Improved Anatomical Specificity of Non-invasive Neuro-stimulation by High Frequency (5 MHz) Ultrasound

Comparison of neural responses to whisker and ultrasound stimulation using a novel dual-stimulation protocol

BACKGROUND

The sensory system allows organisms to perceive and respond to environmental stimuli. This study investigates neural response differences between whisker and ultrasound stimulation in rats to evaluate cortical specificity to sensory inputs.

NEW METHOD

A novel dual-stimulation protocol combining a step motor and ultrasound system was developed to alternately stimulate the C2 whisker and corresponding barrel column region. Experiments were conducted under varying stimulation sequences (whisker-ultrasound and ultrasound-whisker) and time intervals (10 ms, 25 ms, and 100 ms). Neural response signals were recorded, and statistical analyses (ANOVA and T-test) were performed to compare response amplitudes and peak latencies.

RESULTS

Whisker stimulation consistently elicited significantly stronger neural responses than ultrasound stimulation (*p < 0.05), regardless of sequence or interval. The efficiency of neural responses to ultrasound was closely tied to frequency, with higher frequencies producing greater amplitudes and faster latencies. Notably, at a 25 ms interval in the ultrasound-whisker sequence, whisker responses were significantly enhanced compared to whisker stimulation alone, suggesting a pre-activation effect of ultrasound.

COMPARISON WITH EXISTING METHODS

Unlike single-modal whisker or ultrasound stimulation, the dual-stimulation protocol can enhance sensory responses, highlighting its neuromodulatory potential.

CONCLUSION

This study reveals distinct cortical activation patterns induced by whisker and ultrasound stimulation. While whisker stimulation is more sensitive, ultrasound stimulation-when optimized for frequency and timing-can effectively modulate neural responses under dual-stimulation protocol. These findings provide insights into ultrasound-based neuromodulation and sensory processing.

Cortical Plasticity Induced by Pairing Primary Motor Cortex Transcranial Magnetic Stimulation With Subthalamic Nucleus Magneto-Acoustic Coupling Stimulation

Paired cortical and deep stimulation has the potential to induce enhanced cortical plasticity. Ideally, such stimulation should be noninvasive and precisely controlled. A novel paired stimulation method, combining transcranial magnetic stimulation (TMS) with transcranial magneto-acoustic coupled stimulation (TMAS), named TMS-TMAS, was proposed to achieve such stimulations. Although the primary motor cortex (M1) is stimulated using TMS, the pulsed magnetic field is coupled with a focused ultrasound field to achieve TMAS-based focused electrical stimulation of the subthalamic nucleus (STN) via the magneto-acoustic coupling effect. Cortical plasticity is induced by precisely controlling the timing of magnetic pulse and ultrasound emissions based on spike timing-dependent plasticity (STDP). The experimental system achieved cortical-focused magnetic stimulation with a transverse resolution of 4.3 mm, a longitudinal resolution of 2.8 mm, and a magnetic field intensity of 1.6 T in the M1 region. Additionally, deep-focused electrical stimulation with a transverse resolution of 1.6 mm, a longitudinal resolution of 9.9 mm, and a coupled electric field intensity of 280 mV/m in the STN region was realized. In vivo animal experiments demonstrated that TMS-TMAS enhanced the amplitude of motor evoked potential (MEP) and reduced response latency. Simulation and experimental results confirmed that TMS-TMAS achieves high spatial resolution, noninvasive paired stimulation of the cortex and deep nuclei, and induces enhanced cortical plasticity when the stimulation sequence satisfies the STDP criteria. This method provides a promising approach for noninvasive paired stimulation and is expected to advance brain science research and the rehabilitation of neuropsychiatric disorders involving deep brain structures.

Precise Cell Type Electrical Stimulation Therapy Via Force-electric Hydrogel Microspheres for Cartilage Healing

Electrical stimulation enhances cellular activity, promoting tissue regeneration and repair. However, specific cells and maintaining a stable energy supply are challenges for precise cell electrical stimulation therapy. Here, force-electric conversion hydrogel microspheres (Piezo@CR MPs) is devloped to induce specific stem cell aggregation and promote chondrogenic differentiation through localized electrical stimulation. These MPs contain barium titanate (BT) nanoparticles embedded in hyaluronic acid methacrylate hydrogel MPs, with a polydopamine (pDA) coating bound to stem cell recruitment peptides (CR) via π-π conjugation and electrostatic forces. Piezo@CR MPs convert pressure (ultrasound) into electrical stimulation, directing BMSCs for colonization and chondrogenesis. In vitro, directionally migrated stem cells almost covered the Piezo@CR MP surface, generating up to 451 mV of electrical output that enhanced chondrogenic differentiation. In a rabbit osteochondral defect model, Piezo@CR MPs promoted cartilage regeneration, nearly resembling native cartilage. In a rat osteoarthritis model, they reduced cartilage degeneration and improved behavioral outcomes. Additionally, Piezo@CR MPs promoted cartilage regeneration by driving the influx of extracellular calcium and activating the p38 mitogen-activated protein kinase (MAPK) pathway. In conclusion, Piezo@CR MPs offer a new approach for precise cell type electrical stimulation therapy in treating of cartilage injuries and degeneration.

Volumetric Passive Acoustic Mapping and Cavitation Detection of Nanobubbles under Low-Frequency Insonation

Gas bubbles, commonly used in medical ultrasound (US), witness advancements with nanobubbles (NB), providing improved capabilities over microbubbles (MB). NBs offer enhanced penetration into capillaries and the ability to extravasate into tumors following systemic injection, alongside prolonged circulation and persistent acoustic contrast. Low-frequency insonation (<1 MHz) with NBs holds great potential in inducing significant bioeffects, making the monitoring of their acoustic response critical to achieving therapeutic goals. We introduce a US-guided focused US system comprising a one-dimensional (1D) motorized rotating imaging transducer positioned within a low-frequency therapeutic transducer (center frequencies of 105 and 200 kHz), facilitating precise monitoring of NB cavitation activity in three-dimensional (3D) and comparison with MBs. Passive cavitation detection (PCD) revealed frequency-dependent responses, with NBs exhibiting significantly higher stable and inertial cavitation doses compared to MBs of the same gas volume when excited at a center frequency of 105 kHz and peak negative pressures ranging from 100 to 350 kPa. At 200 kHz, MBs showed higher cavitation doses than NBs. PCD showed that 105 kHz enhanced both NBs' and MBs' oscillations compared to 200 kHz. The system was further used for 3D passive acoustic mapping (PAM) to provide spatial resolution alongside PCD monitoring. Two-dimensional PAM was captured for each rotation angle and used to generate a complete 3D PAM reconstruction. Experimental results obtained from a tube phantom demonstrated consistent contrast PAM full-width half-maximum (FWHM) as a function of rotation angle, with similar FWHM between MBs and NBs. Frequency-selective PAM maps distinguished between stable and inertial cavitation via the harmonic, ultraharmonic and broadband content, offering insights into cavitation dynamics. These findings highlight NBs' superior performance at lower frequencies. The developed 3D-PAM technique with a 1D transducer presents a promising technology for real-time, noninvasive monitoring of cavitation-based US therapies.

Focused Ultrasound Combined With Microbubbles Attenuate Symptoms in Heroin-Addicted Mice

OBJECTIVE

To explore the efficacy and mechanisms of stimulating the nucleus accumbens (NAc) in heroin-addicted mice using focused ultrasound and microbubbles (MBs).

METHODS

The conditioned place preference (CPP) method was employed to establish a heroin-addicted mice model. Mice were randomized into control (C), heroin (H), heroin + ultrasound (H + U) and H + U + MBs. Ultrasound (2 MHz fundamental frequency, 1.34 MPa peak-negative pressure, 1 MHz pulse repetition frequency, 5% duty cycle, 15 min/d, over 2 d) was applied to stimulate the NAc in the latter 2 groups. Whereas H + U + MBs received an injection of sulfur hexafluoride MBs during the stimulation. Subsequently, CPP scores, open-field test (OFT), and elevated plus-maze test (EPMT) were conducted to assess behavioral changes in addiction memory, anxiety and exercise status. HE staining was performed to detect pathological structures. Neurotransmitters such as dopamine (DA), serotonin (5-HT) and glutamate (Glu) were detected using ultra-performance liquid chromatography-tandem mass spectrometry (UPLC-MS/MS). Transmission electron microscopy (TEM) was used to observe ultrastructural changes of synapses in NAc. Immunohistochemistry (IHC) was utilized to detect Cleaved Caspase-3 in the NAc region. Western blotting (WB) was used to detect the protein expression of Cleaved Caspase-3, Bax and Bcl-2 in NAc.

RESULTS

HE staining showed small patches of erythrocyte exudation were observed in the NAc and adjacent areas in H + U + MBs. The CPP scores of H + U + MBs were lower (p < 0.05) than H. After ultrasound treatment, all indices of the OFT and EPMT in H + U + MBs were significantly higher than H (p < 0.05). UPLC-MS/MS revealed that the levels of DA, 5-HT and Glu in H + U + MBs were lower than H (p < 0.01). TEM showed decrease the number of synapses (p < 0.05), and noticeable swelling of mitochondria, membrane damage, as well as damage to the cristae. Further detection by IHC and WB showed that the pro-apoptotic proteins Cleaved Caspase-3 and Bax increased and Bcl-2 decreased as anti-apoptotic proteins after ultrasound combined with MBs (p < 0.05).

CONCLUSION

Focused ultrasound combined with MBs stimulate the NAc can weaken the addictive memory and improve anxiety of heroin-related mice. The mechanical effect of ultrasound combined with the cavitation effect may be a potential treatment for addiction.

Transcranial focused ultrasound precise neuromodulation: a review of focal size regulation, treatment efficiency and mechanisms

Ultrasound is a mechanical wave that can non-invasively penetrate the skull to deep brain regions to activate neurons. Transcranial focused ultrasound neuromodulation is a promising approach, with the advantages of noninvasiveness, high-resolution, and deep penetration, which developed rapidly over the past years. However, conventional transcranial ultrasound's spatial resolution is low-precision which hinders its use in precision neuromodulation. Here we focus on methods that could increase the spatial resolution, gain modulation efficiency at the focal spot, and potential mechanisms of ultrasound neuromodulation. In this paper, we summarize strategies to enhance the precision of ultrasound stimulation, which could potentially improve the ultrasound neuromodulation technic.

Advanced Ultrasound Energy Transfer Technologies using Metamaterial Structures

Wireless energy transfer (WET) based on ultrasound-driven generators with enormous beneficial functions, is technologically in progress by the valuation of ultrasonic metamaterials (UMMs) in science and engineering domains. Indeed, novel metamaterial structures can develop the efficiency of mechanical and physical features of ultrasound energy receivers (US-ETs), including ultrasound-driven piezoelectric and triboelectric nanogenerators (US-PENGs and US-TENGs) for advantageous applications. This review article first summarizes the fundamentals, classification, and design engineering of UMMs after introducing ultrasound energy for WET technology. In addition to addressing using UMMs, the topical progress of innovative UMMs in US-ETs is conceptually presented. Moreover, the advanced approaches of metamaterials are reported in the categorized applications of US-PENGs and US-TENGs. Finally, some current perspectives and encounters of UMMs in US-ETs are offered. With this objective in mind, this review explores the potential revolution of reliable integrated energy transfer systems through the transformation of metamaterials into ultrasound-driven active mediums for generators.

Neuromodulatory Responses Elicited by Intermittent versus Continuous Transcranial Focused Ultrasound Stimulation of the Motor Cortex in Rats

Transcranial focused ultrasound stimulation (tFUS) has emerged as a promising neuromodulation technique that delivers acoustic energy with high spatial resolution for inducing long-term potentiation (LTP)- or depression (LTD)-like plasticity. The variability in the primary effects of tFUS-induced plasticity could be due to different stimulation patterns, such as intermittent versus continuous, and is an aspect that requires further detailed exploration. In this study, we developed a platform to evaluate the neuromodulatory effects of intermittent and continuous tFUS on motor cortical plasticity before and after tFUS application. Three groups of rats were exposed to either intermittent, continuous, or sham tFUS. We analyzed the neuromodulatory effects on motor cortical excitability by examining changes in motor-evoked potentials (MEPs) elicited by transcranial magnetic stimulation (TMS). We also investigated the effects of different stimulation patterns on excitatory and inhibitory neural biomarkers, examining c-Fos and glutamic acid decarboxylase (GAD-65) expression using immunohistochemistry staining. Additionally, we evaluated the safety of tFUS by analyzing glial fibrillary acidic protein (GFAP) expression. The current results indicated that intermittent tFUS produced a facilitation effect on motor excitability, while continuous tFUS significantly inhibited motor excitability. Furthermore, neither tFUS approach caused injury to the stimulation sites in rats. Immunohistochemistry staining revealed increased c-Fos and decreased GAD-65 expression following intermittent tFUS. Conversely, continuous tFUS downregulated c-Fos and upregulated GAD-65 expression. In conclusion, our findings demonstrate that both intermittent and continuous tFUS effectively modulate cortical excitability. The neuromodulatory effects may result from the activation or deactivation of cortical neurons following tFUS intervention. These effects are considered safe and well-tolerated, highlighting the potential for using different patterns of tFUS in future clinical neuromodulatory applications.

Study on the Role of Physical Fields in TMAS to Modulate Synaptic Plasticity in Mice

OBJECTIVE

Transcranial magneto-acoustic stimulation (TMAS) is a composite technique combining static magnetic and coupled electric fields with transcranial ultrasound stimulation (TUS) and has shown advantages in neuromodulation. However, the role of these physical fields in neuromodulation is unclear. Synaptic plasticity is the cellular basis for learning and memory. In this paper, we varied the intensity of static magnetic, electric and ultrasonic fields respectively to investigate the modulation of synaptic plasticity by these physical fields.

METHODS

There are control, static magnetic field (0.1 T/0.2 T), TUS (0.15/0.3 MPa), and TMAS (0.15 MPa + 0.2 V/m, 0.3 MPa + 0.2 V/m, 0.3 MPa + 0.4 V/m) groups. Hippocampal areas were stimulated at 5 min daily for 7 days and in vivo electrophysiological experiments were performed.

RESULTS

TMAS induced greater LTP, LTD, and paired-pulse ratio (PPR) than TUS, reflecting that TMAS has a more significant modulation in both long- and short- term synaptic plasticity. In TMAS, a doubling of the electric field amplitude increases LTP, LTD and PPR to a greater extent than a doubling of the acoustic pressure. Increasing the static magnetic field intensity has no significant effect on the modulation of synaptic plasticity.

CONCLUSION

This paper argues that electric fields should be the main reason for the difference in modulation between TMAS and TUS and that changing the amplitude of the electric field affected the modulation of TMAS more than changing the acoustic pressure.

SIGNIFICANCE

This study elucidates the roles of the physical fields in TMAS and provides a parameterisation way to guide TMAS applications based on the dominant roles of the physical fields.

High-spatial-resolution transcranial focused ultrasound neuromodulation using frequency-modulated pattern interference radiation force

Stimulating the brain in a precise location is crucial in ultrasound neuromodulation. However, improving the resolution proves a challenge owing to the characteristics of transcranial focused ultrasound. In this paper, we present a new neuromodulation system that overcomes the existing limitations based on an acoustic radiation force with a frequency-modulated waveform and standing waves. By using the frequency-modulated pattern interference radiation force (FM-PIRF), the axial spatial resolution can be reduced to a single wavelength level and the target location can be controlled in axial direction electronically. A linear frequency-modulated chirp waveform used in the experiment was designed based on the simulation results. The displacement of the polydimethylsiloxane (PDMS) cantilever was measured at intervals of 0.1 mm to visualize the distribution of radiation force. These results and methods experimentally show that FM-PIRF has improved spatial resolution and capability of electrical movement.

Functional ultrasound (fUS) imaging of displacement-guided focused ultrasound (FUS) neuromodulation in mice

Focused ultrasound (FUS) stimulation is a promising neuromodulation technique with the merits of non-invasiveness, high spatial resolution, and deep penetration depth. However, simultaneous imaging of FUS-induced brain tissue displacement and the subsequent effect of FUS stimulation on brain hemodynamics has proven challenging thus far. In addition, earlier studies lack in situ confirmation of targeting except for the magnetic resonance imaging-guided FUS system-based studies. The purpose of this study is 1) to introduce a fully ultrasonic approach to in situ target, modulate neuronal activity, and monitor the resultant neuromodulation effect by respectively leveraging displacement imaging, FUS, and functional ultrasound (fUS) imaging, and 2) to investigate FUS-evoked cerebral blood volume (CBV) response and the relationship between CBV and displacement. We performed displacement imaging on craniotomized mice to confirm the in targeting for neuromodulation site. We recorded hemodynamic responses evoked by FUS and fUS revealed an ipsilateral CBV increase that peaks at 4 s post-FUS. We saw a stronger hemodynamic activation in the subcortical region than cortical, showing good agreement with the brain elasticity map that can also be obtained using a similar methodology. We observed dose-dependent CBV response with peak CBV, activated area, and correlation coefficient increasing with ultrasonic dose. Furthermore, by mapping displacement and hemodynamic activation, we found that displacement colocalizes and linearly correlates with CBV increase. The findings presented herein demonstrated that FUS evokes ipsilateral hemodynamic activation in cortical and subcortical depths and the evoked hemodynamic responses colocalized and correlate with FUS-induced displacement. We anticipate that our findings will help consolidate accurate targeting as well as an understanding of how FUS displaces brain tissue and affects cerebral hemodynamics.

Patient-specific modeling for guided rehabilitation of stroke patients: the BrainX3 use-case

BrainX3 is an interactive neuroinformatics platform that has been thoughtfully designed to support neuroscientists and clinicians with the visualization, analysis, and simulation of human neuroimaging, electrophysiological data, and brain models. The platform is intended to facilitate research and clinical use cases, with a focus on personalized medicine diagnostics, prognostics, and intervention decisions. BrainX3 is designed to provide an intuitive user experience and is equipped to handle different data types and 3D visualizations. To enhance patient-based analysis, and in keeping with the principles of personalized medicine, we propose a framework that can assist clinicians in identifying lesions and making patient-specific intervention decisions. To this end, we are developing an AI-based model for lesion identification, along with a mapping of tract information. By leveraging the patient's lesion information, we can gain valuable insights into the structural damage caused by the lesion. Furthermore, constraining whole-brain models with patient-specific disconnection masks can allow for the detection of mesoscale excitatory-inhibitory imbalances that cause disruptions in macroscale network properties. Finally, such information has the potential to guide neuromodulation approaches, assisting in the choice of candidate targets for stimulation techniques such as Transcranial Ultrasound Stimulation (TUS), which modulate E-I balance, potentiating cortical reorganization and the restoration of the dynamics and functionality disrupted due to the lesion.

A Review of Ultrasound Neuromodulation Technologies

The invasiveness of neuromodulation technologies that require surgical implantation (e.g., electrical and optical stimulation) may limit their clinical application. Thus, alternative technologies that offer similar benefits without surgery are of paramount importance in the field of neuromodulation. Low-intensity ultrasound is an emerging modality for neural stimulation as ultrasound can be focused in deep tissues with millimeter resolution. Transcranial focused ultrasound stimulation (tFUS) has already been demonstrated in a wide range of animals and even humans at different sonication frequencies (mostly in the sub-MHz range due to the presence of the skull). This article first provides some fundamental knowledge in ultrasound, and then reviews various examples of successful tFUS experiments in animals and humans using different stimulation patterns, as well as available tFUS technologies for generating, focusing, and steering ultrasound beams in neural tissues. In particular, phased array technologies for the ultrasound stimulation application are discussed with an emphasis on the design, fabrication, and integration of ultrasound transducer arrays as well as the design and development of phased array electronics with beamformer and high-voltage driver circuitry. The challenges in tFUS, such as its underlying mechanism, indirect auditory response, and skull aberration effects, are also discussed.

Multifocal skull-compensated transcranial focused ultrasound system for neuromodulation applications based on acoustic holography

Transcranial focused ultrasound stimulation is a promising therapeutic modality for human brain disorders because of its noninvasiveness, long penetration depth, and versatile spatial control capability through beamforming and beam steering. However, the skull presents a major hurdle for successful applications of ultrasound stimulation. Specifically, skull-induced focal aberration limits the capability for accurate and versatile targeting of brain subregions. In addition, there lacks a fully functional preclinical neuromodulation system suitable to conduct behavioral studies. Here, we report a miniature ultrasound system for neuromodulation applications that is capable of highly accurate multiregion targeting based on acoustic holography. Our work includes the design and implementation of an acoustic lens for targeting brain regions with compensation for skull aberration through time-reversal recording and a phase conjugation mirror. Moreover, we utilize MEMS and 3D-printing technology to implement a 0.75-g lightweight neuromodulation system and present in vivo characterization of the packaged system in freely moving mice. This preclinical system is capable of accurately targeting the desired individual or multitude of brain regions, which will enable versatile and explorative behavior studies using ultrasound neuromodulation to facilitate widespread clinical adoption.

Experimental demonstration of Fresnel zone plate lens for robust subwavelength focusing at mega hertz

Acoustic focusing with high energy intensity, broadband and few side lobes properties are highly demanded in many fields and can be achieved with Fresnel zone plate (FZP) lens as an artificial structure. However, FZP focusing lenses for high frequency range such as around 5 MHz with precise focusing effect has not been demonstrated and the dependence of the size, especially the full width at half maximum (FWHM), of the focal spot has not been well explored. In this work, we numerically and experimentally study the FZP lenses for robust and subwavelength underwater sound focusing effect around 5 MHz. Stable subwavelength FWHM of the focusing spot over a broad frequency range is achieved. It is found that the FWHM has a quasi-linear relationship with the ratio between the focal length and the diameter of the lens. A quasi-linear relationship between the focal length and working frequency is also validated theoretically, numerically and experimentally. Our study proposes broadband and robust FZP lenses paving the way for high quality acoustic focusing at Mega Hertz frequency range for the development of imaging, non-destructive evaluation, and so on.

Ultrasound-mediated piezoelectric nanoparticle modulation of intrinsic cardiac autonomic nervous system for rate control in atrial fibrillation

Rate control is a cornerstone of atrial fibrillation treatment. Barium titanate nanoparticles (BTNPs) are piezoelectric nanomaterials that can generate local electromagnetic fields under ultrasound activation, stimulating nearby neuronal tissue. This study aimed to modulate the inferior right ganglionated plexus (IRGP) of the heart and reduce the ventricular rate during rapid atrial pacing (RAP)-induced atrial fibrillation using ultrasound-mediated BTNPs. Adult male beagles were randomly divided into a phosphate-buffered saline (PBS) group (n = 6) and a BTNP group (n = 6). PBS or nanoparticles were injected into the IRGP of both groups before RAP. The biological safety of the material was evaluated according to electrophysiology recordings, thermal effects and level of inflammation. Compared to the PBS group, the BaTiO₃ piezoelectric nanoparticle group had reduced ventricular rates in the sinus rhythm and atrial fibrillation models after stimulating the IRGP by applying ultrasound. In addition, transient stimulation by BTNPs did not lead to sustained neuronal excitation in the IRGP. The activation of the BTNPs did not induce inflammation or thermal damage effects in the IRGP. Ultrasound-mediated BTNP neuromodulation can significantly reduce the ventricular rate by stimulating the IRGP. Thus, ultrasound-mediated BTNP neuromodulation is a potential therapy for atrial fibrillation rate control.

Transcranial low-intensity ultrasound stimulation for treating central nervous system disorders: A promising therapeutic application

Transcranial ultrasound stimulation is a neurostimulation technique that has gradually attracted the attention of researchers, especially as a potential therapy for neurological disorders, because of its high spatial resolution, its good penetration depth, and its non-invasiveness. Ultrasound can be categorized as high-intensity and low-intensity based on the intensity of its acoustic wave. High-intensity ultrasound can be used for thermal ablation by taking advantage of its high-energy characteristics. Low-intensity ultrasound, which produces low energy, can be used as a means to regulate the nervous system. The present review describes the current status of research on low-intensity transcranial ultrasound stimulation (LITUS) in the treatment of neurological disorders, such as epilepsy, essential tremor, depression, Parkinson's disease (PD), and Alzheimer's disease (AD). This review summarizes preclinical and clinical studies using LITUS to treat the aforementioned neurological disorders and discusses their underlying mechanisms.

Optically-generated focused ultrasound for noninvasive brain stimulation with ultrahigh precision

High precision neuromodulation is a powerful tool to decipher neurocircuits and treat neurological diseases. Current non-invasive neuromodulation methods offer limited precision at the millimeter level. Here, we report optically-generated focused ultrasound (OFUS) for non-invasive brain stimulation with ultrahigh precision. OFUS is generated by a soft optoacoustic pad (SOAP) fabricated through embedding candle soot nanoparticles in a curved polydimethylsiloxane film. SOAP generates a transcranial ultrasound focus at 15 MHz with an ultrahigh lateral resolution of 83 µm, which is two orders of magnitude smaller than that of conventional transcranial-focused ultrasound (tFUS). Here, we show effective OFUS neurostimulation in vitro with a single ultrasound cycle. We demonstrate submillimeter transcranial stimulation of the mouse motor cortex in vivo. An acoustic energy of 0.6 mJ/cm², four orders of magnitude less than that of tFUS, is sufficient for successful OFUS neurostimulation. OFUS offers new capabilities for neuroscience studies and disease treatments by delivering a focus with ultrahigh precision non-invasively.

Transcranial ultrasound stimulation applied in ischemic stroke rehabilitation: A review

Ischemic stroke is a serious medical condition that is caused by cerebral vascular occlusion and leads to neurological dysfunction. After stroke, patients suffer from long-term sensory, motor and cognitive impairment. Non-invasive neuromodulation technology has been widely studied in the field of stroke rehabilitation. Transcranial ultrasound stimulation (TUS), as a safe and non-invasive technique with deep penetration ability and a tiny focus, is an emerging technology. It can produce mechanical and thermal effects by delivering sound waves to brain tissue that can induce the production of neurotrophic factors (NFs) in the brain, and reduce cell apoptosis and the inflammatory response. TUS, which involves application of an acoustic wave, can also dissolve blood clots and be used to deliver therapeutic drugs to the ischemic region. TUS has great potential in the treatment of ischemic stroke. Future advancements in imaging and parameter optimization will improve the safety and efficacy of this technology in the treatment of ischemic stroke.

Stent with Piezoelectric Transducers for High Spatial Resolution Ultrasound Neuromodulation- a Finite Element Analysis

Deep brain stimulation is currently the only technique used in the clinical setting to modulate the neural activity of deep brain nuclei. Recently, low-intensity transcranial focused ultrasound (LIFU) has been shown to reversibly modulate brain activity through a transcranial pathway. Transcranial LIFU requires a low-frequency ultrasound of around 0.5 MHz due to skull attenuation, thus providing poor axial and lateral resolution. This paper proposes a new conceptual device that would use a stent to place a high-frequency ultrasound array within the brain vasculature to achieve high axial and lateral spatial resolution. The first part of this work identified the most commonly treated deep brain nuclei and examined the human brain vasculature for stent placement. Next, a finite element analysis was carried out using a piezoelectric array that follows the blood vessels curvature, and its ability to focus ultrasound waves in clinically relevant brain nuclei was evaluated. The analytical solution provided promising results for deep brain stimulation via a stent with ultrasound transducers for high spatial resolution neuromodulation.

Enhancement of functional corticomuscular coupling after transcranial ultrasound stimulation in mice

OBJECTIVE

Transcranial ultrasound stimulation (TUS), a large penetration depth and high spatial resolution technology, has developed rapidly in recent years. This study aimed to explore and evaluate the neuromodulation effects of TUS on mouse motor neural circuits under different parameters.

APPROACH

Our study used functional corticomuscular coupling (FCMC) as an index to explore the modulation mechanism for movement control under different TUS parameters (intensity [Isppa] and stimulation duration [SD]). We collected local field potential (LFP) and tail electromyographic (EMG) data under TUS in healthy mice and then introduced the time-frequency coherence method to analyze the FCMC before and after TUS in the time-frequency domain. After that, we defined the relative coherence area (RCA) to quantify the coherence between LFP and EMG under TUS.

MAIN RESULTS

The FCMC at theta, alpha, beta, and gamma bands was enhanced after TUS, and the neuromodulation efficacy mainly occurred in the lower frequency band (theta and alpha band). After TUS with different parameters, the FCMC in all selected frequency bands showed a tendency of increasing first and then decreasing. Further analysis showed that the maximum coupling value of theta band appeared from 0.2 to 0.4 s, and that the maximum coupling value in alpha and gamma band appeared from 0 to 0.2 s.

SIGNIFICANCE

The aforementioned results demonstrate that FCMC in the motor cortex could be modulated by TUS. We provide a theoretical basis for further exploring the modulation mechanism of TUS parameters and clinical application.

Ultrasound Neurostimulation in Mice: Impact of Ultrasound Settings and Beam Properties

Ultrasound neurostimulation (USNS) is being investigated as a treatment approach for neuropsychiatric and neurodegenerative disorders. Indeed, unlike the existing methods that use electric or magnetic stimulation, it offers the possibility to modulate brain activity in a noninvasive way, with good spatial specificity and a high penetration capacity. However, there is no consensus yet on ultrasound parameters and beam properties required for efficient neurostimulation. In this context, this preclinical study aimed to elucidate the effect of frequency, peak negative pressure (PNP), pulse duration (PD), and focal spot diameter, on the USNS efficiency. This was done by targeting the motor cortex (M1) of 70 healthy mice and analyzing the elicited motor responses (visually and with electromyography). Also, a further investigation was performed by assessing the corresponding neuronal activity, using c-Fos immunostaining. The results showed that the success rate, a metric that depicts USNS efficacy, increased with PNP in a sigmoidal way, reaching up to 100%. This was verified at different frequencies (0.5, 1, 1.5, and 2.25 MHz) and PDs (53.3, 160, and 320 ms, at 1.5 MHz fixed frequency). Moreover, it was shown that higher PNP values were required to achieve a constant USNS efficacy not only when frequency increased, but also when the focal spot diameter decreased, emphasizing a close link between these acoustic parameters and USNS efficacy. These findings were confirmed with immunohistochemistry (IHC), which showed a strong relationship between neural activation, the applied PNP, and the focal spot diameter.

Trans-Spinal Focused Ultrasound Stimulation Selectively Modulates Descending Motor Pathway

Compared to current non-invasive methods utilizing magnetic and electrical means, focused ultrasound provides greater spatial resolution and penetration depth. Despite the broad application of ultrasound stimulation, there is a lack of studies dedicated to the investigation of acoustic neuromodulation on the spinal cord. This study aims to apply focused ultrasound on the spinal cord to modulate the descending pathways in a non-invasive fashion. The application of trans-spinal focused ultrasound (tsFUS) was examined on the motor deficit mouse model. tsFUS was achieved using a single-element focused ultrasound transducer operating at 3 MHz. The sonication was performed on anesthetized 6 week-old mice targeting T12 and L3 vertebrae. The effect was analyzed by comparing electromyography responses from the hindlimb induced by electrical stimulation of the motor cortex. Further, the mouse model with the Harmaline-induced essential tremor was selected to investigate the potential clinical application of tsFUS. The safety was verified by histological assessment. Sonication at the T12 area inhibited motor response, while sonication over the L3 region provided signal enhancement. Sonication of T12 of the ET mouse also showed the ability of ultrasound to suppress tremors. Meanwhile, the histological examination did not show any abnormalities with the highest applied acoustic pressure. In this work, a non-invasive motor signal modulation was achieved using tsFUS. Moreover, the results showed the ability of focused ultrasound to manage tremors in a safe manner. This study provides a stepping stone for the trans-spinal application of focused ultrasound to motor-related disorders.

Region-specific effects of ultrasound on individual neurons in the awake mammalian brain

Ultrasound modulates brain activity. However, it remains unclear how ultrasound affects individual neurons in the brain, where neural circuit architecture is intact and different brain regions exhibit distinct tissue properties. Using a high-resolution calcium imaging technique, we characterized the effect of ultrasound stimulation on thousands of individual neurons in the hippocampus and the motor cortex of awake mice. We found that brief 100-ms-long ultrasound pulses increase intracellular calcium in a large fraction of individual neurons in both brain regions. Ultrasound-evoked calcium response in hippocampal neurons exhibits a rapid onset with a latency shorter than 50 ms. The evoked response in the hippocampus is shorter in duration and smaller in magnitude than that in the motor cortex. These results demonstrate that noninvasive ultrasound stimulation transiently increases intracellular calcium in individual neurons in awake mice, and the evoked response profiles are brain region specific.

A Wearable Ultrasonic Neurostimulator-Part II: A 2D CMUT Phased Array System With a Flip-Chip Bonded ASIC

A 2D ultrasonic array is the ultimate form of a focused ultrasonic system, which enables electronically focusing beams in a 3D space. A 2D array is also a versatile tool for various applications such as 3D imaging, high-intensity focused ultrasound, particle manipulation, and pattern generation. However, building a 2D system involves complicated technologies: fabricating a 2D transducer array, developing a pitch-matched ASIC, and interconnecting the transducer and the ASIC. Previously, we successfully demonstrated 2D capacitive micromachined ultrasonic transducer (CMUT) arrays using various fabrication technologies. In this paper, we present a 2D ultrasonic transmit phased array based on a 32 × 32 CMUT array flip-chip bonded to a pitch-matched pulser ASIC for ultrasonic neuromodulation. The ASIC consists of 32 × 32 unipolar high-voltage (HV) pulsers, each of which occupies an area of 250 μm × 250 μm. The phase of each pulser output is individually programmable with a resolution of 1/f_C/16, where f_C is less than 10 MHz. This enables the fine granular control of a focus. The ASIC was fabricated in the TSMC 0.18- μm HV BCD process within an area of 9.8 mm × 9.8 mm, followed by a wafer-level solder bumping process. After flip-chip bonding an ASIC and a CMUT array, we identified shorted elements in the CMUT array using the built-in test function in the ASIC, which took approximately 9 minutes to scan the entire 32 × 32 array. A compact-form-factor wireless neural stimulator system-only requiring a connected 15-V DC power supply-was also developed, integrating a power management unit, a clock generator, and a Bluetooth Low-Energy enabled microcontroller. The focusing and steering capability of the system in a 3D space is demonstrated, while achieving a spatial-peak pulse-average intensity ( I_SPPA) of 12.4 and 33.1 W/ cm²; and a 3-dB focal volume of 0.2 and 0.05 mm³-at a depth of 5 mm-at 2 and 3.4 MHz, respectively. We also characterized transmission of ultrasound through a mouse skull and compensated the phase distortion due to the skull by using the programmable phase-delay function in the ASIC, achieving 10% improvement in pressure and a tighter focus. Finally, we demonstrated a ultrasonic arbitrary pattern generation on a 5 mm × 5 mm plane at a depth of 5 mm.

A Wearable Ultrasonic Neurostimulator - Part I: A 1D CMUT Phased Array System for Chronic Implantation in Small Animals

In this work, we present a wireless ultrasonic neurostimulator, aiming at a truly wearable device for brain stimulation in small behaving animals. A 1D 5-MHz capacitive micromachined ultrasonic transducer (CMUT) array is adopted to implement a head-mounted stimulation device. A companion ASIC with integrated 16-channel high-voltage (60-V) pulsers was designed to drive the 16-element CMUT array. The ASIC can generate excitation signals with element-wise programmable phases and amplitudes: 1) programmable sixteen phase delays enable electrical beam focusing and steering, and 2) four scalable amplitude levels, implemented with a symmetric pulse-width-modulation technique, are sufficient to suppress unwanted sidelobes (apodization). The ASIC was fabricated in the TSMC 0.18- μm HV BCD process within a die size of 2.5 × 2.5 mm². To realize a completely wearable system, the system is partitioned into two parts for weight distribution: 1) a head unit (17 mg) with the CMUT array, 2) a backpack unit (19.7 g) that includes electronics such as the ASIC, a power management unit, a wireless module, and a battery. Hydrophone-based acoustic measurements were performed to demonstrate the focusing and beam steering capability of the proposed system. Also, we achieved a peak-to-peak pressure of 2.1 MPa, which corresponds to a spatial peak pulse average intensity ( I_SPPA) of 33.5 W/cm², with a lateral full width at half maximum (FWHM) of 0.6 mm at a depth of 3.5 mm.

Wearable Transcranial Ultrasound System for Remote Stimulation of Freely Moving Animal

OBJECTIVE

Transcranial focused ultrasound (tFUS) has drawn considerable attention in the neuroscience field as a noninvasive approach to modulate brain circuits. However, the conventional approach requires the use of anesthetized or immobilized animal models, which places considerable restrictions on behavior and affects treatment. Thus, this work presents a wireless, wearable system to achieve ultrasound brain stimulation in freely behaving animals.

METHODS

The wearable tFUS system was developed based on a microcontroller and amplifier circuit. Brain activity induced by tFUS was monitored through cerebral hemodynamic changes using near-infrared spectroscopy. The system was also applied to stroke rehabilitation after temporal middle cerebral artery occlusion (tMCAO) in rats. Temperature calculations and histological results showed the safety of the application even with prolonged 40 min sonication.

RESULTS

The output ultrasonic wave produced from a custom PZT transducer had a central frequency of 457 kHz and peak to peak pressure of 426 kPa. The device weight was 20 g, allowing a full range of motion. The stimulation was found to induce hemodynamic changes in the sonicated area, while open-field tests showed that ultrasound applied to the ipsilateral hemisphere for 5 consecutive days after the stroke facilitated recovery.

CONCLUSION

The wearable tFUS system has been designed and implemented on moving rats. The results showed the ability of device to cause both short- and long lasting effects.

SIGNIFICANCE

The proposed device provides a more natural environment to investigate the effects of tFUS for behavioral and long-term studies.

Implication of auditory confounding in interpreting somatosensory and motor responses in low-intensity focused transcranial ultrasound stimulation

Low-intensity transcranial focused ultrasound (LI-tFUS) stimulation is a noninvasive neuromodulation tool that demonstrates high target localization accuracy and depth penetration. It has been shown to modulate activities in the primary motor and somatosensory cortex. Previous studies in animals as well as in humans, illustrated in the recently published paper in Brain Stimulation by Braun et al. [Braun V, Blackmore J, Cleveland RO, Butler CR. Brain Stimul 13: 1527-1534, 2020], acknowledged the possibility of indirect stimulation of the peripheral auditory pathway that could confound the somatosensory and motor responses observed with LI-tFUS stimulation. Here, we discuss the implications and interpretations of auditory confounding in the context of neuromodulation.

Transcranial Ultrasound Stimulation Suppresses Neuroinflammation in a Chronic Mouse Model of Parkinson's Disease

OBJECTIVE

Neuroinflammation contributes to the development and progression of Parkinson's disease (PD). The aim of this study was to examine whether ultrasound (US) stimulation of the subthalamic nucleus (STN) could suppress the neuroinflammation in a chronic PD mouse model induced by 1-Methyl-4-phenyl-1, 2, 3, 6-tetrahydropyridine (MPTP).

METHODS

A chronic PD mouse model was built by injections of 20mg/kg MPTP and 250 mg/kg probenecid at 3.5-day intervals for 5 weeks. Mice were randomized into control+sham, MPTP+sham and MPTP+STN+US group. For MPTP+STN+US group, ultrasound wave (3.8 MHz, 50% duty cycle, 1 kHz pulse repetition frequency, 30 min/day) was delivered to the STN the day after MPTP and probenecid injection (the early stage of PD progression). The rotarod test and pole test were performed to evaluate the behavioral changes after ultrasound treatment. Then, the activity of microglia and astrocyte were measured to evaluate the inflammation level in the brain.

RESULTS

Ultrasound stimulation improved the latency to falls in the rotarod test (p = 0.033) and decreased the climbing time in the pole test (p = 0.016) compared with MPTP+sham group. Moreover, ultrasound stimulation reduced the chronic inflammation response as shown in microglia (p = 0.007) and astrocyte (p = 0.032) activation. In addition, HE, Nissl and Tunel staining showed that no brain tissue injury was induced by US.

CONCLUSION

These findings demonstrated that ultrasound stimulation could suppress neuroinflammation in PD mice.

SIGNIFICANCE

Transcranial ultrasound neuromodulation offers a novel approach for Parkinson's disease intervention, potentially through its anti-neuroinflammation functions.

High-Resolution Focused Ultrasound Neuromodulation Induces Limb-Specific Motor Responses in Mice in Vivo

Ultrasound can modulate activity in the central nervous system, including the induction of motor responses in rodents. Recent studies investigating ultrasound-induced motor movements have described mostly bilateral limb responses, but quantitative evaluations have failed to reveal lateralization or differences in response characteristics between separate limbs or how specific brain targets dictate distinct limb responses. This study uses high-resolution focused ultrasound (FUS) to elicit motor responses in anesthetized mice in vivo and four-limb electromyography (EMG) to evaluate the latency, duration and power of paired motor responses (n = 1768). The results indicate that FUS generates target-specific differences in electromyographic characteristics and that brain targets separated by as little as 1 mm can modulate the responses in individual limbs differentially. Exploiting these differences may provide a tool for quantifying the susceptibility of underlying neural volumes to FUS, understanding the functioning of the targeted neuroanatomy and aiding in mechanistic studies of this non-invasive neuromodulation technique.

Targeted peripheral focused ultrasound stimulation attenuates obesity-induced metabolic and inflammatory dysfunctions

Obesity, a growing health concern, is associated with an increased risk of morbidity and mortality. Chronic low-grade inflammation is implicated in obesity-driven metabolic complications. Peripheral focused ultrasound stimulation (pFUS) is an emerging non-invasive technology that modulates inflammation. Here, we reasoned that focused ultrasound stimulation of the liver may alleviate obesity-related inflammation and other comorbidities. After 8 weeks on a high-fat high-carbohydrate "Western" diet, C57BL/6J mice were subjected to either sham stimulation or focused ultrasound stimulation at the porta hepatis. Daily liver-focused ultrasound stimulation for 8 weeks significantly decreased body weight, circulating lipids and mitigated dysregulation of adipokines. In addition, liver-focused ultrasound stimulation significantly reduced hepatic cytokine levels and leukocyte infiltration. Our findings demonstrate the efficacy of hepatic focused ultrasound for alleviating obesity and obesity-associated complications in mice. These findings suggest a previously unrecognized potential of hepatic focused ultrasound as a possible novel noninvasive approach in the context of obesity.

Transcranial focused ultrasound stimulation with high spatial resolution

BACKGROUND

Low-intensity transcranial focused ultrasound stimulation is a promising candidate for noninvasive brain stimulation and accurate targeting of brain circuits because of its focusing capability and long penetration depth. However, achieving a sufficiently high spatial resolution to target small animal sub-regions is still challenging, especially in the axial direction.

OBJECTIVE

To achieve high axial resolution, we designed a dual-crossed transducer system that achieved high spatial resolution in the axial direction without complex microfabrication, beamforming circuitry, and signal processing.

METHODS

High axial resolution was achieved by crossing two ultrasound beams of commercially available piezoelectric curved transducers at the focal length of each transducer. After implementation of the fixture for the dual-crossed transducer system, three sets of in vivo animal experiments were conducted to demonstrate high target specificity of ultrasound neuromodulation using the dual-crossed transducer system (n = 38).

RESULTS

The full-width at half maximum (FWHM) focal volume of our dual-crossed transducer system was under 0.52 μm³. We report a focal diameter in both lateral and axial directions of 1 mm. To demonstrate successful in vivo brain stimulation of wild-type mice, we observed the movement of the forepaws. In addition, we targeted the habenula and verified the high spatial specificity of our dual-crossed transducer system.

CONCLUSIONS

Our results demonstrate the ability of the dual-crossed transducer system to target highly specific regions of mice brains using ultrasound stimulation. The proposed system is a valuable tool to study the complex neurological circuitry of the brain noninvasively.

A High-Frequency Phased Array System for Transcranial Ultrasound Delivery in Small Animals

Existing systems for applying transcranial focused ultrasound (FUS) in small animals produce large focal volumes relative to the size of cerebral structures available for interrogation. The use of high ultrasonic frequencies can improve targeting specificity; however, the aberrations induced by rodent calvaria at megahertz frequencies severely distort the acoustic fields produced by single-element focused transducers. Here, we present the design, fabrication, and characterization of a high-frequency phased array system for transcranial FUS delivery in small animals. A transducer array was constructed by micromachining a spherically curved PZT-5H bowl (diameter = 25 mm, radius of curvature = 20 mm, fundamental frequency = 3.3 MHz) into 64 independent elements of equal surface area. The acoustic field generated by the phased array was measured at various target locations using a calibrated fiber-optic hydrophone, both in free-field conditions as well as through ex vivo rat skullcaps with and without hydrophone-assisted phase aberration corrections. Large field-of-view acoustic field simulations were carried out to investigate potential grating lobe formation. The focal beam size obtained when targeting the array's geometric focus was [Formula: see text] mm in water. The array can steer the FUS beam electronically over cylindrical volumes of 4.5 mm in diameter and 6 mm in height without introducing grating lobes. Insertion of a rat skullcap resulted in substantial distortion of the acoustic field ( [Formula: see text]% [Formula: see text]); however, phase corrections restored partial focal quality ( [Formula: see text]% [Formula: see text]). Using phase corrections, the array is capable of generating a trans-rat skull peak negative focal pressure of up to ~2.0 MPa, which is sufficient for microbubble-mediated blood-brain barrier permeabilization at this frequency.

Spherical Array System for High-Precision Transcranial Ultrasound Stimulation and Optoacoustic Imaging in Rodents

Ultrasound can be delivered transcranially to ablate brain tissue, open the blood-brain barrier, or affect neural activity. Transcranial focused ultrasound in small rodents is typically done with low-frequency single-element transducers, which results in unspecific targeting and impedes the concurrent use of fast neuroimaging methods. In this article, we devised a wide-angle spherical array bidirectional interface for high-resolution parallelized optoacoustic imaging and transcranial ultrasound (POTUS) delivery in the same target regions. The system operates between 3 and 9 MHz, allowing to generate and steer focal spots with widths down to [Formula: see text] across a field of view covering the entire mouse brain, while the same array is used to capture high-resolution 3-D optoacoustic data in real time. We showcase the system's versatile beam-forming capacities as well as volumetric optoacoustic imaging capabilities and discuss its potential to noninvasively monitor brain activity and various effects of ultrasound emission.

Ultrasound for the Brain: A Review of Physical and Engineering Principles, and Clinical Applications

The emergence of new ultrasound technologies has improved our understanding of the brain functions and offered new opportunities for the treatment of brain diseases. Ultrasound has become a valuable tool in preclinical animal and clinical studies as it not only provides information about the structure and function of brain tissues but can also be used as a therapy alternative for brain diseases. High-resolution cerebral flow images with high sensitivity can be acquired using novel functional ultrasound and super-resolution ultrasound imaging techniques. The noninvasive treatment of essential tremors has been clinically approved and it has been demonstrated that the ultrasound technology can revolutionize the currently existing treatment methods. Microbubble-mediated ultrasound can remotely open the blood-brain barrier enabling targeted drug delivery in the brain. More recently, ultrasound neuromodulation received a great amount of attention due to its noninvasive and deep penetration features and potential therapeutic benefits. This review provides a thorough introduction to the current state-of-the-art research on brain ultrasound and also introduces basic knowledge of brain ultrasound including the acoustic properties of the brain/skull and engineering techniques for ultrasound. Ultrasound is expected to play an increasingly important role in the diagnosis and therapy of brain diseases.

Stereotactic Transcranial Focused Ultrasound Targeting System for Murine Brain Models

An inexpensive, accurate focused ultrasound stereotactic targeting method guided by pretreatment magnetic resonance imaging (MRI) images for murine brain models is presented. An uncertainty of each sub-component of the stereotactic system was analyzed. The entire system was calibrated using clot phantoms. The targeting accuracy of the system was demonstrated with an in vivo mouse glioblastoma (GBM) model. The accuracy was quantified by the absolute distance difference between the prescribed and ablated points visible on the pre treatment and posttreatment MR images, respectively. A precalibration phantom study ( N = 6 ) resulted in an error of 0.32 ± 0.31, 0.72 ± 0.16, and 1.06 ± 0.38 mm in axial, lateral, and elevational axes, respectively. A postcalibration phantom study ( N = 8 ) demonstrated a residual error of 0.09 ± 0.01, 0.15 ± 0.09, and 0.47 ± 0.18 mm in axial, lateral, and elevational axes, respectively. The calibrated system showed significantly reduced ( ) error of 0.20 ± 0.21, 0.34 ± 0.24, and 0.28 ± 0.21 mm in axial, lateral, and elevational axes, respectively, in the in vivo GBM tumor-bearing mice ( N = 10 ).

A fiber optoacoustic emitter with controlled ultrasound frequency for cell membrane sonoporation at submillimeter spatial resolution

Focused ultrasound has attracted great attention in minimally invasive therapeutic and mechanism studies. Frequency below 1 MHz is identified preferable for high-efficiency bio-modulation. However, the poor spatial confinement of several millimeters and large device diameter of ∼25 mm of typical sub-MHz ultrasound technology suffered from the diffraction limit, severely hindering its further applications. To address it, a fiber-based optoacoustic emitter (FOE) is developed, serving as a miniaturized ultrasound point source, with sub-millimeter confinement, composed of an optical diffusion layer and an expansion layer on an optical fiber. By modifying acoustic damping and light absorption performance, controllable frequencies in the range of 0.083 MHz-5.500 MHz are achieved and further induce cell membrane sonoporation with frequency dependent efficiency. By solving the problem of compromise between sub-MHz frequency and sub-millimeter precision via breaking the diffraction limit, the FOE shows a great potential in region-specific drug delivery, gene transfection and neurostimulation.

Ultrasonic Retinal Neuromodulation and Acoustic Retinal Prosthesis

Ultrasound is an emerging method for non-invasive neuromodulation. Studies in the past have demonstrated that ultrasound can reversibly activate and inhibit neural activities in the brain. Recent research shows the possibility of using ultrasound ranging from 0.5 to 43 MHz in acoustic frequency to activate the retinal neurons without causing detectable damages to the cells. This review recapitulates pilot studies that explored retinal responses to the ultrasound exposure, discusses the advantages and limitations of the ultrasonic stimulation, and offers an overview of engineering perspectives in developing an acoustic retinal prosthesis. For comparison, this article also presents studies in the ultrasonic stimulation of the visual cortex. Despite that, the summarized research is still in an early stage; ultrasonic retinal stimulation appears to be a viable technology that exhibits enormous therapeutic potential for non-invasive vision restoration.

Transcranial ultrasound stimulation in humans is associated with an auditory confound that can be effectively masked

Background

Transcranial ultrasound stimulation (TUS) is emerging as a potentially powerful, non-invasive technique for focal brain stimulation. Recent animal work suggests, however, that TUS effects may be confounded by indirect stimulation of early auditory pathways.

Objective

We aimed to investigate in human participants whether TUS elicits audible sounds and if these can be masked by an audio signal.

Methods

In 18 healthy participants, T1-weighted magnetic resonance brain imaging was acquired for 3D ultrasound simulations to determine optimal transducer placements and source amplitudes. Thermal simulations ensured that temperature rises were <0.5 °C at the target and <3 °C in the skull. To test for non-specific auditory activation, TUS (500 kHz, 300 ms burst, modulated at 1 kHz with 50% duty cycle) was applied to primary visual cortex and participants were asked to distinguish stimulation from non-stimulation trials. EEG was recorded throughout the task. Furthermore, ex-vivo skull experiments tested for the presence of skull vibrations during TUS.

Results

We found that participants can hear sound during TUS and can distinguish between stimulation and non-stimulation trials. This was corroborated by EEG recordings indicating auditory activation associated with TUS. Delivering an audio waveform to participants through earphones while TUS was applied reduced detection rates to chance level and abolished the TUS-induced auditory EEG signal. Ex vivo skull experiments demonstrated that sound is conducted through the skull at the pulse repetition frequency of the ultrasound.

Conclusion

Future studies using TUS in humans need to take this auditory confound into account and mask stimulation appropriately.

•

Transcranial ultrasound stimulation elicits auditory signals in humans.

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Healthy human participants can distinguish stimulation from non-stimulation trials.

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Auditory masking reduces detection rates.

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Skull vibrations are present during transcranial ultrasound stimulation.

•

The auditory signal is likely due to bone conduction at the pulse repetition frequency.

Neuronavigated Repetitive Transcranial Ultrasound Stimulation Induces Long-Lasting and Reversible Effects on Oculomotor Performance in Non-human Primates

Since the late 2010s, Transcranial Ultrasound Stimulation (TUS) has been used experimentally to carryout safe, non-invasive stimulation of the brain with better spatial resolution than Transcranial Magnetic Stimulation (TMS). This innovative stimulation method has emerged as a novel and valuable device for studying brain function in humans and animals. In particular, single pulses of TUS directed to oculomotor regions have been shown to modulate visuomotor behavior of non-human primates during 100 ms ultrasound pulses. In the present study, a sustained effect was induced by applying 20-s trains of neuronavigated repetitive Transcranial Ultrasound Stimulation (rTUS) to oculomotor regions of the frontal cortex in three non-human primates performing an antisaccade task. With the help of MRI imaging and a frame-less stereotactic neuronavigation system (SNS), we were able to demonstrate that neuronavigated TUS (outside of the MRI scanner) is an efficient tool to carry out neuromodulation procedures in non-human primates. We found that, following neuronavigated rTUS, saccades were significantly modified, resulting in shorter latencies compared to no-rTUS trials. This behavioral modulation was maintained for up to 20 min. Oculomotor behavior returned to baseline after 18-31 min and could not be significantly distinguished from the no-rTUS condition. This study is the first to show that neuronavigated rTUS can have a persistent effect on monkey behavior with a quantified return-time to baseline. The specificity of the effects could not be explained by auditory confounds.

A Sub-Millimeter Lateral Resolution Ultrasonic Beamforming System for Brain Stimulation in Behaving Animals

In this paper, we present a second-generation wireless ultrasonic beamforming system, aiming for a truly wearable device for brain stimulation in small behaving animals. The fully-integrated, battery-operated system enables a self- contained untethered system. The system is partitioned into two parts for weight distribution: (1) a 1D capacitive micromachined transducer (CMUT) array on a separate head-mountable flexible printed circuit board (PCB), (2) a rigid back-mountable PCB including electronics such as a custom ASIC, a power management unit, a wireless module, and a battery. The newly developed ASIC not only enables a compact electronic system (30.5 mm x 63.5 mm) but also generates 3.4 times higher acoustic pressure (1.89 MPa_PP), which corresponds to a spatial-peak pulse-average intensity (I_SPPA) of 33.5 W/cm², at a depth of 5 mm, compared to the first-generation ASIC. The full width at half maximum (FWHM) of the pressure is estimated to be 0.6 mm, achieving a sub-millimeter lateral resolution by using 5-MHz focused waves.

Effect of scattered pressures from oscillating microbubbles on neuronal activity in mouse brain under transcranial focused ultrasound stimulation

Previous studies have indicated that the presence of microbubbles (MBs) during sonication has an impact on neuronal activity, while the underlying mechanisms remain to be revealed. In this study, a model for the scattered pressures produced by the pulsating lipid-encapsulated MBs in mouse brain was developed to numerically investigate the effect of MBs on neuronal activity during transcranial focused ultrasound stimulation. The additional summed scattered pressure (P_{summed_scat}) from the oscillating MBs was calculated from the model. The level of neuronal activity was experimentally verified using an immunofluorescence assay with antibodies against c-fos. The pressure difference (ΔP) between acoustic pressures at which the same level of neuronal activity is excited by ultrasound stimulation with and without MBs was obtained from the experiments. The results showed that P_{summed_scat} accounts for about half of the ΔP when the MBs experience a "compression-only" response. The P_{summed_scat} suddenly increased at a critical acoustic pressure, around which a rapid enhancement of ΔP obtained from experiment also occurred. This work suggested that the additional scattered pressures from pulsating MBs are probably a mechanism that affects neuronal activity under transcranial focused ultrasound stimulation.

Ultrasound neuromodulation: mechanisms and the potential of multimodal stimulation for neuronal function assessment

Focused ultrasound (FUS) neuromodulation has shown that mechanical waves can interact with cell membranes and mechanosensitive ion channels, causing changes in neuronal activity. However, the thorough understanding of the mechanisms involved in these interactions are hindered by different experimental conditions for a variety of animal scales and models. While the lack of complete understanding of FUS neuromodulation mechanisms does not impede benefiting from the current known advantages and potential of this technique, a precise characterization of its mechanisms of action and their dependence on experimental setup (e.g., tuning acoustic parameters and characterizing safety ranges) has the potential to exponentially improve its efficacy as well as spatial and functional selectivity. This could potentially reach the cell type specificity typical of other, more invasive techniques e.g., opto- and chemogenetics or at least orientation-specific selectivity afforded by transcranial magnetic stimulation. Here, the mechanisms and their potential overlap are reviewed along with discussions on the potential insights into mechanisms that magnetic resonance imaging sequences along with a multimodal stimulation approach involving electrical, magnetic, chemical, light, and mechanical stimuli can provide.

Remote, brain region-specific control of choice behavior with ultrasonic waves

The ability to modulate neural activity in specific brain circuits remotely and systematically could revolutionize studies of brain function and treatments of brain disorders. Sound waves of high frequencies (ultrasound) have shown promise in this respect, combining the ability to modulate neuronal activity with sharp spatial focus. Here, we show that the approach can have potent effects on choice behavior. Brief, low-intensity ultrasound pulses delivered noninvasively into specific brain regions of macaque monkeys influenced their decisions regarding which target to choose. The effects were substantial, leading to around a 2:1 bias in choices compared to the default balanced proportion. The effect presence and polarity was controlled by the specific target region. These results represent a critical step towards the ability to influence choice behavior noninvasively, enabling systematic investigations and treatments of brain circuits underlying disorders of choice.

Skull Impact on the Ultrasound Beam Profile of Transcranial Focused Ultrasound Stimulation

Transcranial focused ultrasound stimulation (tFUS) is a promising noninvasive neuromodulation tool for targeting brain regions with millimeter-scale spatial resolutions. In conventional tFUS studies, a focused ultrasound beam generated by an external ultrasound transducer is delivered to the neural target. In tFUS, ultrasound should travel through the skull that features large attenuation and different acoustic impedance compared with the soft tissue, thereby resulting in acoustic impedance mismatch and reflections. In this paper, we study the impact of the rat skull on the ultrasound beam profile generated by both focused and unfocused (but with a natural focus) transducers at a wide sonication frequency (f_p) range of 0.5-9.5 MHz. At different axial and lateral distances from the transducers, we measured ultrasound intensity profiles of three transducers operating at f_ps of 0.5 MHz, 1.2 MHz and 9.5 MHz with and without the skull. Our results showed that ultrasound beam profiles were significantly distorted by the skull. The transmission factor due to skull attenuation was measured 0.79,0.34 and 0.03 at f_ps of 0.5 MHz, 1.2 MHz and 9.5 MHz, respectively, when the skull was close to transducers focal zones.

A Soft Housing Needle Ultrasonic Transducer for Focal Stimulation to Small Animal Brain

Conventional acoustic brain stimulators that transmit low frequency (< 1 MHz) bursts in a pulse repetition frequency with large-sized transducers are barely compatible with small animal models because of broad beam width, possible stimulation of auditory pathways, and blocking of field-of-view for in vivo imaging of brain hemodynamics and neuronal activities. A miniaturized ultrasound stimulator with higher stimulation frequencies will enhance spatial specificity and enable simultaneous eliciting and monitoring brain activities. Moreover, the use of non-periodic pulse sequences may reduce unintended stimulations on auditory cortex, which might be caused by transmitting periodic bursting patterns. A platform for ultrasound brain stimulations for small animal models, including a soft housing 10 MHz needle transducer with a beam size of 680 μm, random transmission sequences, and optical imaging systems, was developed. The platform can deliver focal stimulations to the visual and barrel cortex of mice and monitor subsequent brain activities. The stimulated sites in both the visual and primary somatosensory cortices (S1) showed approximately two to three times higher neuronal calcium signal levels than those in peripheral regions. Activities in the auditory cortex were elicited by periodic sequence stimulation, while it was reduced by 67 and 35% for barrel and visual cortex stimulation with the random sequence, respectively.

High-Resolution Transcranial Electrical Simulation for Living Mice Based on Magneto-Acoustic Effect

Transcranial electrical stimulation is an important neuromodulation tool, which has been widely applied in the cognitive sciences and in the treatment of neurological and psychiatric diseases. In this work, a novel non-invasive method of transcranial electrical stimulation with high-resolution transcranial magneto-acoustic stimulation (TMAS) method has been tested experimentally in living mice for the first time. It can achieve spatial resolution of 2 mm in the cortex and even in the deep brain regions. The induced electrical field of TMAS was simulated and measured using a test sample. Then, an animal experimental system was built, and the healthy as well as Parkinson’s disease (PD) mice were simulated by TMAS in vivo. To investigate the effect of transcranial ultrasound stimulation (TUS) at the same time as TMAS, a TUS group was added in the experiments and its results compared with those of the TMAS group. The results not only demonstrate the high-resolution ability and safety of TMAS, but also show that both TMAS and TUS improved the synaptic plasticity of the PD mice and might improve the spatial learning and memory ability of the healthy mice and the PD mice, although the improvement performance of the TMAS group was superior to that of the TUS-group. Based on the in vivo TMAS studies, we propose that TMAS functions as a dual-mode stimulation combining the electric field of the magneto-acoustic effect and the mechanical force of TUS. Our results also provide an explanation of the mechanism of TMAS. This research suggests that future use of US stimulation in magnetic resonance imaging (MRI)-guided studies should involve careful consideration of the induced magneto-acoustic electrical field caused by the static magnetic field of MRI.

Enhanced neuronal activity in mouse motor cortex with microbubbles' oscillations by transcranial focused ultrasound stimulation

Microbubbles (MBs) are known to serve as an amplifier of the mechanical effects of ultrasound, which combined with ultrasound are widely used in brain. The goal of this study is to investigate the effect of oscillating MBs on the neuronal activity in the central nervous system (CNS) of mammals. The motor cortex of mice brain was subjected to ultrasound stimulation with and without MBs, and evoked electromyogram signals were recorded. A c-fos immunofluorescence assay was performed to evaluate the neuronal activation in the region of ultrasound stimulation. BBB integrity during ultrasound stimulation with MBs was assessed in this study. Moreover, the safety of ultrasound stimulation with MBs was examined. Using ultrasound at 620 kHz, the injection of MBs significantly increased the success rate of motor response from 0.065 ± 0.06 to 0.28 ± 0.10 when stimulation was applied at 0.12 MPa and from 0.38 ± 0.09 to 0.77 ± 0.18 at 0.25 MPa (p < 0.001). The results of the c-fos immunofluorescence assay showed that the mean densities of c-fos⁺ cells were significantly increased from 15.67 ± 3.51 to 53.01 ± 9.54 at 0.12 MPa acoustic pressure. At 0.25 MPa, the mean density of c-fos + cells was 81 ± 10.97 without MBs and it significantly increased to 124.12 ± 25.71 with MBs (p < 0.05). Enhanced neuronal activities were observed with 0.12 MPa ultrasound stimulation with MBs, while the integrity of BBB was not compromised, but 0.25 MPa ultrasound stimulation with MBs resulted in BBB disruption. These findings reveal that the oscillations of MBs can enhance neuronal activity in the CNS of mammals, and may provide an insight into the application of MBs combined with ultrasound in brain.

Improved Target Specificity of Transcranial Focused Ultrasound Stimulation (TFUS) using Double-Crossed Ultrasound Transducers

Ultrasound neuromodulation is a promising stimulation modality because of its non-invasiveness, focusing and steering capability, and relatively high spatial resolution compared to the other stimulation modalities. However, despite the high lateral resolution, the ultrasound beam in the axial direction is relatively long, especially when compared to the small size of the mouse brain. Here, we report a new ultrasound focusing technique for small animal in vivo experiments where a high spatial resolution in both lateral and axial directions is achieved by crossing two ultrasound beams. The focal volume of a full width half maximum (FWHM) of our proposed system is only 0.161 mm3 and the focal diameter in the axial direction is about 1 mm, which is ten times smaller than the previously reported ultrasound neuromodulation system. Thus, the proposed system enables targeting a sub-region of a mouse brain using ultrasound for the first time. We also demonstrate successful stimulation of the motor cortex through in vivo mice experiments where the movement of forepaw of the mouse was observed using the double-crossed ultrasound transducers. Moreover, by sweeping the focal point in the z-axis and measuring the success rate of stimulated movements, we show that our double-transducer system targeted a region with 2 mmresolution in the dorsal-ventral (DV) coordinates. The success rate of the double-crossed ultrasound stimulation was quantified by recording the electromyography (EMG) signals during the stimulation. Our results show that the double-crossed ultrasound transducer system with a ten times higher spatial resolution enables highly specific and noninvasive stimulation of small animals and thus enables versatile in vivo experiments to study functional connectivities of brain circuits.