU. HIFU also promotes thrombolysis and conversely induces platelet activation at low frequencies and higher intensities. Various clinical studies have attempted to translate some of these properties and demonstrated positive clinical outcomes for arterial recanalization after thrombotic stroke, decreased atherosclerotic plaque burden in carotid arteries, increase in tissue perfusion and a decrease in diameter stenosis in patients with atherosclerotic arterial disease.

Conclusions

In current preclinical and clinical data, the safety and efficacy of HIFU shows great promise in the treatment of atherosclerotic arterial disease. Future focused studies are warranted to guide the refinement of HIFU settings for more widespread adoption of this technology.

Safety and feasibility study of non-invasive robot-assisted high-intensity focused ultrasound therapy for the treatment of atherosclerotic plaques in the femoral artery: protocol for a pilot study

INTRODUCTION

Peripheral arterial disease (PAD) is an atherosclerotic disease leading to stenosis and/or occlusion of the arterial circulation of the lower extremities. The currently available revascularisation methods have an acceptable initial success rate, but the long-term patency is limited, while surgical revascularisation is associated with a relatively high perioperative risk. This urges the need for development of less invasive and more effective treatment modalities. This protocol article describes a study investigating a new non-invasive technique that uses robot assisted high-intensity focused ultrasound (HIFU) to treat atherosclerosis in the femoral artery.

METHODS AND ANALYSIS

A pilot study is currently performed in 15 symptomatic patients with PAD with a significant stenosis in the common femoral and/or proximal superficial femoral artery. All patients will be treated with the dual-mode ultrasound array system to deliver imaging-guided HIFU to the atherosclerotic plaque. Safety and feasibility are the primary objectives assessed by the technical feasibility of this therapy and the 30-day major complication rate as primary endpoints. Secondary endpoints are angiographic and clinical success and quality of life.

ETHICS AND DISSEMINATION

Ethical approval for this study was obtained in 2019 from the Medical Ethics Committee of the University Medical Center Utrecht, the Netherlands. Data will be presented at national and international conferences and published in a peer-reviewed journal.

TRIAL REGISTRATION NUMBER

NL7564.

Multi-Pathway Microenvironment Regulation for Atherosclerosis Therapy Based on Beta-Cyclodextrin/L-Arginine/Au Nanomotors with Dual-Mode Propulsion

Most of the current non-pharmacological treatment strategies for atherosclerosis (AS) suffer from poor penetration into the plaque and only aim at a certain factor in its formation process, resulting in limited therapeutic effect. Herein, a kind of nanomotor with dual-mode propulsion is constructed, which is sensitive to higher reactive oxygen species (ROS) at the AS site and near-infrared (NIR) laser by the covalent binding and self-assembly of β-cyclodextrin (β-CD) and L-arginine (LA) with immobilization of Au nanoparticles. NIR laser irradiation can be used as a driving force and to ablate inflammatory macrophages through the photothermal effect. The nitric oxide (NO) released by the nanomotors can be used as another driving force and a therapeutic agent to promote endothelial repair in the plaque site. LA can eliminate ROS in the inflammatory site, and β-CD can promote the removal of cholesterol from foam cells. In particular, the two driving modes of nanomotors synergistically promote their aggregation and penetration in the plaque. This kind of nanomotor can regulate the microenvironment of AS in multiple ways, including combination therapy for endothelial repair, lipid clearance, and reducing ROS, which is expected to become a potential non-pharmacological strategy in the treatment of AS.

Safety and feasibility of arterial wall targeting with robot-assisted high intensity focused ultrasound: a preclinical study

PURPOSE

High-intensity focused ultrasound (HIFU) is a potential noninvasive thermal ablation method for the treatment of peripheral artery disease. Dual-mode ultrasound arrays (DMUA) offer the possibility of simultaneous imaging and treatment. In this study, safety and feasibility of femoral artery robot-assisted HIFU/DMUA therapy was assessed.

METHODS

In 18 pigs (∼50kg), angiography and diagnostic ultrasound were used to visualize diameter and blood flow of the external femoral arteries (EFA). HIFU/DMUA-therapy was unilaterally applied to the EFA dorsal wall using a 3.5 MHz, 64-element transducer, closed-loop-control was used to automatically adjust energy delivery to control thermal lesion formation. A continuous lesion of at least 25 mm was created by delivering 6-8 HIFU shots per imaging plane perpendicular to the artery spaced 1 mm apart. Directly after HIFU/DMUA-therapy and after 0, 3 or 14 days follow up, diameter and blood flow were measured and the skin was macroscopically examined for thermal damage. The tissue was removed for histological analysis.

RESULTS

No complications were observed. The most frequently observed treatment effect was formation of scar tissue, predominantly in the adventitia and the surrounding tissue. No damage to the endothelium or excessive damage of the surrounding tissue was observed. There was no significant decrease in the mean arterial diameter after HIFU/DMUA-therapy.

CONCLUSION

HIFU/DMUA therapy successfully targeted the vessel walls of healthy porcine arteries, without causing endothelial damage or other vascular complications. Therefore, this therapy can be safely applied to healthy arterial walls in animals. Future studies should focus on safety and dose-finding in atherosclerotic diseased arteries.

Precision Targeted Ablation of Fine Neurovascular Structures In Vivo Using Dual-mode Ultrasound Arrays

Carotid bodies (CBs) are chemoreceptors that monitor and register changes in the blood, including the levels of oxygen, carbon dioxide, and pH, and regulate breathing. Enhanced activity of CBs was shown to correlate with a significant elevation in the blood pressure of patients with hypertension. CB removal or denervation were previously shown to reduce hypertension. Here we demonstrate the feasibility of a dual-mode ultrasound array (DMUA) system to safely ablate the CB in vivo in a spontaneously hypertensive rat (SHR) model of hypertension. DMUA imaging was used for guiding and monitoring focused ultrasound (FUS) energy delivered to the target region. In particular, 3D imaging was used to identify the carotid bifurcation for targeting the CBs. Intermittent, high frame rate imaging during image-guided FUS (IgFUS) delivery was used for monitoring the lesion formation. DMUA imaging provided feedback for closed-loop control (CLC) of the lesion formation process to avoid overexposure. The procedure was tolerated well in over 100 SHR and normotensive rats that received unilateral and bilateral treatments. The measured mean arterial pressure (MAP) exhibited measurable deviation from baseline 2–4 weeks post IgFUS treatment. The results suggest that the direct unilateral FUS treatment of the CB might be sufficient to reduce the blood pressure in hypertensive rats and justify further investigation in large animals and eventually in human patients.

A New RF Heating Strategy for Thermal Treatment of Atherosclerosis

OBJECTIVES

Restenosis remains a challenge for the treatment of atherosclerosis due to the damage of the endothelial layer and induced proliferation of the smooth muscle cell.

METHODS

A new RF heating strategy was proposed to selectively ablate the atherosclerosis plaque, and to thermally inhibit the proliferation of smooth muscle cells, while keeping the endothelial cells intact. To achieve the goal, an internal cooling agent and distributed electrodes have been integrated in the new designed balloon catheter to focus the shape conformal energy onto the plaque shape. A three-dimensional (3-D) model with experimentally fitted parameters has been established to demonstrate the heating ability of the design and evaluate the microelectrodes configurations for different plaque geometries.

RESULTS

The 3-D shape of the lesions resulting from different electrodes settings is obtained. It is found that by individual control of the micro-electrodes, special shapes of the lesions can be formed, which can match the eccentric crescent plaques. Besides, through changing of the polarity of the electrodes, separate lesions can be reached. This suggests the possibility for treatment of disconnected plaques in situ.

CONCLUSION

By the control of RF heating and convection coefficient of the internal cooling agent, a targeted heating region away from the inner surface of the blood vessel can be realized.

SIGNIFICANCE

This study has illustrated the possibility of achieving a precision thermal treatment of atherosclerosis in favor of inhibiting further restenosis.

Feasibility study for removing calcified material using a planar rectangular ultrasound transducer

BACKGROUND

The aim of the proposed study was to conduct a feasibility study using a flat rectangular (3 mm × 10 mm) MRI compatible transducer operating at 5.3 MHz for destroying calcified material in an in vitro model. The proposed method can be used in the future for treating atherosclerosis plaques of the coronary, carotid or peripheral arteries.

METHODS

The system was tested initially on calcium rods. Another test was performed in a hydroxyapatite-polylactide model.

RESULTS

A parametric study was performed where the mass of calcified material removed was studied as a function of intensity, pulse repetition frequency (PRF), duty factor (DF) and presence of bubbles.

CONCLUSIONS

The amount of calcified material removed is directly related to the intensity, PRF and DF. It was found that the presence of bubbles accelerates the removal of calcified material. In order to ensure that pure mechanical mode ultrasound was used, the protocols were designed so that the temperature does not exceed 1 °C.

Ultrasound-guided therapeutic focused ultrasound: current status and future directions

This paper reviews ultrasound imaging methods for the guidance of therapeutic focused ultrasound (USgFUS), with emphasis on real-time preclinical methods. Guidance is interpreted in the broadest sense to include pretreatment planning, siting of the FUS focus, real-time monitoring of FUS-tissue interactions, and real-time control of exposure and damage assessment. The paper begins with an overview and brief historical background of the early methods used for monitoring FUS-tissue interactions. Current imaging methods are described, and discussed in terms of sensitivity and specificity of the localisation of the FUS effects in both therapeutic and sub-therapeutic modes. Thermal and non-thermal effects are considered. These include cavitation-enhanced heating, tissue water boiling and cavitation. Where appropriate, USgFUS methods are compared with similar methods implemented using other guidance modalities, e.g. magnetic resonance imaging. Conclusions are drawn regarding the clinical potential of the various guidance methods, and the feasibility and current status of real-time implementation.

External ultrasound for carotid atherosclerotic plaque treatment

OBJECTIVES

The purpose of this study was to evaluate the efficacy of external ultrasound in the treatment of carotid atherosclerotic plaques.

METHODS

In the prospective study, 357 patients with 363 carotid atherosclerotic plaques were divided into an ultrasound treatment group and a control group. For 30 days, conventional medical treatment was conducted on 54 plaques in the control group, whereas irradiation therapy in addition to conventional medical treatment was conducted on 309 plaques in the ultrasound group. Carotid sonography was conducted before and after treatment, and the maximum plaque thickness and area were measured in a longitudinal section.

RESULTS

No patients withdrew from the treatment because of related side effects. After treatment, the maximum thickness and area of 79.94% of the plaques in the ultrasound group were reduced, whereas in the control group, the thickness and area of 18.52% were reduced. The mean changes in plaque thickness and area ± SD in the ultrasound and control groups were 0.22 ± 0.19 mm (7.61% ± 5.67%) versus 0.02 ± 0.05 mm (0.74% ± 1.64%) and 0.047 ± 0.039 cm(2) (13.28% ± 9.8%) versus 0.0044 ± 0.0102 cm(2) (1.1% ± 2.46%), respectively. Changes in both plaque thickness and area in the ultrasound group were significantly greater than those in the control group (P< .0001). Furthermore, the plaque echo type was another prognostic factor affecting efficacy (P < .05).

CONCLUSIONS

External ultrasound treatment is safe and effective for carotid atherosclerotic plaques and is worthy of further research and applications. The efficacy in anechoic/hypoechoic plaques is significantly higher than that in mixed echoic and calcified echoic plaques.

Removing atherosclerotic plaque created using high cholesterol diet in rabbit using ultrasound

Background

The aim of the proposed study was to conduct a feasibility study using a flat rectangular (3 × 10 mm²) transducer operating at 5 MHz for removing atherosclerotic plaque in an in vivo model. The proposed method can be used in the future for treating atherosclerotic plaques in humans.

Methods and results

The plaque in the rabbits was created using high cholesterol diet for 4 months. The amount of plaque removed was studied as a function of intensity, with a fixed pulse repetition frequency (PRF), and duty factor (DF).

Conclusions

The amount of plaque removed is directly related to the acoustic intensity. It was found that the presence of bubbles accelerates the removal of plaque. In order to ensure that pure mechanical mode ultrasound was used, the intensity used does not produce temperatures that exceed 1°C.

High-intensity focused ultrasound for potential treatment of polycystic ovary syndrome: toward a noninvasive surgery

OBJECTIVE

To investigate the feasibility of using high-intensity focused ultrasound (HIFU), under dual-mode ultrasound arrays (DMUAs) guidance, to induce localized thermal damage inside ovaries without damage to the ovarian surface.

DESIGN

Laboratory feasibility study.

SETTING

University-based laboratory.

ANIMAL(S)

Ex vivo canine and bovine ovaries.

INTERVENTION(S)

DMUA-guided HIFU.

MAIN OUTCOME MEASURE(S)

Detection of ovarian damage by ultrasound imaging, gross pathology, and histology.

RESULT(S)

It is feasible to induce localized thermal damage inside ovaries without damage to the ovarian surface. DMUA provided sensitive imaging feedback regarding the anatomy of the treated ovaries and the ablation process. Different ablation protocols were tested, and thermal damage within the treated ovaries was histologically characterized.

CONCLUSION(S)

The absence of damage to the ovarian surface may eliminate many of the complications linked to current laparoscopic ovarian drilling (LOD) techniques. HIFU may be used as a less traumatic tool to perform LOD.