Biomedical applications of radiation force of ultrasound: historical roots and physical basis

Research status of biomaterials based on physical signals for bone injury repair

Bone defects repair continues to be a significant challenge facing the world. Biological scaffolds, bioactive molecules, and cells are the three major elements of bone tissue engineering, which have been widely used in bone regeneration therapy, especially with the rise of bioactive molecules in recent years. According to their physical properties, they can be divided into force, magnetic field (MF), electric field (EF), ultrasonic wave, light, heat, etc. However, the transmission of bioactive molecules has obvious shortcomings that hinder the development of the tissue-rearing process. This paper reviews the mechanism of physical signal induction in bone tissue engineering in recent years. It summarizes the application strategies of physical signal in bone tissue engineering, including biomaterial designs, physical signal loading strategies and related pathways. Finally, the ongoing challenges and prospects for the future are discussed.

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.

Method for measuring acoustic radiation force of a focused ultrasound beam acting on an elastic spherea)

Acoustic radiation force (ARF) is a nonlinear phenomenon resulting from the wave momentum transfer to an absorbing or scattering target. ARF allows objects to be remotely manipulated, pushed, trapped, or pulled, which is used in medical applications such as kidney stone expulsion or acoustic tweezers. Such applications require development of methods for precision ARF measurements and calculations. The purpose of this paper is to present a method for direct measurement of the axial component of the ARF exerted by an ultrasound beam on its axis acting on a millimeter-sized spherical particle in a liquid. The method consists of weighing a rigid frame with a scatterer on electronic scales, similar to the radiation force balance method of measuring the total acoustic beam power. The capabilities of the method are demonstrated by applying it to spheres of different diameters (2-8 mm) and compositions (steel, glass). The additional objective is to provide experimental validation of the theoretical model of Sapozhnikov and Bailey [J. Acoust. Soc. Am. 133, (2013)], previously developed to calculate the ARF of an arbitrary acoustic beam on an elastic sphere in a liquid or gaseous medium based on the angular spectrum approach.

A Comprehensive Review of Low-Intensity Focused Ultrasound Parameters and Applications in Neurologic and Psychiatric Disorders

OBJECTIVES

Low-intensity focused ultrasound (LIFU) is gaining increased interest as a potential therapeutic modality for a range of neuropsychiatric diseases. Current neuromodulation modalities often require a choice between high spatial fidelity or invasiveness. LIFU is unique in this regard because it provides high spatial acuity of both superficial and deep neural structures while remaining noninvasive. This new form of noninvasive brain stimulation may provide exciting potential treatment options for a variety of neuropsychiatric disorders involving aberrant neurocircuitry within deep brain structures, including pain and substance use disorders. Furthermore, LIFU is compatible with noninvasive neuroimaging techniques, such as functional magnetic resonance imaging and electroencephalography, making it a useful tool for more precise clinical neuroscience research to further understand the central nervous system.

MATERIALS AND METHODS

In this study, we provide a review of the most recent LIFU literature covering three key domains: 1) the history of focused ultrasound technology, comparing it with other forms of neuromodulation, 2) the parameters and most up-to-date proposed mechanisms of LIFU, and finally, 3) a consolidation of the current literature to date surrounding the clinical research that has used LIFU for the modification or amelioration of several neuropsychiatric conditions.

RESULTS

The impact of LIFU including poststroke motor changes, pain, mood disorders, disorders of consciousness, dementia, and substance abuse is discussed.

CONCLUSIONS

Although still in its infancy, LIFU is a promising tool that has the potential to change the way we approach and treat neuropsychiatric disorders. In this quickly evolving field, this review serves as a snapshot of the current understanding of LIFU in neuropsychiatric research.

Pressure estimation of ultra-high frequency ultrasound using gas vesicles

Acoustic microscopy uses ultra-high frequency (UHF) ultrasound transducers over 80 MHz to perform high-resolution imaging. The pressure output of these transducers is unknown, as commercial calibrated hydrophones can measure pressure for transducers with frequencies only up to 80 MHz. This study used gas vesicle nanostructures (GVs) that collapse at 571 kPa to estimate the pressure of UHF transducers at 40, 80, 200, and 375 MHz. Agarose phantoms containing GVs were made, and a baseline ultrasound image was performed at low pressure to prevent GV collapse. Sections within the phantom were scanned at varying voltage to determine the GV collapse threshold. The pressure at full driving voltage was then calculated, assuming a linear relation between transducer voltage and pressure. The pressure calculated for the 40 MHz transducer was 2.2 ± 0.1 MPa at 21 °C. Using a hydrophone, the measured pressure was 2.1 ± 0.3 MPa, a difference of <2%, validating the method at this frequency. The pressure calculated for the other transducers was 2.0 ± 0.1 MPa (80 MHz), 1.2 ± 0.1 (200 MHz), and 1.05 ± 0.17 (375 MHz at 37 °C). This study addresses the challenge of estimating pressure output from UHF ultrasound transducers, demonstrating that the pressure output in the 40-400 MHz frequency range can be quantified.

Miniaturized therapeutic systems for ultrasound-modulated drug delivery to the central and peripheral nervous system

Ultrasound is a promising technology to address challenges in drug delivery, including limited drug penetration across physiological barriers and ineffective targeting. Here we provide an overview of the significant advances made in recent years in overcoming technical and pharmacological barriers using ultrasound-assisted drug delivery to the central and peripheral nervous system. We commence by exploring the fundamental principles of ultrasound physics and its interaction with tissue. The mechanisms of ultrasonic-enhanced drug delivery are examined, as well as the relevant tissue barriers. We highlight drug transport through such tissue barriers utilizing insonation alone, in combination with ultrasound contrast agents (e.g., microbubbles), and through innovative particulate drug delivery systems. Furthermore, we review advances in systems and devices for providing therapeutic ultrasound, as their practicality and accessibility are crucial for clinical application.

Dual-Frequency Ultrasound Assisted Thrombolysis in Interventional Therapy of Deep Vein Thrombosis

Deep vein thrombosis (DVT) is one of the main causes of disability and death worldwide. Currently, the treatment of DVT still needs a long time and faces a high risk of major bleeding. It is necessary to find a rapid and safe method for the therapy of DVT. Here, a dual-frequency ultrasound assisted thrombolysis (DF-UAT) is reported for the interventional treatment of DVT. A series of piezoelectric elements are placed in an interventional catheter to emit ultrasound waves with two independent frequencies in turn. The low-frequency ultrasound drives the drug-loaded droplets into the thrombus, while the high-frequency ultrasound causes the cavitation of the droplets in the thrombus. With the joint effect of the enhanced drug diffusion and the cavitation under the dual-frequency ultrasound, the thrombolytic efficacy can be improved. In a proof-of-concept experiment performed with living sheep, the recanalization of the iliac vein is realized in 15 min using the DF-UAT technology. Therefore, the DF-UAT can be one of the most promising methods in the interventional treatment of DVT.

Rapid and Efficient Computation of Cell Paths During Ultrasonic Focusing

This biophysical analysis explores the first-principles physics of movement of white blood cell sized particles, suspended in an aqueous fluid and experiencing progressive or standing waves of acoustic pressure. In many current applications the cells are gradually nudged or herded toward the nodes of the standing wave, providing a degree of acoustic focusing and concentration of the cells in layers perpendicular to the direction of sound propagation. Here the underlying biomechanics of this phenomenon are analyzed specifically for the viscous regime of water and for small diameter microscopic spheroids such as living cells. The resulting mathematical model leads to a single algebraic expression for the creep or drift velocity as a function of sound frequency, amplitude, wavelength, fluid viscosity, boundary dimensions, and boundary reflectivity. This expression can be integrated numerically by a simple and fast computer algorithm to demonstrate net movement of particles as a function of time, providing a guide to optimization in a variety of emerging applications of ultrasonic cell focusing.

The impact of amplitude modulation frequency in harmonic motion imaging on inclusion characterization

OBJECTIVE

Ultrasound elasticity imaging techniques aim to provide a non-invasive characterization of tissue mechanical properties to detect pathological changes and monitor disease progression. Harmonic motion imaging (HMI) is an ultrasound-based elasticity imaging technique that utilizes an oscillatory acoustic radiation force to induce localized displacements and estimate relative tissue stiffness. Previous studies have applied a low amplitude modulation (AM) frequency of 25 or 50 Hz in HMI to assess the mechanical properties of different tissue types. In this study, we investigate the dependence of AM frequency in HMI and whether the frequency can be adjusted based on the size and mechanical properties of the underlying medium for enhanced image contrast and inclusion detection.

METHODS

A tissue-mimicking phantom with embedded inclusions at different sizes and stiffnesses was imaged within a range of AM frequencies from 25 to 250 Hz at 25-Hz step size.

DISCUSSION

The AM frequency at which the maximum contrast and CNR are achieved depends on the size and stiffness of the inclusions. A general trend shows that contrast and CNR peak at higher frequencies for smaller inclusions. In addition, for some inclusions with the same size but different stiffnesses, the optimized AM frequency increases with the stiffness of the inclusion. Nevertheless, there is a shift between the frequencies at which the contrast peaks and those with maximum CNR. Finally, in agreement with the phantom findings, imaging an ex-vivo human specimen with a 2.7-cm breast tumor at a range of AM frequencies showed that the highest contrast and CNR are achieved at the AM frequency of 50 Hz.

CONCLUSION

These findings indicate that the AM frequency can be optimized in different applications of HMI, especially in the clinic, for improved detection and characterization of tumors with different geometries and mechanical properties.

Evaporation issues of acoustically levitated fuel droplets

Fuel droplet evaporation is essential to the generation of flammable mixtures in thermal engines. Generally, liquid fuel is injected directly into the hot, high-pressure atmosphere to form scattered droplets. Many investigations on droplet evaporation have been conducted with techniques involving the influence of boundaries, such as suspended wires. Ultrasonic levitation is a non-contact and non-destructive technology that can avoid the impact of hanging wire on droplet shape and heat transfer. Besides, it can simultaneously levitate multiple droplets and allow them to associate with each other or be used to study droplet instability behaviors. This paper reviews the influences of the acoustic field on levitated droplets, the evaporation characteristics of acoustically levitated droplets, and the prospects and limitations of ultrasonic suspension methods for droplet evaporation, which can serve as references for relevant studies.

Acoustic Radiation Forces at the Crossroads of Ultrasound Exposimetry, HIFU, and Elastography

The applications of the acoustic radiation force (ARF) continue to multiply and extend from elastography into high-intensity focused ultrasound (HIFU), diagnostic imaging, lithotripsy, sonochemistry, levitation, and microsonics yet fundamental principles remain shrouded in mystery. A well-known and popular equation often used for calculating ARF in elastography is in conflict with the equation commonly employed for calculating ARF for determining acoustic power in radiation force balances (RFBs). Controversies have sparked debate for over a century concerning the physical mechanisms underlying ARFs. For over four decades, the science of ultrasound exposimetry has steadily improved and has provided clues in terms of accumulated data about the characteristics of transmitted ultrasound fields. Concurrently, the availability and capability of predicting these fields have improved significantly. The author draws on these sources to re-examine the physical principles behind ARFs. Conflicts are shown to stem from idealized configurations and simplistic assumptions. By more fully accounting for the pulse shape and spectrum, the effect of frequency power law attenuation, diffraction, and nonlinearity, more accurate equations are developed for ARFs for practical applications which are more consistent with exposimetry observations. Simulations compare well to corrected 1.5 MHz RFB data. While some questions await resolution, the approach presented here settles several longstanding conflicts and provides a new broadband framework for future ARF work.

Exosomes and ultrasound: The future of theranostic applications

Biomaterials and pertaining formulations have been very successful in various diagnostic and therapeutic applications because of its ability to overcome pharmacological limitations. Some of them have gained significant focus in the recent decade for their theranostic properties. Exosomes can be grouped as biomaterials, since they consist of various biological micro/macromolecules and possess all the properties of a stable biomaterial with size in nano range. Significant research has gone into isolation and exploitation of exosomes as potential theranostic agent. However, the limitations in terms of yield, efficacy, and target specificity are continuously being addressed. On the other hand, several nano/microformulations are responsive to physical or chemical alterations and were successfully stimulated by tweaking the physical characteristics of the surrounding environment they are in. Some of them are termed as photodynamic, sonodynamic or thermodynamic therapeutic systems. In this regard, ultrasound and acoustic systems were extensively studied for its ability towards altering the properties of the systems to which they were applied on. In this review, we have detailed about the diagnostic and therapeutic applications of exosomes and ultrasound separately, consisting of their conventional applications, drawbacks, and developments for addressing the challenges. The information were categorized into various sections that provide complete overview of the isolation strategies and theranostic applications of exosomes in various diseases. Then the ultrasound-based disease diagnosis and therapy were elaborated, with special interest towards the use of ultrasound in enhancing the efficacy of nanomedicines and nanodrug delivery systems, Finally, we discussed about the ability of ultrasound in enhancing the diagnostic and therapeutic properties of exosomes, which could be the future of theranostics.

Interstitial fluid streaming in deep tissue induced by ultrasound momentum transfer for accelerating nanoagent transport and controlling its distribution

Objective. This study aims to theoretically investigate the dynamics of ultrasound-induced interstitial fluid streaming and tissue recovery after ultrasound exposure for potentially accelerating nanoagent transport and controlling its distribution in tissue. Approach. Starting from fundamental equations, the dynamics of ultrasound-induced interstitial fluid streaming and tissue relaxation after an ultrasound exposure were modeled, derived and simulated. Also, both ultrasound-induced mechanical and thermal effects were considered in the models. Main results. The proposed new mechanism was named squeezing interstitial fluid via transfer of ultrasound momentum (SIF-TUM). It means that an ultrasound beam can squeeze the tissue in a small focal volume from all the directions, and generate a macroscopic streaming of interstitial fluid and a compression of tissue solid matrix. After the ultrasound is turned off, the solid matrix will recover and can generate a backflow. Rather than the ultrasound pressure itself or intensity, the streaming velocity is determined by the dot product of the ultrasound pressure gradient and its conjugate. Tissue and nanoagent properties also affect the streaming and recovery velocities. Significance. The mobility of therapeutic or diagnostic agents, such as drugs, drug carriers, or imaging contrast agents, in the interstitial space of many diseased tissues, such as tumors, is usually extremely low because of the inefficiency of the natural transport mechanisms. Therefore, the interstitial space is one of the major barriers hindering agent deliveries. The ability to externally accelerate agent transport and control its distribution is highly desirable. Potentially, SIF-TUM can be a powerful technology to accelerate agent transport in deep tissue and control the distribution if appropriate parameters are selected.

Stable Cavitation Interferes with Aβ16-22 Oligomerization

Ultrasound and microbubbles are used for many medical applications nowadays. Scanning ultrasound can remove amyloid-β (Aβ) aggregates in the mouse brain and restores memory in an Alzheimer's disease mouse model. In vitro studies showed that amyloid fibrils are fragmented due to the ultrasound-induced bubble inertial cavitation, and ultrasonic pulses accelerate the depolymerization of Aβ fibrils into monomers at 1 μM of concentration. Under applied ultrasound, microbubbles can be in a stable oscillating state or unstable inertial cavitation state. The latter occurs when ultrasound causes a dramatic change of bubble sizes above a certain acoustic pressure. We have developed and implemented a nonequilibrium molecular dynamics simulation algorithm to the AMBER package, to facilitate the investigation of the molecular mechanism of Aβ oligomerization under stable cavitation. Our results indicated that stable cavitation not only inhibited oligomeric formation, but also prevented the formation of β-rich oligomers. The network analysis of state transitions revealed that stable cavitation altered the oligomerization pathways of Aβ_16-22 peptides. Our simulation tool may be applied to optimize the experimental conditions to achieve the best therapeutical effect.

A Dual-modal Imaging Method Combining Ultrasound and Electromagnetism for Simultaneous Measurement of Tissue Elasticity and Electrical Conductivity

The mechanical and electrical properties of soft tissues are relative to soft tissues' pathological state. Modern medical imaging devices have shown a trend to multi-modal imaging, which will provide complementary functional information to improve the accuracy of disease diagnosis. However, no method or system can simultaneously measure the mechanical and electrical properties of the soft tissue. In this study, we proposed a novel dual-modal imaging method integrated by shear wave elasticity imaging (SWEI) and Magneto-acousto-electrical tomography (MAET) to measure soft tissue's elasticity and conductivity simultaneously. A dual-modal imaging system based on a linear array transducer is built, and the imaging performances of MAET and SWEI were respectively evaluated by phantoms experiment and \textit{in vitro} experiment. Conductivity phantom experiments show that the MAET in this dual-modal system can image conductivity gradient as low as 0.4 S/m. The phantom experiments show that the reconstructed 2-D elasticity maps of the phantoms with inclusions with a diameter larger than 5 mm are relatively accurate. \textit{In vitro} experiments show that the elasticity parameter can significantly distinguish the changes in tissue before and after heating. This study first proposes a method that can simultaneously obtain tissue elasticity and electrical conductivity to the best of our knowledge. Although this paper just carried out the proof of concept experiments of the new method, it demonstrates great potential for disease diagnosis in the future.

Nonlinear large deformation of a spherical red blood cell induced by ultrasonic standing wave

A computational model is developed to investigate the nonlinear static deformation of a spherical (osmotically swollen) red blood cell (RBC) induced by ultrasonic standing wave. The ultrasonic standing wave can generate steady acoustic radiation stress to deform the cell, and in turn, the deformed cell reshapes the acoustic field. This is a real-time coupling problem between the acoustic field and the mechanical field. In the computational model, the acoustic radiation stress acting on the RBC membrane is modeled by adopting the nonviscous momentum flux theory. The RBC membrane is modeled as a hyperelastic shell considering the in-plane elasticity, bending elasticity, and surface tension of the membrane. The volume conservation constraint of the membrane sealing fluid is applied to ensure the osmotic balance of the membrane. To address this real-time coupling problem, the computational model is implemented by a finite element method algorithm. The numerical results are compared with the existing theoretical model and experimental data, and the strain hardening trend of the experimental data is successfully predicted, which verifies the accuracy and effectiveness of the computational model. The computational model can accurately extract the mechanical properties of cells from acoustic deformation experiments, which is helpful for the diagnosis of some human diseases.

Perfluorocarbon loaded fluorinated covalent organic polymers with effective sonosensitization and tumor hypoxia relief enable synergistic sonodynamic-immunotherapy

Relieving tumor hypoxia has recently been found to be a promising approach to reverse tumor immunosuppression and thus enhance the treatment outcomes of diverse cancer treatments. Herein, we prepared a type of fluorinated covalent conjugate polymers (COPs) with sonosensitizer meso-5, 10, 15, 20-tetra (4-hydroxylphenyl) porphyrin (THPP) and perfluorosebacic acid (PFSEA) as cross-linkers, yielding THPP_pf-COPs with efficient sonodynamic efficacy and loading capacity towards perfluoro-15-crown-5-ether (PFCE), a model perfluorocarbon molecule. Upon intratumoral injection, such PFCE@THPP_pf-COPs could not only attenuate tumor hypoxia, but also exhibit the most effective suppression effect on tumor growth in the presence of ultrasound exposure by inducing immunogenic cell death of cancer cells. Furthermore, we found that the sonodynamic therapy of PFCE@THPP_pf-COPs together with anti-CD47 immunotherapy would synergistically suppress tumor growth by increasing the tumor-infiltrating frequencies of phagocytic M1 macrophages and cytotoxic CD3⁺CD8⁺ T cells, while reducing the frequency of immunosuppressive regulatory T cells. Moreover, such combination treatment could also elicit potent protective memory antitumor immunity to prevent tumor challenge. Therefore, this work presents PFCE@THPP_pf-COPs are a type of multifunctional nano-sonosensitizers potent in removing negative impacts of inherent tumor hypoxia and immunosuppression, and suppressing tumor growth and tumor recurrence by priming host's antitumor immunity, particularly in synergizing with anti-CD47 immunotherapy.

Acoustic Radiation Force: A Review of Four Mechanisms for Biomedical Applications

Radiation force is a universal phenomenon in any wave motion where the wave energy produces a static or transient force on the propagation medium. The theory of acoustic radiation force (ARF) dates back to the early 19th century. In recent years, there has been an increasing interest in the biomedical applications of ARF. Following a brief history of ARF, this article describes a concise theory of ARF under four physical mechanisms of radiation force generation in tissue-like media. These mechanisms are primarily based on the dissipation of acoustic energy of propagating waves, the reflection of the incident wave, gradients of the compressional wave speeds, and the spatial variations of energy density in standing acoustic waves. Examples describing some of the practical applications of ARF under each mechanism are presented. This article concludes with a discussion on selected ideas for potential future applications of ARF in biomedicine.

Spike frequency-dependent inhibition and excitation of neural activity by high-frequency ultrasound

Ultrasound can modulate action potential firing in vivo and in vitro, but the mechanistic basis of this phenomenon is not well understood. To address this problem, we used patch-clamp recording to quantify the effects of focused, high-frequency (43 MHz) ultrasound on evoked action potential firing in CA1 pyramidal neurons in acute rodent hippocampal brain slices. We find that ultrasound can either inhibit or potentiate firing in a spike frequency–dependent manner: at low (near-threshold) input currents and low firing frequencies, ultrasound inhibits firing, while at higher input currents and higher firing frequencies, ultrasound potentiates firing. The net result of these two competing effects is that ultrasound increases the threshold current for action potential firing, the slope of frequency-input curves, and the maximum firing frequency. In addition, ultrasound slightly hyperpolarizes the resting membrane potential, decreases action potential width, and increases the depth of the after-hyperpolarization. All of these results can be explained by the hypothesis that ultrasound activates a sustained potassium conductance. According to this hypothesis, increased outward potassium currents hyperpolarize the resting membrane potential and inhibit firing at near-threshold input currents but potentiate firing in response to higher-input currents by limiting inactivation of voltage-dependent sodium channels during the action potential. This latter effect is a consequence of faster action potential repolarization, which limits inactivation of voltage-dependent sodium channels, and deeper (more negative) after-hyperpolarization, which increases the rate of recovery from inactivation. Based on these results, we propose that ultrasound activates thermosensitive and mechanosensitive two-pore-domain potassium (K2P) channels through heating or mechanical effects of acoustic radiation force. Finite-element modeling of the effects of ultrasound on brain tissue suggests that the effects of ultrasound on firing frequency are caused by a small (<2°C) increase in temperature, with possible additional contributions from mechanical effects.

Comprehensive review on ultrasound-responsive theranostic nanomaterials: mechanisms, structures and medical applications

The field of theranostics has been rapidly growing in recent years and nanotechnology has played a major role in this growth. Nanomaterials can be constructed to respond to a variety of different stimuli which can be internal (enzyme activity, redox potential, pH changes, temperature changes) or external (light, heat, magnetic fields, ultrasound). Theranostic nanomaterials can respond by producing an imaging signal and/or a therapeutic effect, which frequently involves cell death. Since ultrasound (US) is already well established as a clinical imaging modality, it is attractive to combine it with rationally designed nanoparticles for theranostics. The mechanisms of US interactions include cavitation microbubbles (MBs), acoustic droplet vaporization, acoustic radiation force, localized thermal effects, reactive oxygen species generation, sonoluminescence, and sonoporation. These effects can result in the release of encapsulated drugs or genes at the site of interest as well as cell death and considerable image enhancement. The present review discusses US-responsive theranostic nanomaterials under the following categories: MBs, micelles, liposomes (conventional and echogenic), niosomes, nanoemulsions, polymeric nanoparticles, chitosan nanocapsules, dendrimers, hydrogels, nanogels, gold nanoparticles, titania nanostructures, carbon nanostructures, mesoporous silica nanoparticles, fuel-free nano/micromotors.

Therapeutic Angiogenesis for Ovarian Transplantation through Ultrasound-Targeted Microbubble Destruction

Timely angiogenesis and effective microcirculation perfusion are essential for the survival and functional recovery of transplanted ovaries. Ultrasound-targeted microbubble destruction (UTMD) can lead to angiogenesis and increase flow perfusion by causing transient inflammation. The purpose of this study was to evaluate the effects of UTMD on transplanted ovarian revascularization and survival. In vitro, for the criteria of cell viability and tube formation capability, the optimal exposure parameters were determined to be a microbubble concentration of 1 × 10⁸/mL, mechanical index of 1 and exposure time of 30 s. After ovarian transplantation, 40 female Sprague Dawley rats were divided into four groups: transplantation alone, ultrasound alone, microbubbles alone and ultrasound and microbubbles (UTMD). At 7 d after transplantation, ovarian perfusion was assessed using qualitative and quantitative methods. The effect of angiogenesis was assessed by contrast-enhanced ultrasound, laser Doppler perfusion imaging and histologic analysis. The results, in which ovarian perfusion was highest in the UTMD group, suggest that UTMD can effectively improve ovarian perfusion. Compared with the other three groups, the number of follicles, microvascular density and rate of Ki-67-positive cells increased significantly in the UTMD group, while apoptosis decreased significantly (p < 0.05). The study indicates that UTMD promoted ovarian re-vascularization after ovarian transplantation and maintained follicular reserve.

Feasibility of Harmonic Motion Imaging Using a Single Transducer: In Vivo Imaging of Breast Cancer in a Mouse Model and Human Subjects

Harmonic motion imaging (HMI) interrogates the mechanical properties of tissues by simultaneously generating and tracking harmonic oscillation using focused ultrasound and imaging transducers, respectively. Instead of using two transducers, the objective of this work is to develop a single transducer HMI (ST-HMI) to both generate and track harmonic motion at "on-axis" to the force for facilitating data acquisition. In ST-HMI, the amplitude-modulated force was generated by modulating excitation pulse duration and tracking of motion was performed by transmitting tracking pulses interleaved between excitation pulses. The feasibility of ST-HMI was performed by imaging two elastic phantoms with three inclusions (N = 6) and comparing it with acoustic radiation force impulse (ARFI) imaging, in vivo longitudinal monitoring of 4T1, orthotropic breast cancer mice (N = 4), and patients (N = 3) with breast masses in vivo. Six inclusions with Young's moduli of 8, 10, 15, 20, 40, and 60 kPa were embedded in a 5 kPa background. The ST-HMI-derived peak-to-peak displacement (P2PD) successfully detected all inclusions with [Formula: see text] of the linear regression between the P2PD ratio of background to inclusion versus Young's moduli ratio of inclusion to background. The contrasts of 10 and 15 kPa inclusions were higher in ST-HMI than ARFI-derived images. In the mouse study, the median P2PD ratio of tumor to non-cancerous tissues was 3.0, 5.1, 6.1, and 7.7 at 1, 2, 3, and 4 weeks post-injection of the tumor cells, respectively. In the clinical study, ST-HMI detected breast masses including fibroadenoma, pseudo angiomatous stromal hyperplasia, and invasive ductal carcinoma with a P2PD ratio of 1.37, 1.61, and 1.78, respectively. These results indicate that ST-HMI can assess the mechanical properties of tissues via generation and tracking of harmonic motion "on-axis" to the ARF. This study is the first step towards translating ST-HMI in clinics.

Polymer Nanoparticles and Nanomotors Modified by DNA/RNA Aptamers and Antibodies in Targeted Therapy of Cancer

Polymer nanoparticles and nano/micromotors are novel nanostructures that are of increased interest especially in the diagnosis and therapy of cancer. These structures are modified by antibodies or nucleic acid aptamers and can recognize the cancer markers at the membrane of the cancer cells or in the intracellular side. They can serve as a cargo for targeted transport of drugs or nucleic acids in chemo- immuno- or gene therapy. The various mechanisms, such as enzyme, ultrasound, magnetic, electrical, or light, served as a driving force for nano/micromotors, allowing their transport into the cells. This review is focused on the recent achievements in the development of polymer nanoparticles and nano/micromotors modified by antibodies and nucleic acid aptamers. The methods of preparation of polymer nanoparticles, their structure and properties are provided together with those for synthesis and the application of nano/micromotors. The various mechanisms of the driving of nano/micromotors such as chemical, light, ultrasound, electric and magnetic fields are explained. The targeting drug delivery is based on the modification of nanostructures by receptors such as nucleic acid aptamers and antibodies. Special focus is therefore on the method of selection aptamers for recognition cancer markers as well as on the comparison of the properties of nucleic acid aptamers and antibodies. The methods of immobilization of aptamers at the nanoparticles and nano/micromotors are provided. Examples of applications of polymer nanoparticles and nano/micromotors in targeted delivery and in controlled drug release are presented. The future perspectives of biomimetic nanostructures in personalized nanomedicine are also discussed.

Acoustic Microfluidics

Acoustic microfluidic devices are powerful tools that use sound waves to manipulate micro- or nanoscale objects or fluids in analytical chemistry and biomedicine. Their simple device designs, biocompatible and contactless operation, and label-free nature are all characteristics that make acoustic microfluidic devices ideal platforms for fundamental research, diagnostics, and therapeutics. Herein, we summarize the physical principles underlying acoustic microfluidics and review their applications, with particular emphasis on the manipulation of macromolecules, cells, particles, model organisms, and fluidic flows. We also present future goals of this technology in analytical chemistry and biomedical research, as well as challenges and opportunities.

Pulsatile Drug Delivery System Triggered by Acoustic Radiation Force

Since biological systems exhibit a circadian rhythm (24-hour cycle), they are susceptible to the timing of drug administration. Indeed, several disorders require a therapy that synchronizes with the onset of symptoms. A targeted therapy with spatially and temporally precise controlled drug release can guarantee a considerable gain in terms of efficacy and safety of the treatment compared to traditional pharmacological methods, especially for chronotherapeutic disorders. This paper presents a proof of concept of an innovative pulsatile drug delivery system remotely triggered by the acoustic radiation force of ultrasound. The device consists of a case, in which a drug-loaded gel can be embedded, and a sliding top that can be moved on demand by the application of an acoustic stimulus, thus enabling drug release. Results demonstrate for the first time that ultrasound acoustic radiation force (up to 0.1 N) can be used for an efficient pulsatile drug delivery (up to 20 μg of drug released for each shot).

Ultrafast ultrasound imaging of surface acoustic waves induced by laser excitation compared with acoustic radiation force

Two generation mechanisms-optical perturbation and acoustic radiation force (ARF)-were investigated where high frame rate ultrasound imaging was used to track the propagation of induced SAWs. We compared ARF-induced SAWs with laser-induced SAWs generated by laser beam irradiation of the uniformly absorbing tissue-like viscoelastic phantom, where light was preferentially absorbed at the surface. We also compared the frequency content of SAWs generated by ARF versus pulsed laser light, using the same duration of excitation. Differences in the SAW bandwidth were expected because, in general, laser light can be focused into a smaller area. Finally, we compared wave generation and propagation when the wave's origin was below the surface. We also investigated the relationship between shear wave amplitude and optical fluence. The investigation reported here can potentially extend the applications of laser-induced SAW generation and imaging in life sciences and other applications.

Numerical modeling and verification of a sonobioreactor and its application on two model microorganisms

Ultrasound has many uses, such as in medical imaging, monitoring of crystallization, characterization of emulsions and suspensions, and disruption of cell membranes in the food industry. It can also affect microbial cells by promoting or slowing their growth and increasing the production of some metabolites. However, the exact mechanism explaining the effect of ultrasound has not been identified yet. Most equipment employed to study the effect of ultrasound on microorganisms has been designed for other applications and then only slightly modified. This results in limited control over ultrasound frequency and input power, or pressure distribution in the reactor. The present study aimed to obtain a well-defined reactor by simulating the pressure distribution of a sonobioreactor. Specifically, we optimized a sonotrode to match the bottle frequency and compared it to measured results to verify the accuracy of the simulation. The measured pressure distribution spectrum presented the same overall trend as the simulated spectrum. However, the peaks were much less intense, likely due to non-linear events such as the collapse of cavitation bubbles. To test the application of the sonobioreactor in biological systems, two biotechnologically interesting microorganisms were assessed: an electroactive bacterium, Geobacter sulfurreducens, and a lignocellulose-degrading fungus, Fusarium oxysporum. Sonication resulted in increased malate production by G. sulfurreducens, but no major effect on growth. In comparison, morphology and growth of F. oxysporum were more sensitive to ultrasound intensity. Despite considerable morphological changes at 4 W input power, the growth rate was not adversely affected; however, at 12 W, growth was nearly halted. The above findings indicate that the novel sonobioreactor provides an effective tool for studying the impact of ultrasound on microorganisms.

Acoustic Fountains and Atomization at Liquid Surfaces Excited by Diagnostic Ultrasound

Pulmonary capillary hemorrhage (PCH) has been found in mammalian lungs exposed to diagnostic ultrasound (DUS), although the mechanism is poorly understood. This work investigates acoustic atomization and fountains at liquid-air interfaces subjected to DUS, which has been suggested as a possible PCH damage mechanism. Primarily using a SuperSonic Imagine Aixplorer DUS machine (SuperSonic Imagine, Aix-en-Provence, France), blood and water surfaces were excited in vitro by DUS and recorded with a high-speed camera. The surface was driven by B-mode, color Doppler, pulsed Doppler, and shear wave elastography imaging modes with center frequencies from 5.0-7.2 MHz and mechanical indexes (MI) up to 1.7. Fountains and atomization were only observed for SWE, for MI as low as 1.0. A comparison of the SWE waveforms with the surface dynamics suggests that fountains and atomization were driven by push-pulses and depended on pulse duration and intensity. However, we conclude that atomization and fountaining are unlikely primary mechanisms behind all DUS-induced PCH because neither phenomenon occurred for traditional diagnostic imaging modes.

Interaction mechanism between the focused ultrasound and lipid membrane at the molecular level

Focused ultrasound (FUS) has a wide range of medical applications. Nowadays, the diagnostic and therapeutic ultrasound procedures are routinely used; effects of ultrasound on biological systems at the molecular level are, however, not fully understood. Experimental results on the interaction of the cell membrane, a simplest but important system component, with ultrasound are controversial. Molecular dynamics (MD) simulations could provide valuable insights, but there is no single study on the mechanism of the FUS induced structural changes in cell membranes. With this in mind, we develop a simple method to include FUS into a standard MD simulation. Adopting the 1,2-dioleoyl-sn-glycero-3-phosphocholine lipid membrane as a representative model described by the MARTINI coarse-grained force field, and using experimental values of the ultrasound frequency and intensity, we show that the heat and bubble cavitation are not the primary direct mechanisms that cause structural changes in the membrane. The spatial pressure gradients between the focused and free regions and between the parallel and perpendicular directions to the membrane are the origin of the mechanism. These gradients force lipids to move out of the focused region, forming a lipid flow along the membrane diagonal. Lipids in the free region move in the opposite direction due to the conservation of the total momentum. These opposite motions create wrinkles along the membrane diagonal at low FUS intensities and tear up the membrane at high FUS intensities. Once the membrane is torn up, it is not easy to reform. The implication of our findings in the FUS-induced drug delivery is discussed in some detail.

Acoustofluidic separation of cells and particles

Acoustofluidics, the integration of acoustics and microfluidics, is a rapidly growing research field that is addressing challenges in biology, medicine, chemistry, engineering, and physics. In particular, acoustofluidic separation of biological targets from complex fluids has proven to be a powerful tool due to the label-free, biocompatible, and contact-free nature of the technology. By carefully designing and tuning the applied acoustic field, cells and other bioparticles can be isolated with high yield, purity, and biocompatibility. Recent advances in acoustofluidics, such as the development of automated, point-of-care devices for isolating sub-micron bioparticles, address many of the limitations of conventional separation tools. More importantly, advances in the research lab are quickly being adopted to solve clinical problems. In this review article, we discuss working principles of acoustofluidic separation, compare different approaches of acoustofluidic separation, and provide a synopsis of how it is being applied in both traditional applications, such as blood component separation, cell washing, and fluorescence activated cell sorting, as well as emerging applications, including circulating tumor cell and exosome isolation.

Displacement of a bubble located at a fluid-viscoelastic medium interface

A model for estimating the displacement of a bubble located at a fluid-viscoelastic medium interface in response to acoustic radiation force is presented by extending the model for a spherical object embedded in a bulk material. The effects of the stiffness and viscosity of the viscoelastic medium and the amplitude and duration of the excitation force on bubble displacement were investigated using the proposed model. The results show that bubble displacement has a nonlinear relationship with excitation duration and viscosity. The time at which the steady state is reached increases with increasing medium viscosity and decreasing medium stiffness.

Focused Ultrasound for Neuromodulation

For more than 70 years, the promise of noninvasive neuromodulation using focused ultrasound has been growing while diagnostic ultrasound established itself as a foundation of clinical imaging. Significant technical challenges have been overcome to allow transcranial focused ultrasound to deliver spatially restricted energy into the nervous system at a wide range of intensities. High-intensity focused ultrasound produces reliable permanent lesions within the brain, and low-intensity focused ultrasound has been reported to both excite and inhibit neural activity reversibly. Despite intense interest in this promising new platform for noninvasive, highly focused neuromodulation, the underlying mechanism remains elusive, though recent studies provide further insight. Despite the barriers, the potential of focused ultrasound to deliver a range of permanent and reversible neuromodulation with seamless translation from bench to the bedside warrants unparalleled attention and scientific investment. Focused ultrasound boasts a number of key features such as multimodal compatibility, submillimeter steerable focusing, multifocal, high temporal resolution, coregistration, and the ability to monitor delivered therapy and temperatures in real time. Despite the technical complexity, the future of noninvasive focused ultrasound for neuromodulation as a neuroscience and clinical platform remains bright.

Mitigation of Articular Cartilage Degeneration and Subchondral Bone Sclerosis in Osteoarthritis Progression Using Low-Intensity Ultrasound Stimulation

The purpose of this study was to evaluate the effect of low-intensity ultrasound on articular cartilage and subchondral bone alterations in joints under normal and functional disuse conditions during osteoarthritis (OA) progression. Total of thirty 5-mo-old female Sprague-Dawley rats were randomly assigned to six groups (n = 5/group): age-matched group, OA group, OA + ultrasound (US) group, hindlimb suspension (HLS) group, HLS + OA group and HLS + OA + US group. The surgical anterior cruciate ligament was used to induce OA in the right knee joints. After 2 wk of OA induction, low-intensity ultrasound generated with a 3-MHz transducer with 20% pulse duty cycle and 30 mW/cm² acoustic intensity was delivered to the right knee joints for 20 min a day, 5 d a week for a total of 6 wk. Then, the right tibias were harvested for micro-computed tomography, histologic and mechanical analysis. Micro-computed tomography results indicated that the thickness and sulfated glycosaminoglycan content of cartilage decreased, but the thickness of the subchondral cortical bone plate and the formation of subchondral trabecular bone increased in the OA group under the normal joint use condition. Furthermore, histologic results revealed that chondrocyte density and arrangement in cartilage corrupted and the underlying subchondral bone increased during OA progression. These changes were accompanied by reductions in mechanical parameters in OA cartilage. However, fewer OA symptoms were observed in the HLS + OA group under the joint disuse condition. The cartilage degeneration and subchondral bone sclerosis were alleviated in the US treatment group, especially under normal joint use condition. In conclusion, low-intensity ultrasound could improve cartilage degeneration and subchondral sclerosis during OA progression. Also, it could provide a promising strategy for future clinical treatment for OA patients.

Real-time Optical Imaging of Microbubble Destruction with an Acoustic Lens Attached Ultrasonic Diagnostic Probe in Microfluidic Capillary Models

In this work, an ultrasonic diagnostic system with an attachable acoustic lens was demonstrated for real-time optical imaging of ultrasound-mediated microbubble destruction in a microfluidic capillary model using an inverted microscope. Microbubble destruction under ultrasonic pressure was monitored via an EM-CCD camera with the frame rate of 70 fps. The acoustic field distribution of the transducer with the attachable acoustic lens was simulated via a finite element method (FEM) and measured by a hydrophone. The result of acoustic field distribution shows unfocused beam profiles with 50% decreased pressure of original focal area. With the unfocused beam, inertial cavitation of the microbubbles as a function of transducer input voltages of 30-60 Vpp was studied. In addition, the acoustic cavitation parameters such as frequency of 2 MHz, pulse length of 16 μs, and pulse repetition frequency (PRF) of 1 kHz were investigated under static and dynamic flow conditions in the microfluidic model. In our system, above 45 Vpp, the microbubbles were destroyed more than 50% within 20 seconds so that the threshold for the inertial cavitation was determined to be 45 Vpp in the channel without flow. In the microfluidic capillary model with fluidic flow, it is investigated that shape of microbubble mass continuously changed with acoustic pressure with 60 Vpp.

Production of acoustic radiation force using ultrasound: methods and applications

INTRODUCTION

Acoustic radiation force (ARF) is used in many biomedical applications. The transfer of momentum in acoustic waves can be used in a multitude of ways to perturb tissue and manipulate cells.

AREAS COVERED

This review will briefly cover the acoustic theory related to ARF, particularly that related to application in tissues. The use of ARF in measurement of mechanical properties will be treated in detail with emphasis on the spatial and temporal modulation of the ARF. Additional topics covered will be the manipulation of particles with ARF, correction of phase aberration with ARF, modulation of cellular behavior with ARF, and bioeffects related to ARF use.

EXPERT COMMENTARY

The use of ARF can be tailored to specific applications for measurements of mechanical properties or correction of focusing for ultrasound beams. Additionally, noncontact manipulation of particles and cells with ARF enables a wide array of applications for tissue engineering and biosensing.

Biomolecular Ultrasound and Sonogenetics

Visualizing and modulating molecular and cellular processes occurring deep within living organisms is fundamental to our study of basic biology and disease. Currently, the most sophisticated tools available to dynamically monitor and control cellular events rely on light-responsive proteins, which are difficult to use outside of optically transparent model systems, cultured cells, or surgically accessed regions owing to strong scattering of light by biological tissue. In contrast, ultrasound is a widely used medical imaging and therapeutic modality that enables the observation and perturbation of internal anatomy and physiology but has historically had limited ability to monitor and control specific cellular processes. Recent advances are beginning to address this limitation through the development of biomolecular tools that allow ultrasound to connect directly to cellular functions such as gene expression. Driven by the discovery and engineering of new contrast agents, reporter genes, and bioswitches, the nascent field of biomolecular ultrasound carries a wave of exciting opportunities.

Activation of Piezo1 but Not NaV1.2 Channels by Ultrasound at 43 MHz

Ultrasound (US) can modulate the electrical activity of the excitable tissues, but the mechanisms underlying this effect are not understood at the molecular level or in terms of the physical modality through which US exerts its effects. Here, we report an experimental system that allows for stable patch-clamp recording in the presence of US at 43 MHz, a frequency known to stimulate neural activity. We describe the effects of US on two ion channels proposed to be involved in the response of excitable cells to US: the mechanosensitive Piezo1 channel and the voltage-gated sodium channel Na_V1.2. Our patch-clamp recordings, together with finite-element simulations of acoustic field parameters indicate that Piezo1 channels are activated by continuous wave US at 43 MHz and 50 or 90 W/cm² through cell membrane stress caused by acoustic streaming. Na_V1.2 channels were not affected through this mechanism at these intensities, but their kinetics could be accelerated by US-induced heating.

Evaluation of a Three Hydrophones Method for 2-Dimensional Cavitation Localization

Cavitation is a critical parameter in various therapeutic applications involving ultrasound (US) such as histotrispy, lithothripsy, drug delivery, and cavitation-enhanced hyperthermia. A cavitation exposure outside the region of interest may lead to suboptimal treatment efficacy or in a worse case, to safety issues. Current methods of localizing cavitation are based on imaging approaches, such as beamforming the cavitation signals received passively by a US imager. These methods, although efficient, require expensive equipment, which may discourage potential future developments. We propose a threehydrophone method to localize the cavitation cloud source. Firstly, the delays between the three receptors are measured by detecting the maximum of their inter-correlations. Then, the position of the source is calculated by either minimizing a cost function or solving hyperbolic equations. After a numerical validation, the method was assessed experimentally. This method was able to track a source displacement with accuracy similar to the size of the cavitation cloud (2-4 millimeters). This light and versatile method provides interesting perspectives since localization can be executed in real time and the extension to three-dimensional localization seems straightforward.

Recent technological advancements in cardiac ultrasound imaging

About 92.1 million Americans suffer from at least one type of cardiovascular disease. Worldwide, cardiovascular diseases are the number one cause of death (about 31% of all global deaths). Recent technological advancements in cardiac ultrasound imaging are expected to aid in the clinical diagnosis of many cardiovascular diseases. This article provides an overview of such recent technological advancements, specifically focusing on tissue Doppler imaging, strain imaging, contrast echocardiography, 3D echocardiography, point-of-care echocardiography, 3D volumetric flow assessments, and elastography. With these advancements ultrasound imaging is rapidly changing the domain of cardiac imaging. The advantages offered by ultrasound imaging include real-time imaging, imaging at patient bed-side, cost-effectiveness and ionizing-radiation-free imaging. Along with these advantages, the steps taken towards standardization of ultrasound based quantitative markers, reviewed here, will play a major role in addressing the healthcare burden associated with cardiovascular diseases.

Ultrasound Elicits Behavioral Responses through Mechanical Effects on Neurons and Ion Channels in a Simple Nervous System

Focused ultrasound has been shown to stimulate excitable cells, but the biophysical mechanisms behind this phenomenon remain poorly understood. To provide additional insight, we devised a behavioral-genetic assay applied to the well-characterized nervous system of Caenorhabditis elegans nematodes. We found that pulsed ultrasound elicits robust reversal behavior in wild-type animals in a pressure-, duration-, and pulse protocol-dependent manner. Responses were preserved in mutants unable to sense thermal fluctuations and absent in mutants lacking neurons required for mechanosensation. Additionally, we found that the worm's response to ultrasound pulses rests on the expression of MEC-4, a DEG/ENaC/ASIC ion channel required for touch sensation. Consistent with prior studies of MEC-4-dependent currents in vivo, the worm's response was optimal for pulses repeated 300-1000 times per second. Based on these findings, we conclude that mechanical, rather than thermal, stimulation accounts for behavioral responses. Further, we propose that acoustic radiation force governs the response to ultrasound in a manner that depends on the touch receptor neurons and MEC-4-dependent ion channels. Our findings illuminate a complete pathway of ultrasound action, from the forces generated by propagating ultrasound to an activation of a specific ion channel. The findings further highlight the importance of optimizing ultrasound pulsing protocols when stimulating neurons via ion channels with mechanosensitive properties.SIGNIFICANCE STATEMENT How ultrasound influences neurons and other excitable cells has remained a mystery for decades. Although it is widely understood that ultrasound can heat tissues and induce mechanical strain, whether or not neuronal activation depends on heat, mechanical force, or both physical factors is not known. We harnessed Caenorhabditis elegans nematodes and their extraordinary sensitivity to thermal and mechanical stimuli to address this question. Whereas thermosensory mutants respond to ultrasound similar to wild-type animals, mechanosensory mutants were insensitive to ultrasound stimulation. Additionally, stimulus parameters that accentuate mechanical effects were more effective than those producing more heat. These findings highlight a mechanical nature of the effect of ultrasound on neurons and suggest specific ways to optimize stimulation protocols in specific tissues.

Neuromodulation with transcranial focused ultrasound

The understanding of brain function and the capacity to treat neurological and psychiatric disorders rest on the ability to intervene in neuronal activity in specific brain circuits. Current methods of neuromodulation incur a tradeoff between spatial focus and the level of invasiveness. Transcranial focused ultrasound (FUS) is emerging as a neuromodulation approach that combines noninvasiveness with focus that can be relatively sharp even in regions deep in the brain. This may enable studies of the causal role of specific brain regions in specific behaviors and behavioral disorders. In addition to causal brain mapping, the spatial focus of FUS opens new avenues for treatments of neurological and psychiatric conditions. This review introduces existing and emerging FUS applications in neuromodulation, discusses the mechanisms of FUS effects on cellular excitability, considers the effects of specific stimulation parameters, and lays out the directions for future work.

Mechanistic understanding the bioeffects of ultrasound-driven microbubbles to enhance macromolecule delivery

Ultrasound-driven microbubbles can trigger reversible membrane perforation (sonoporation), open interendothelial junctions and stimulate endocytosis, thereby providing a temporary and reversible time-window for the delivery of macromolecules across biological membranes and endothelial barriers. This time-window is related not only to cavitation events, but also to biological regulatory mechanisms. Mechanistic understanding of the interaction between cavitation events and cells and tissues, as well as the subsequent cellular and molecular responses will lead to new design strategies with improved efficacy and minimized side effects. Recent important progress on the spatiotemporal characteristics of sonoporation, cavitation-induced interendothelial gap and endocytosis, and the spatiotemporal bioeffects and the preliminary biological mechanisms in cavitation-enhanced permeability, has been made. On the basis of the summary of this research progress, this Review outlines the underlying bioeffects and the related biological regulatory mechanisms involved in cavitation-enhanced permeability; provides a critical commentary on the future tasks and directions in this field, including developing a standardized methodology to reveal mechanism-based bioeffects in depth, and designing biology-based treatment strategies to improve efficacy and safety. Such mechanistic understanding the bioeffects that contribute to cavitation-enhanced delivery will accelerate the translation of this approach to the clinic.

Optical coherence elastography in ophthalmology

Optical coherence elastography (OCE) can provide clinically valuable information based on local measurements of tissue stiffness. Improved light sources and scanning methods in optical coherence tomography (OCT) have led to rapid growth in systems for high-resolution, quantitative elastography using imaged displacements and strains within soft tissue to infer local mechanical properties. We describe in some detail the physical processes underlying tissue mechanical response based on static and dynamic displacement methods. Namely, the assumptions commonly used to interpret displacement and strain measurements in terms of tissue elasticity for static OCE and propagating wave modes in dynamic OCE are discussed with the ultimate focus on OCT system design for ophthalmic applications. Practical OCT motion-tracking methods used to map tissue elasticity are also presented to fully describe technical developments in OCE, particularly noting those focused on the anterior segment of the eye. Clinical issues and future directions are discussed in the hope that OCE techniques will rapidly move forward to translational studies and clinical applications.

Modeling of the acoustic radiation force in elastography

Elastography is a non-invasive imaging technique that can assess in vivo tissue stiffness. In shear wave elastography imaging, the acoustic radiation force (ARF) produced by focused ultrasound generates a local force that produces shear waves. The authors compare three existing formulations for the ARF: its full expression in the second-order approximation and two simplified formulations using a quasi-plane wave and an attenuated plane wave approximation. Analytical expressions for the ARF are derived for the special cases of a concave spherical source and a quasi-Gaussian beam. They provide expressions for the resulting ARF and show discrepancies between the different formulations. For strongly divergent or highly focused beams the ARF expressed by the second-order approximation significantly differs from both simplified formulations. However, despite those differences the second-order and quasi-plane wave approximations create identical shear displacements in tissue. To compute the ARF and the displacements produced by a conventional ultrasound probe, the three formulations were incorporated into the k-Wave simulation package. The second-order and quasi-plane wave approximations give different forces but nearly identical displacements while the plane wave approximation significantly differs. It is concluded that to properly take into account the ultrasound field structure, the second-order or quasi-plane wave approximations should be preferably used.

Nonlinear dispersive waves in repulsive lattices

The propagation of nonlinear waves in a lattice of repelling particles is studied theoretically and experimentally. A simple experimental setup is proposed, consisting of an array of coupled magnetic dipoles. By driving harmonically the lattice at one boundary, we excite propagating waves and demonstrate different regimes of mode conversion into higher harmonics, strongly influenced by dispersion and discreteness. The phenomenon of acoustic dilatation of the chain is also predicted and discussed. The results are compared with the theoretical predictions of the α-Fermi-Pasta-Ulam equation, describing a chain of masses connected by nonlinear quadratic springs and numerical simulations. The results can be extrapolated to other systems described by this equation.

Acoustic Radiation Force of a Quasi-Gaussian Beam on an Elastic Sphere in a Fluid

Acoustic radiation force has many applications. One of the related technologies is the ability to noninvasively expel stones from the kidney. To optimize the procedure it is important to develop theoretical approaches that can provide rapid calculations of the radiation force depending in stone size and elastic properties, together with ultrasound beam diameter, intensity, and frequency. We hypothesize that the radiation force nonmonotonically depends on the ratio between the acoustic beam width and stone diameter because of coupling between the acoustic wave in the fluid and shear waves in the stone. Testing this hypothesis by considering a spherical stone and a quasi-Gaussian beam was performed in the current work. The calculation of the radiation force was conducted for elastic spheres of two types. Dependence of the magnitude of the radiation force on the beam diameter at various fixed values of stone diameters was modeled. In addition to using real material properties, speed of shear wave in the stone was varied to reveal the importance of shear waves in the stone. It was found that the radiation force reaches its maximum at the beamwidth comparable to the stone diameter; the gain in the force magnitude can reach 40% in comparison with the case of a narrow beam.