Ultrasound Neuromodulation: A Review of Results, Mechanisms and Safety

A Wearable, Steerable, Transcranial Low-Intensity Focused Ultrasound System

OBJECTIVES

Transcranial low-intensity focused ultrasound (LIFU) offers unique opportunities for precisely neuromodulating small and/or deep targets within the human brain, which may be useful for treating psychiatric and neurological disorders. This article presents a novel ultrasound system that delivers focused ultrasound through the forehead to anterior brain targets and evaluates its safety and usability in a volunteer study.

METHODS

The ultrasound system and workflow are described, including neuronavigation, LIFU planning, and ultrasound delivery components. Its capabilities are analyzed through simulations and experiments in water to establish its safe steering range. A cohort of 20 healthy volunteers received a LIFU protocol aimed at the anterior medial prefrontal cortex (amPFC), using imaging and questionnaires to screen for adverse effects. Additional development after the study also analyzes the effect of the skull and sinus cavities on delivered ultrasound energy.

RESULTS

Simulations and hydrophone readings agreed with <5% error, and the safe steering range was found to encompass a 1.8 cm × 2.5 cm × 2 cm volume centered at a depth 5 cm from the surface of the skin. There were no adverse effects evident on qualitative assessments, nor any signs of damage in susceptibility-weighted imaging scans. All participants tolerated the treatment well. The interface effectively enabled the users to complete the workflow with all participants. In particular, the amPFC of every participant was within the steering limits of the system. A post hoc analysis showed that "virtual fitting" could aid in steering the beams around subjects' sinuses.

CONCLUSIONS

The presented system safely delivered LIFU through the forehead while targeting the amPFC in all volunteers, and was well-tolerated. With the capabilities validated here and positive results of the study, this technology appears well-suited to explore LIFU's efficacy in clinical neuromodulation contexts.

Flexible micromachined ultrasound transducers (MUTs) for biomedical applications

The use of bulk piezoelectric transducer arrays in medical imaging is a well-established technology that operates based on thickness mode piezoelectric vibration. Meanwhile, advancements in fabrication techniques have led to the emergence of micromachined alternatives, namely, piezoelectric micromachined ultrasound transducer (PMUT) and capacitive micromachined ultrasound transducer (CMUT). These devices operate in flexural mode using piezoelectric thin films and electrostatic forces, respectively. In addition, the development of flexible ultrasound transducers based on these principles has opened up new possibilities for biomedical applications, including biomedical imaging, sensing, and stimulation. This review provides a detailed discussion of the need for flexible micromachined ultrasound transducers (MUTs) and potential applications, their specifications, materials, fabrication, and electronics integration. Specifically, the review covers fabrication approaches and compares the performance specifications of flexible PMUTs and CMUTs, including resonance frequency, sensitivity, flexibility, and other relevant factors. Finally, the review concludes with an outlook on the challenges and opportunities associated with the realization of efficient MUTs with high performance and flexibility.

Stimulation of dorsal root ganglion with low-intensity focused ultrasound ameliorates pain responses through the GABA inhibitory pathway

AIMS

Chronic pain is a critical public health issue that severely impacts quality of life and poses significant treatment challenges, particularly due to the risk of adverse effects associated with pharmacological therapies. The search for effective non-invasive treatment alternatives has become increasingly relevant. Low-intensity focused ultrasound (LIFU) has been identified as an effective non-invasive strategy for pain management, although the underlying mechanism remains unclear. This study aimed to evaluate the analgesic effects and cellular mechanisms of LIFU-induced neuromodulation targeted on dorsal root ganglia (DRG) in both inflammatory and neuropathic pain models.

MATERIALS AND METHODS

The study adopted in vivo complete Freund's adjuvant-induced inflammatory pain and chronic constriction injury-induced neuropathic pain rat models, along with ex vivo DRG primary cultures.

KEY FINDINGS

The application of LIFU resulted in a time-dependent analgesic response in both pain models. The neurophysiological analyses revealed that LIFU activated GABAergic neurons and deactivated CGRP-expressing neurons triggered by noxious stimuli. The analgesic effects of LIFU and the LIFU-deactivated CGRP-containing neurons were reversed by injecting GABA antagonist to the L5 DRG, confirming the mediation of these effects through the GABAergic pathways. Ex vivo LIFU stimulation of DRG resulted in increased GABA release and decreased capsaicin-triggered CGRP cascades, further supporting in vivo findings.

SIGNIFICANCE

We demonstrate, for the first time, that LIFU modulates the GABA-CGRP pathways in the peripheral sensory neurons to alleviate pain, which underscores the importance of GABAergic systems in LIFU-induced analgesia. Our results reveal potential clinical applications of FUS for pain management through the peripheral nervous system.

Enhancing Equanimity with Non-Invasive Brain Stimulation: A Novel Framework for Mindfulness Interventions

Mindfulness has gained widespread recognition for its benefits to mental health, cognitive performance, and wellbeing. However, the multifaceted nature of mindfulness, encompassing elements like attentional focus, emotional regulation, and present-moment awareness, complicates its definition and measurement. A key component that may underlie its broad benefits is equanimity - the ability to maintain an open and non-reactive attitude toward all sensory experiences. Empirical research suggests that mindfulness works through a combination of top-down attentional control and bottom-up sensory and emotional processes, and that equanimity's role in regulating those bottom-up processes drives the psychological and physiological benefits, making it a promising target for both theoretical and practical exploration. Given these findings, the development of interventions that specifically augment equanimity could improve the impact of mindfulness practices. Research into non-invasive brain stimulation (NIBS) suggests that it is a potential tool for altering neural circuits involved in mindfulness. However, most NIBS studies to date have focused on improving cognitive control systems, leaving equanimity relatively unexplored. Preliminary findings from focused ultrasound interventions targeting the posterior cingulate cortex (PCC) suggest that NIBS can directly facilitate equanimity by inhibiting self-referential processing in the default mode network (DMN) to promote a more present-centered state of awareness. Future research should prioritize the integration of NIBS with well-defined mindfulness training protocols, focusing on equanimity as a core target. This approach could provide a novel framework for advancing both contemplative neuroscience and clinical applications, offering new insights into the mechanisms of mindfulness and refining NIBS methodologies to support individualized, precision wellness interventions.

Remotely induced electrical modulation of deep brain circuits in non-human primates

Introduction

The combination of magnetic and focused ultrasonic fields generates focused electric fields at depth entirely noninvasively. This noninvasive method may find particularly important applications in targeted treatments of the deep brain circuits involved in mental and neurological disorders. Due to the novelty of this method, it is nonetheless unknown which parameters modulate neural activity effectively.

Methods

We have investigated this issue by applying the combination of magnetic and focused ultrasonic fields to deep brain visual circuits in two non-human primates, quantifying the electroencephalographic gamma activity evoked in the visual cortex. We hypothesized that the pulse repetition frequency of the ultrasonic stimulation should be a key factor in modulating the responses, predicting that lower frequencies should elicit inhibitory effects and higher frequencies excitatory effects.

Results

We replicated the results of a previous study, finding an inhibition of the evoked gamma responses by a strong magnetic field. This inhibition was only observed for the lowest frequency tested (5 Hz), and not for the higher frequencies (10 kHz and 50 kHz). These neuromodulatory effects were transient and no safety issues were noted.

Discussion

We conclude that this new method can be used to transiently inhibit evoked neural activity in deep brain regions of primates, and that delivering the ultrasonic pulses at low pulse repetition frequencies maximizes the effect.

Alterations in large-scale resting-state network nodes following transcranial focused ultrasound of deep brain structures

Background

Low-intensity transcranial focused ultrasound (tFUS) is a brain stimulation approach that holds promise for the treatment of brain-based disorders. Studies in humans have shown that tFUS can successfully modulate perfusion in focal sonication targets, including the amygdala; however, limited research has explored how tFUS impacts large-scale neural networks.

Objective

The aim of the current study was to address this gap and examine changes in resting-state connectivity between large-scale network nodes using a randomized, double-blind, within-subjects crossover study design.

Methods

Healthy adults (n = 18) completed two tFUS sessions, 14 days apart. Each session included tFUS of either the right amygdala or the left entorhinal cortex (ErC). The inclusion of two active targets allowed for within-subjects comparisons as a function of the locus of sonication. Resting-state functional magnetic resonance imaging was collected before and after each tFUS session.

Results

tFUS altered resting-state functional connectivity (rsFC) within and between rs-network nodes. Pre-to-post sonication of the right amygdala modulated connectivity within nodes of the salience network (SAN) and between nodes of the SAN and the default mode network (DMN) and frontoparietal network (FRP). A decrease in SAN to FPN connectivity was specific to the amygdala target. Pre-to-post sonication of the left ErC modulated connectivity between the dorsal attention network (DAN) and FPN and DMN. An increase in DAN to DMN connectivity was specific to the ErC target.

Conclusion

These preliminary findings may suggest that tFUS induces neuroplastic changes beyond the immediate sonication target. Additional studies are needed to determine the long-term stability of these effects.

Dynamic changes in human brain connectivity following ultrasound neuromodulation

Non-invasive neuromodulation represents a major opportunity for brain interventions, and transcranial focused ultrasound (FUS) is one of the most promising approaches. However, some challenges prevent the community from fully understanding its outcomes. We aimed to address one of them and unravel the temporal dynamics of FUS effects in humans. Twenty-two healthy volunteers participated in the study. Eleven received FUS in the right inferior frontal cortex while the other 11 were stimulated in the right thalamus. Using a temporal dynamic approach, we compared resting-state fMRI seed-based functional connectivity obtained before and after FUS. We also assessed behavioural changes as measured with a task of reactive motor inhibition. Our findings reveal that the effects of FUS are predominantly time-constrained and spatially distributed in brain regions functionally connected with the directly stimulated area. In addition, mediation analysis highlighted that FUS applied in the right inferior cortex was associated with behavioural alterations which was directly explained by the applied acoustic pressure and the brain functional connectivity change we observed. Our study underscored that the biological effects of FUS are indicative of behavioural changes observed more than an hour following stimulation and are directly related to the applied acoustic pressure.

The mechanical properties of nerves, the size of the action potential, and consequences for the brain

The action potential is widely regarded as a purely electrical phenomenon. However, one also finds mechanical and thermal changes that can be observed experimentally. In particular, nerve membranes become thicker and axons contract. The spatial length of the action potential can be quite large, ranging from millimeters to many centimeters. This suggests the use of macroscopic thermodynamics methods to understand its properties. The pulse length is several orders of magnitude larger than the synaptic gap, larger than the distance of the nodes of Ranvier and even larger than the size of many neurons such as pyramidal cells or brain stem motor neurons. Here, we review the mechanical changes in nerves, we discuss theoretical possibilities to explain them and implications of a mechanical nerve pulse for neurons and for the brain. In particular, the contraction of nerves leads to the possibility of fast mechanical synapses.

Advances in non-invasive brain stimulation: enhancing sports performance function and insights into exercise science

The cerebral cortex, as the pinnacle of human complexity, poses formidable challenges to contemporary neuroscience. Recent advancements in non-invasive brain stimulation have been pivotal in enhancing human locomotor functions, a burgeoning area of interest in exercise science. Techniques such as transcranial direct current stimulation, transcranial alternating current stimulation, transcranial random noise stimulation, and transcranial magnetic stimulation are widely recognized for their neuromodulator capabilities. Despite their broad applications, these methods are not without limitations, notably in spatial and temporal resolution and their inability to target deep brain structures effectively. The advent of innovative non-invasive brain stimulation modalities, including transcranial focused ultrasound stimulation and temporal interference stimulation technology, heralds a new era in neuromodulation. These approaches offer superior spatial and temporal precision, promising to elevate athletic performance, accelerate sport science research, and enhance recovery from sports-related injuries and neurological conditions. This comprehensive review delves into the principles, applications, and future prospects of non-invasive brain stimulation in the realm of exercise science. By elucidating the mechanisms of action and potential benefits, this study aims to arm researchers with the tools necessary to modulate targeted brain regions, thereby deepening our understanding of the intricate interplay between brain function and human behavior.

Overview of Therapeutic Ultrasound Applications and Safety Considerations: 2024 Update

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

Focused ultrasound for treatment of epilepsy: a systematic review and meta-analysis of preclinical and clinical studies

Various preclinical and clinical studies have demonstrated the neuromodulatory and ablative effects of focused ultrasound (FUS). However, the safety and efficacy of FUS in clinical settings for treating epilepsy have not been well established. This study aims to provide a systematic review of all preclinical and clinical studies that have used FUS for the treatment of epilepsy. A systematic search was conducted using Scopus, Web of Science, PubMed, and Embase databases. All preclinical and clinical studies reporting outcomes of FUS in the treatment of epilepsy were included in the systematic review. Random-effect meta-analysis was performed to determine safety in clinical studies and seizure activity reduction in preclinical studies. A total of 24 articles were included in the study. Meta-analysis demonstrated that adverse events occurred in 13% (95% CI = 2-57%) of patients with epilepsy who underwent FUS. The frequency of adverse events was higher with the use of FUS for lesioning (36%, 95% CI = 4-88%) in comparison to neuromodulation (5%, 95% CI = 0-71%), although this difference was not significant (P = 0.31). Three-level meta-analysis in preclinical studies demonstrated a reduced spike rate in neuromodulating FUS compared to the control group (P = 0.02). According to this systematic review and meta-analysis, FUS can be considered a safe and feasible approach for treating epileptic seizures, especially in drug-resistant patients. While the efficacy of FUS has been demonstrated in several preclinical studies, further research is necessary to confirm its effectiveness in clinical practice and to determine the adverse events.

Low-intensity focused ultrasound stimulation promotes stroke recovery via astrocytic HMGB1 and CAMK2N1 in mice

BACKGROUND

Low-intensity focused ultrasound stimulation (LIFUS) has been developed to enhance neurological repair and remodelling during the late acute stage of ischaemic stroke in rodents. However, the cellular and molecular mechanisms of neurological repair and remodelling after LIFUS in ischaemic stroke are unclear.

METHODS

Ultrasound stimulation was treated in adult male mice 7 days after transient middle cerebral artery occlusion. Angiogenesis was measured by laser speckle imaging and histological analyses. Electromyography and fibre photometry records were used for synaptogenesis. Brain atrophy volume and neurobehaviour were assessed 0-14 days after ischaemia. iTRAQ proteomic analysis was performed to explore the differentially expressed protein. scRNA-seq was used for subcluster analysis of astrocytes. Fluorescence in situ hybridisation and Western blot detected the expression of HMGB1 and CAMK2N1.

RESULTS

Optimal ultrasound stimulation increased cerebral blood flow, and improved neurobehavioural outcomes in ischaemic mice (p<0.05). iTRAQ proteomic analysis revealed that the expression of HMGB1 increased and CAMK2N1 decreased in the ipsilateral hemisphere of the brain at 14 days after focal cerebral ischaemia with ultrasound treatment (p<0.05). scRNA-seq revealed that this expression pattern belonged to a subcluster of astrocytes after LIFUS in the ischaemic brain. LIFUS upregulated HMGB1 expression, accompanied by VEGFA elevation compared with the control group (p<0.05). Inhibition of HMGB1 expression in astrocytes decreased microvessels counts and cerebral blood flow (p<0.05). LIFUS reduced CAMK2N1 expression level, accompanied by increased extracellular calcium ions and glutamatergic synapses (p<0.05). CAMK2N1 overexpression in astrocytes decreased dendritic spines, and aggravated neurobehavioural outcomes (p<0.05).

CONCLUSION

Our results demonstrated that LIFUS promoted angiogenesis and synaptogenesis after focal cerebral ischaemia by upregulating HMGB1 and downregulating CAMK2N1 in a subcluster of astrocytes, suggesting that LIFUS activated specific astrocyte subcluster could be a key target for ischaemic brain therapy.

Examination of the interaction of parameters for low-intensity focused ultrasound of the human motor cortex

BACKGROUND

Low-intensity focused ultrasound (LIFU) is a promising form of non-invasive neuromodulation characterized by a rich parameter space that includes intensity, duration, duty cycle and pulsing strategy. The effect and interaction of these parameters to affect human brain activity is poorly understood. A better understanding of how parameters interact is critical to advance LIFU as a potential therapeutic.

OBJECTIVE/HYPOTHESIS

To determine how intensity, duration, and duty cycle interact to produce neuromodulation effects in the human motor cortex. Further, this study assesses the effect of pulsing versus continuous ultrasound. We hypothesize that higher duty cycles will confer excitation. Increasing intensity or duration will increase the magnitude of effect. Pulsing LIFU will not be more effective than continuous wave ultrasound.

METHODS

N = 18 healthy human volunteers underwent 20 different parameter combinations that included a fully parametrized set of two intensities (ISPPA: 6 & 24 W/cm2), five duty cycles (1, 10, 30, 50, 70 %) and two durations (100, 500 msec) with a constant pulse repetition frequency of 1 kHz delivered concurrently with transcranial magnetic stimulation (TMS) to the primary motor cortex (M1). Four of these parameter combinations were also delivered continuously, matched on the number of cycles. Motor-evoked potential (MEP) amplitude was the primary outcome measure. All parameter combinations were collected time-locked to MEP generation.

RESULTS

There was no evidence of excitation from any parameter combination. 3 of the 24 parameter sets resulted in significant inhibition. The parameter set that resulted in the greatest inhibition (∼30 %) was an intensity of 6W/cm2 with a duty cycle of 30 % and a duration of 500 msec. A three-way ANOVA revealed an interaction of intensity and duty cycle. The analysis of continuous versus pulsed ultrasound revealed a 3-way interaction of intensity, pulsing, and the number of cycles such that under the 6W/cm2 condition higher cycles of pulsed ultrasound resulted in inhibition whereas lower number of cycles using continuous LIFU resulted in inhibition.

CONCLUSIONS

LIFU to M1 in humans, in the range employed, either conferred inhibition or had no effect. Significant excitation was not observed. In general, lower intensity looks to be more efficacious for inhibition that depends on duration. In addition, pulsed ultrasound looks to be more effective for inhibition as compared to continuous wave after controlling for total energy delivered. Non-specific auditory effects may contribute to these results.

Investigating the hypotensive effect of focused ultrasound neuromodulation and barbiturate-loaded nanodroplets in healthy and hypertensive rats

BACKGROUND

Current strategies for reducing blood pressure (BP) are ineffective and unsafe for many patient populations, including drug-resistant hypertension and during pregnancy. Stimulating the periaqueductal grey (PAG) region has shown promise in treating drug-resistant hypertension in patients using deep brain stimulation.

OBJECTIVE

To develop a minimally invasive neuromodulation technique for the sustained treatment of hypertension.

METHODS

We have investigated BP reduction using focused ultrasound (FUS) (540 kHz) and anesthetic-loaded ultrasound-responsive nanodroplets to deliver pentobarbital to the PAG in normotensive (N = 27) and hypertensive (N = 20) male and female rats. BP, heart rate and plasma hormone content were collected before and after FUS exposure, and neuronal activity was mapped in the PAG using C-Fos and neuron subtype staining. Cavitation activity was monitored by detecting acoustic emissions from vaporizing nanodroplets, and neuromodulation was verified with immunohistochemistry.

RESULTS

Systolic and diastolic BP were reduced for 6 h following a single sonication of the PAG (-37/-28 mmHg systolic/diastolic), and the offline effect was extended to 4 days with consecutive sonications combined with systemically injected pentobarbital-loaded nanodroplets. In contrast, FUS applied to the frontal cortex had no effect on BP. Immunohistochemistry revealed stimulation of inhibitory neurons in the PAG region, indicating that the hypotensive effect was associated with a GABAergic pathway. The acoustic emissions from vaporizing droplets were found to correlate with neuron activity and change in BP, offering the potential for real-time treatment monitoring using ultrasound.

CONCLUSIONS

This work has implications for developing a new treatment for hypertension that has greater safety and broader applicability for vulnerable patient populations.

The relationship between parameters and effects in transcranial ultrasonic stimulation

Transcranial ultrasonic stimulation (TUS) is rapidly gaining traction for non-invasive human neuromodulation, with a pressing need to establish protocols that maximise neuromodulatory efficacy. In this review, we aggregate and examine empirical evidence for the relationship between tunable TUS parameters and in vitro and in vivo outcomes. Based on this multiscale approach, TUS researchers can make better informed decisions about optimal parameter settings. Importantly, we also discuss the challenges involved in extrapolating results from prior empirical work to future interventions, including the translation of protocols between models and the complex interaction between TUS protocols and the brain. A synthesis of the empirical evidence suggests that larger effects will be observed at lower frequencies within the sub-MHz range, higher intensities and pressures than commonly administered thus far, and longer pulses and pulse train durations. Nevertheless, we emphasise the need for cautious interpretation of empirical data from different experimental paradigms when basing protocols on prior work as we advance towards refined TUS parameters for human neuromodulation.

Dynamic Acoustic Holography: One-Shot High-Precision and High-Information Methodology

Acoustic holography technology is widely used in the field of ultrasound due to its capability to achieve complex acoustic fields. The traditional acoustic holography method based on single-phase holograms is limited due to its inability to complete acoustic field control with high dynamics and accuracy. Here, we propose a method for constructing an acoustic holographic model, introducing an ultrasonic array to provide dynamic amplitude control degrees of freedom, and combining the dynamically controllable ultrasonic array and high-precision acoustic hologram to achieve the highest acoustic field accuracy and dynamic range. This simulation method has been proven to be applicable to both simple linear patterns and complex surface patterns. Moreover, it is possible to reconstruct the degree of freedom of the target plane amplitude effectively and achieve a breakthrough in high information content. This high-efficiency acoustic field control capability has potential applications in ultrasound imaging, acoustic tweezers, and neuromodulation.

Noninvasive Modulation of the Subcallosal Cingulate and Depression With Focused Ultrasonic Waves

BACKGROUND

Severe forms of depression have been linked to excessive subcallosal cingulate cortex (SCC) activity. Stimulation of the SCC with surgically implanted electrodes can alleviate depression, but current noninvasive techniques cannot directly and selectively modulate deep targets. We developed a new noninvasive neuromodulation approach that can deliver low-intensity focused ultrasonic waves to the SCC.

METHODS

Twenty-two individuals with treatment-resistant depression participated in a randomized, double-blind, sham-controlled study. Ultrasonic stimulation was delivered to the bilateral SCC during concurrent functional magnetic resonance imaging to quantify target engagement. Mood state was measured with the Sadness subscale of the Positive and Negative Affect Schedule before and after 40 minutes of real or sham SCC stimulation. Change in depression severity was measured with the 6-item Hamilton Depression Rating Scale at 24 hours and 7 days.

RESULTS

Functional magnetic resonance imaging demonstrated a target-specific decrease in SCC activity during stimulation (p = .028, n = 16). In 7 of 16 participants, SCC neuromodulation was detectable at the individual participant level with a single 10-minute scan (p < .05, small-volume correction). Mood and depression scores improved more with real than with sham stimulation. In the per-protocol sample (n = 19), real stimulation was superior to sham for 6-item Hamilton Depression Rating Scale scores at 24 hours and for Sadness scores (both p < .05, d > 1). Nonsignificant trends were found in the intent-to-treat sample.

CONCLUSIONS

This small pilot study indicates that ultrasonic stimulation modulates SCC activity and can rapidly reduce depressive symptoms. The capability to noninvasively and selectively target deep brain areas creates new possibilities for the future development of circuit-directed therapeutics and for the analysis of deep-brain circuit function in humans.

Thalamic Roles in Conscious Perception Revealed by Low-Intensity Focused Ultrasound Neuromodulation

The neural basis of conscious perception remains incompletely understood. While cortical mechanisms of conscious content have been extensively investigated, the role of subcortical structures, including the thalamus, remains less explored. We aim to elucidate the causal contributions of different thalamic regions to conscious perception using transcranial low-intensity focused ultrasound (LIFU) neuromodulation. We hypothesize that modulating different thalamic regions would result in distinct perceptual outcomes. We apply LIFU in human volunteers to investigate region-specific and sonication parameter-dependent effects. We target anterior (transmodal-dominant) and posterior (unimodal-dominant) thalamic regions, further divided into ventral and dorsal regions, while participants perform a near-threshold visual perception task. Task performance is evaluated using Signal Detection Theory metrics. We find that the high duty cycle stimulation of the ventral anterior thalamus enhanced object recognition sensitivity. We also observe a general (i.e., region-independent) effect of LIFU on decision bias (i.e., a tendency toward a particular response) and object categorization accuracy. Specifically, high duty cycle stimulation decreases categorization accuracy, whereas low duty cycle shifts decision bias towards a more conservative stance. In conclusion, our results provide causal insight into the functional organization of the thalamus in shaping human visual experience and highlight the unique role of the transmodal-dominant ventral anterior thalamus.

Therapeutic ultrasound: an innovative approach for targeting neurological disorders affecting the basal ganglia

The basal ganglia are involved in motor control and action selection, and their impairment manifests in movement disorders such as Parkinson's disease (PD) and dystonia, among others. The complex neuronal circuitry of the basal ganglia is located deep inside the brain and presents significant treatment challenges. Conventional treatment strategies, such as invasive surgeries and medications, may have limited effectiveness and may result in considerable side effects. Non-invasive ultrasound (US) treatment approaches are becoming increasingly recognized for their therapeutic potential for reversibly permeabilizing the blood-brain barrier (BBB), targeting therapeutic delivery deep into the brain, and neuromodulation. Studies conducted on animals and early clinical trials using ultrasound as a therapeutic modality have demonstrated promising outcomes for controlling symptom severity while preserving neural tissue. These results could improve the quality of life for patients living with basal ganglia impairments. This review article explores the therapeutic frontiers of ultrasound technology, describing the brain mechanisms that are triggered and engaged by ultrasound. We demonstrate that this cutting-edge method could transform the way neurological disorders associated with the basal ganglia are managed, opening the door to less invasive and more effective treatments.

Revealing physical interactions of ultrasound waves with the body through photoelasticity imaging

Ultrasound is a ubiquitous technology in medicine for screening, diagnosis, and treatment of disease. The functionality and efficacy of different ultrasound modes relies strongly on our understanding of the physical interactions between ultrasound waves and biological tissue structures. This article reviews the use of photoelasticity imaging for investigating ultrasound fields and interactions. Physical interactions are described for different ultrasound technologies, including those using linear and nonlinear ultrasound waves, as well as shock waves. The use of optical modulation of light by ultrasound is presented for shadowgraphic and photoelastic techniques. Investigations into shock wave and burst wave lithotripsy using photoelastic methods are summarized, along with other endoscopic forms of lithotripsy. Photoelasticity in soft tissue surrogate materials is reviewed, and its deployment in investigating tissue-bubble interactions, generated ultrasound waves, and traumatic brain injury, are discussed. With the continued growth of medical ultrasound, photoelasticity imaging can play a role in elucidating the physical mechanisms leading to useful bioeffects of ultrasound for imaging and therapy.

Getting stress-related disorders under control: the untapped potential of neurofeedback

Stress-related disorders are among the biggest global health challenges. Despite significant progress in understanding their neurocognitive basis, the promise of applying insights from fundamental research to prevention and treatment remains largely unfulfilled. We argue that neurofeedback - a method for training voluntary control over brain activity - has the potential to fill this translational gap. We provide a contemporary perspective on neurofeedback as endogenous neuromodulation that can target complex brain network dynamics, is transferable to real-world scenarios outside a laboratory or treatment facility, can be trained prospectively, and is individually adaptable. This makes neurofeedback a prime candidate for a personalized preventive neuroscience-based intervention strategy that focuses on the ecological momentary neuromodulation of stress-related brain networks in response to actual stressors in real life.

Progress in Noninvasive Low-Intensity Focused Ultrasound Neuromodulation

Low-intensity focused ultrasound represents groundbreaking medical advancements, characterized by its noninvasive feature, safety, precision, and broad neuromodulatory capabilities. This technology operates through mechanisms, for example, acoustic radiation force, cavitation, and thermal effects. Notably, with the evolution of medical technology, ultrasound neuromodulation has been gradually applied in treating central nervous system diseases, especially stroke. Furthermore, burgeoning research areas such as sonogenetics and nanotechnology show promising potential. Despite the benefit of low-intensity focused ultrasound the precise biophysical mechanism of ultrasound neuromodulation still need further exploration. This review discusses the recent and ongoing developments of low-intensity focused ultrasound for neurological regulation, covering the underlying rationale to current utility and the challenges that impede its further development and broader adoption of this promising alternative to noninvasive therapy.

The future of transcranial ultrasound as a precision brain interface

Our understanding of brain circuit operations and disorders has rapidly outpaced our ability to intervene and restore them. Developing technologies that can precisely interface with any brain region and circuit may combine diagnostics with therapeutic intervention, expediting personalised brain medicine. Transcranial ultrasound stimulation (TUS) is a promising noninvasive solution to this challenge, offering focal precision and scalability. By exploiting the biomechanics of pressure waves on brain tissue, TUS enables multi-site targeted neuromodulation across distributed circuits in the cortex and deeper areas alike. In this Essay, we explore the emergent evidence that TUS can functionally test and modify dysfunctional regions, effectively serving as a search and rescue tool for the brain. We define the challenges and opportunities faced by TUS as it moves towards greater target precision and integration with advanced brain monitoring and interventional technology. Finally, we propose a roadmap for the evolution of TUS as it progresses from a research tool to a clinically validated therapeutic for brain disorders.

Focal Volume, Acoustic Radiation Force, and Strain in Two-Transducer Regimes

Transcranial ultrasound stimulation (TUS) holds promise for noninvasive neural modulation in treating neurological disorders. Most clinically relevant targets are deep within the brain (near or at its geometric center), surrounded by other sensitive regions that need to be spared clinical intervention. However, in TUS, increasing frequency with the goal of improving spatial resolution reduces the effective penetration depth. We show that by using a pair of 1-MHz orthogonally arranged transducers, we improve the spatial resolution afforded by each of the transducers individually, by nearly 40 folds, achieving a subcubic millimeter target volume of [Formula: see text]. We show that orthogonally placed transducers generate highly localized standing waves with acoustic radiation force (ARF) arranged into periodic regions of compression and tension near the target. We further present an extended capability of the orthogonal setup, which is to impart selective pressures-either positive or negative, but not both-on the target. Finally, we share our preliminary findings that strain can arise from both particle motion (PM) and ARF with the former reaching its maximum value at the focus and the latter remaining null at the focus and reaching its maximum around the focus. As the field is investigating the mechanism of interaction in TUS by way of elucidating the mapping between ultrasound parameters and neural response, orthogonal transducers expand our toolbox by making it possible to conduct these investigations at much finer spatial resolutions, with localized and directed (compression versus tension) ARF and the capability of applying selective pressures at the target.

Ultrasound pulse repetition frequency preferentially activates different neuron populations independent of cell type

Objective. Transcranial ultrasound (US) stimulation serves as an external input to a neuron, and thus the evoked response relies on neurons' intrinsic properties. Neural activity is limited to a couple hundred hertz and often exhibits preference to input frequencies. Accordingly, US pulsed at specific physiologic pulse repetition frequencies (PRFs) may selectively engage neurons with the corresponding input frequency preference. However, most US parametric studies examine the effects of supraphysiologic PRFs. It remains unclear whether pulsing US at different physiologic PRFs could activate distinct neurons in the awake mammalian brain.Approach. We recorded cellular calcium responses of individual motor cortex neurons to US pulsed at PRFs of 10, 40, and 140 Hz in awake mice. We compared the evoked responses across these PRFs in the same neurons. To further understand the cell-type dependent effects, we categorized the recorded neurons as parvalbumin positive fast spiking interneurons or putative excitatory neurons and analyzed single-cell mechanosensitive channel expression in mice and humans using the Allen Brain Institute's RNA-sequencing databases.Main results. We discovered that many neurons were preferentially activated by only one PRF and different PRFs selectively engaged distinct neuronal populations. US-evoked cellular calcium responses exhibited the same characteristics as those naturally occurring during spiking, suggesting that US increases intrinsic neuronal activity. Furthermore, evoked responses were similar between fast-spiking inhibitory neurons and putative excitatory neurons. Thus, variation in individual neuron's cellular properties dominates US-evoked response heterogeneity, consistent with our observed cell-type independent expression patterns of mechanosensitive channels across individual neurons in mice and humans. Finally, US transiently increased network synchrony without producing prolonged over-synchronization that could be detrimental to neural circuit functions.Significance. These results highlight the feasibility of activating distinct neuronal subgroups by varying PRF and the potential to improve neuromodulation effects by combining physiologic PRFs.

Displacement and 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 situ targeting for neuromodulation site. We recorded hemodynamic responses evoked by FUS while fUS imaging revealed an ipsilateral CBV increase that peaks at 4 s post-FUS. We report a stronger hemodynamic activation in the subcortical region than cortical, showing good agreement with a brain elasticity map that can also be obtained using a similar methodology. We observed dose-dependent CBV responses with peak CBV, activated area, and correlation coefficient increasing with the ultrasonic dose. Furthermore, by mapping displacement and hemodynamic activation, we found that displacement colocalized and linearly correlated with CBV increase. The findings presented herein demonstrated that FUS evokes ipsilateral hemodynamic activation in cortical and subcortical depths while the evoked hemodynamic responses colocalize and correlate with FUS-induced displacement. We anticipate that our findings will help consolidate accurate targeting as well as shedding light on one of the mechanisms behind FUS modulation, i.e., how FUS mechanically displaces brain tissue affecting cerebral hemodynamics and thereby its associated connectivity.

Controlling action potentials with magnetoelectric nanoparticles

Non-invasive or minutely invasive and wireless brain stimulation that can target any region of the brain is an open problem in engineering and neuroscience with serious implications for the treatment of numerous neurological diseases. Despite significant recent progress in advancing new methods of neuromodulation, none has successfully replicated the efficacy of traditional wired stimulation and improved on its downsides without introducing new complications. Due to the capability to convert magnetic fields into local electric fields, MagnetoElectric NanoParticle (MENP) neuromodulation is a recently proposed framework based on new materials that can locally sensitize neurons to specific, low-strength alternating current (AC) magnetic fields (50Hz 1.7 kOe field). However, the current research into this neuromodulation concept is at a very early stage, and the theoretically feasible game-changing advantages remain to be proven experimentally. To break this stalemate phase, this study leveraged understanding of the non-linear properties of MENPs and the nanoparticles' field interaction with the cellular microenvironment. Particularly, the applied magnetic field's strength and frequency were tailored to the M - H hysteresis loop of the nanoparticles. Furthermore, rectangular prisms instead of the more traditional "spherical" nanoparticle shapes were used to: (i) maximize the magnetoelectric effect and (ii) improve the nanoparticle-cell-membrane surface interface. Neuromodulation performance was evaluated in a series of exploratory in vitro experiments on 2446 rat hippocampus neurons. Linear mixed effect models were used to ensure the independence of samples by accounting for fixed adjacency effects in synchronized firing. Neural activity was measured over repeated 4-min segments, containing 90 s of baseline measurements, 90 s of stimulation measurements, and 60 s of post stimulation measurements. 87.5 % of stimulation attempts produced statistically significant (P < 0.05) changes in neural activity, with 58.3 % producing large changes (P < 0.01). In negative controls using either zero or 1.7 kOe-strength field without nanoparticles, no experiments produced significant changes in neural activity (P > 0.05 and P > 0.15 respectively). Furthermore, an exploratory analysis of a direct current (DC) magnetic field indicated that the DC field could be used with MENPs to inhibit neuron activity (P < 0.01). These experiments demonstrated the potential for magnetoelectric neuromodulation to offer a near one-to-one functionality match with conventional electrode stimulation without requiring surgical intervention or genetic modification to achieve success, instead relying on physical properties of these nanoparticles as "On/Off" control mechanisms. ONE-SENTENCE SUMMARY: This in vitro neural cell culture study explores how to exploit the non-linear and anisotropic properties of magnetoelectric nanoparticles for wireless neuromodulation, the importance of magnetic field strength and frequency matching for optimization, and demonstrates, for the first time, that magnetoelectric neuromodulation can inhibit neural responses.

Low frequency ultrasound elicits broad cortical responses inhibited by ketamine in mice

The neuromodulatory effects of >250 kHz ultrasound have been well-demonstrated, but the impact of lower-frequency ultrasound, which can transmit better through air and the skull, on the brain is unclear. This study investigates the biological impact of 40 kHz pulsed ultrasound on the brain using calcium imaging and electrophysiology in mice. Our findings reveal burst duration-dependent neural responses in somatosensory and auditory cortices, resembling responses to 12 kHz audible tone, in vivo. In vitro brain slice experiments show no neural responses to 300 kPa 40 kHz ultrasound, implying indirect network effects. Ketamine fully blocks neural responses to ultrasound in both cortices but only partially affects 12 kHz audible tone responses in the somatosensory cortex and has no impact on auditory cortex 12 kHz responses. This suggests that low-frequency ultrasound's cortical effects rely heavily on NMDA receptors and may involve mechanisms beyond indirect auditory cortex activation. This research uncovers potential low-frequency ultrasound effects and mechanisms in the brain, offering a path for future neuromodulation.

Mitochondria-targeted sonodynamic modulation of neuroinflammation to protect against myocardial ischemia‒reperfusion injury

Sympathetic hyperactivation and inflammatory responses are the main causes of myocardial ischemia‒reperfusion (I/R) injury and myocardial I/R-related ventricular arrhythmias (VAs). Previous studies have demonstrated that light-emitting diodes (LEDs) could modulate post-I/R neuroinflammation, thus providing protection against myocardial I/R injury. Nevertheless, further applications of LEDs are constrained due to the low penetration depth (<1 cm) and potential phototoxicity. Low-intensity focused ultrasound (LIFU), an emerging noninvasive neuromodulation strategy with deeper penetration depth (∼10 cm), has been confirmed to modulate sympathetic nerve activity and inflammatory responses. Sonodynamic therapy (SDT), which combines LIFU with sonosensitizers, confers additional advantages, including superior therapeutic efficacy, precise localization of neuronal modulation and negligible side effects. Herein, LIFU and SDT were introduced to modulate post-myocardial I/R neuroinflammation to protect against myocardial I/R injury. The results indicated that LIFU and SDT inhibited sympathetic neural activity, suppressed the activation of astrocytes and microglia, and promoted microglial polarization towards the M2 phenotype, thereby attenuating myocardial I/R injury and preventing I/R-related malignant VAs. These insights suggest that LIFU and SDT inspire a noninvasive and efficient neuroinflammatory modulation strategy with great clinical translation potential thus benefiting more patients with myocardial I/R in the future. STATEMENT OF SIGNIFICANCE: Myocardial ischemia-reperfusion (I/R) may cause I/R injury and I/R-induced ventricular arrhythmias. Sympathetic hyperactivation and inflammatory response play an adverse effect in myocardial I/R injury. Previous studies have shown that light emitting diode (LED) can regulate I/R-induced neuroinflammation, thus playing a myocardial protective role. However, due to the low penetration depth and potential phototoxicity of LED, it is difficult to achieve clinical translation. Herein, we introduced sonodynamic modulation of neuroinflammation to protect against myocardial I/R injury, based on mitochondria-targeted nanosonosensitizers (CCNU980 NPs). We demonstrated that sonodynamic modulation could promote microglial autophagy, thereby preventing myocardial I/R injury and I/R-induced ventricular arrhythmias. This is the first example of sonodynamic modulation of myocardial I/R-induced neuroinflammation, providing a novel strategy for clinical translation.

Application of Data-Driven computing to patient-specific prediction of the viscoelastic response of human brain under transcranial ultrasound stimulation

We present a class of model-free Data-Driven solvers that effectively enable the utilization of in situ and in vivo imaging data directly in full-scale calculations of the mechanical response of the human brain to sonic and ultrasonic stimulation, entirely bypassing the need for analytical modeling or regression of the data. The well-posedness of the approach and its convergence with respect to data are proven analytically. We demonstrate the approach, including its ability to make detailed spatially resolved patient-specific predictions of wave patterns, using public-domain MRI images, MRE data and commercially available solid-mechanics software.

High-Precision Photoacoustic Neural Modulation Uses a Non-Thermal Mechanism

Neuromodulation is a powerful tool for fundamental studies in neuroscience and potential treatments of neurological disorders. Both photoacoustic (PA) and photothermal (PT) effects are harnessed for non-genetic high-precision neural stimulation. Using a fiber-based device excitable by a nanosecond pulsed laser and a continuous wave laser for PA and PT stimulation, respectively, PA and PT neuromodulation is systematically investigated at the single neuron level. These results show that to achieve the same level of neuron activation recorded by Ca2+ imaging, the laser energy needed for PA stimulation is 1/40 of that needed for PT stimulation. The threshold energy for PA stimulation is found to be further reduced in neurons overexpressing mechano-sensitive channels, indicating direct involvement of mechano-sensitive channels in PA stimulation. Electrophysiology study of single neurons upon PA and PT stimulation is performed by patch clamp recordings. Electrophysiological features induced by PA are distinct from those by PT, confirming that PA and PT stimulation operate through different mechanisms. These insights offer a foundation for the rational design of more efficient and safer non-genetic neural modulation approaches.

A silicon diode-based optoelectronic interface for bidirectional neural modulation

The development of advanced neural modulation techniques is crucial to neuroscience research and neuroengineering applications. Recently, optical-based, nongenetic modulation approaches have been actively investigated to remotely interrogate the nervous system with high precision. Here, we show that a thin-film, silicon (Si)-based diode device is capable to bidirectionally regulate in vitro and in vivo neural activities upon adjusted illumination. When exposed to high-power and short-pulsed light, the Si diode generates photothermal effects, evoking neuron depolarization and enhancing intracellular calcium dynamics. Conversely, low-power and long-pulsed light on the Si diode hyperpolarizes neurons and reduces calcium activities. Furthermore, the Si diode film mounted on the brain of living mice can activate or suppress cortical activities under varied irradiation conditions. The presented material and device strategies reveal an innovated optoelectronic interface for precise neural modulations.

The relationship between parameters and effects in transcranial ultrasonic stimulation

Transcranial ultrasonic stimulation (TUS) is rapidly gaining traction for non-invasive human neuromodulation, with a pressing need to establish protocols that maximise neuromodulatory efficacy. In this review, we aggregate and examine empirical evidence for the relationship between tunable TUS parameters and in vitro and in vivo outcomes. Based on this multiscale approach, TUS researchers can make better informed decisions about optimal parameter settings. Importantly, we also discuss the challenges involved in extrapolating results from prior empirical work to future interventions, including the translation of protocols between models and the complex interaction between TUS protocols and the brain. A synthesis of the empirical evidence suggests that larger effects will be observed at lower frequencies within the sub-MHz range, higher intensities and pressures than commonly administered thus far, and longer pulses and pulse train durations. Nevertheless, we emphasise the need for cautious interpretation of empirical data from different experimental paradigms when basing protocols on prior work as we advance towards refined TUS parameters for human neuromodulation.

Coupling of photovoltaics with neurostimulation electrodes-optical to electrolytic transduction

Objective.The wireless transfer of power for driving implantable neural stimulation devices has garnered significant attention in the bioelectronics field. This study explores the potential of photovoltaic (PV) power transfer, utilizing tissue-penetrating deep-red light-a novel and promising approach that has received less attention compared to traditional induction or ultrasound techniques. Our objective is to critically assess key parameters for directly powering neurostimulation electrodes with PVs, converting light impulses into neurostimulation currents.Approach.We systematically investigate varying PV cell size, optional series configurations, and coupling with microelectrodes fabricated from a range of materials such as Pt, TiN, IrOx, Ti, W, PtOx, Au, or poly(3,4 ethylenedioxythiophene):poly(styrene sulfonate). Additionally, two types of PVs, ultrathin organic PVs and monocrystalline silicon PVs, are compared. These combinations are employed to drive pairs of electrodes with different sizes and impedances. The readout method involves measuring electrolytic current using a straightforward amplifier circuit.Main results.Optimal PV selection is crucial, necessitating sufficiently large PV cells to generate the desired photocurrent. Arranging PVs in series is essential to produce the appropriate voltage for driving current across electrode/electrolyte impedances. By carefully choosing the PV arrangement and electrode type, it becomes possible to emulate electrical stimulation protocols in terms of charge and frequency. An important consideration is whether the circuit is photovoltage-limited or photocurrent-limited. High charge-injection capacity electrodes made from pseudo-faradaic materials impose a photocurrent limit, while more capacitive materials like Pt are photovoltage-limited. Although organic PVs exhibit lower efficiency than silicon PVs, in many practical scenarios, stimulation current is primarily limited by the electrodes rather than the PV driver, leading to potential parity between the two types.Significance.This study provides a foundational guide for designing a PV-powered neurostimulation circuit. The insights gained are applicable to bothin vitroandin vivoapplications, offering a resource to the neural engineering community.

Low-intensity focused ultrasound to the posterior insula reduces temporal summation of pain

BACKGROUND

The insula and dorsal anterior cingulate cortex (dACC) are core brain regions involved in pain processing and central sensitization, a shared mechanism across various chronic pain conditions. Methods to modulate these regions may serve to reduce central sensitization, though it is unclear which target may be most efficacious for different measures of central sensitization.

OBJECTIVE/HYPOTHESIS

Investigate the effect of low-intensity focused ultrasound (LIFU) to the anterior insula (AI), posterior insula (PI), or dACC on conditioned pain modulation (CPM) and temporal summation of pain (TSP).

METHODS

N = 16 volunteers underwent TSP and CPM pain tasks pre/post a 10 min LIFU intervention to either the AI, PI, dACC or Sham stimulation. Pain ratings were collected pre/post LIFU.

RESULTS

Only LIFU to the PI significantly attenuated pain ratings during the TSP protocol. No effects were found for the CPM task for any of the LIFU targets. LIFU pressure modulated group means but did not affect overall group differences.

CONCLUSIONS

LIFU to the PI reduced temporal summation of pain. This may, in part, be due to dosing (pressure) of LIFU. Inhibition of the PI with LIFU may be a future potential therapy in chronic pain populations demonstrating central sensitization. The minimal effective dose of LIFU for efficacious neuromodulation will help to translate LIFU for therapeutic options.

Development of a Wearable Ultrasonic Auricular Vagus Nerve Stimulator

This article discusses the design and development of a wearable and portable ultrasound auricular nerve stimulator. The device is in the form of a headset that presses against the cymba concha depression in the ear and stimulates the auricular vagus nerve with ultrasound energy. This article reviews the development, design, and material structures for such a system, including the characterization of the working prototypes. The devices are being supplied to various organizations and universities for clinical trials, in which effectiveness will be assessed. The device is light, compact, and an effective neurostimulator to induce calm. To the best of our knowledge, the device is the first fully built ultrasonic auricular nerve stimulator in the world.

tFUSFormer: Physics-Guided Super-Resolution Transformer for Simulation of Transcranial Focused Ultrasound Propagation in Brain Stimulation

Transcranial focused ultrasound (tFUS) has emerged as a new mode of non-invasive brain stimulation (NIBS), with its exquisite spatial precision and capacity to reach the deep regions of the brain. The placement of the acoustic focus onto the desired part of the brain is critical for successful tFUS procedures; however, acoustic wave propagation is severely affected by the skull, distorting the focal location/shape and the pressure level. High-resolution (HR) numerical simulation allows for monitoring of acoustic pressure within the skull but with a considerable computational burden. To address this challenge, we employed a 4× super-resolution (SR) Swin Transformer method to improve the precision of estimating tFUS acoustic pressure field, targeting operator-defined brain areas. The training datasets were obtained through numerical simulations at both ultra-low (2.0 [Formula: see text]) and high (0.5 [Formula: see text]) resolutions, conducted on in vivo CT images of 12 human skulls. Our multivariable datasets, which incorporate physical properties of the acoustic pressure field, wave velocity, and skull CT images, were utilized to train three-dimensional SR models. We found that our method yielded 87.99 ± 4.28% accuracy in terms of focal volume conformity under foreseen skull data, and accuracy of 82.32 ± 5.83% for unforeseen skulls, respectively. Moreover, a significant improvement of 99.4% in computational efficiency compared to the traditional 0.5 [Formula: see text] HR numerical simulation was shown. The presented technique, when adopted in guiding the placement of the FUS transducer to engage specific brain targets, holds great potential in enhancing the safety and effectiveness of tFUS therapy.

Contrast factor for standing-wave radiation forces on spheres: Series expansion in powers of sphere radius

Recently researchers often normalize the radiation force on spheres in standing waves in inviscid fluids using an acoustic contrast factor (typically denoted by Φ) that is independent of kR where k is the wave number and R is the sphere radius. An alternative normalization uses a function Ys that depends on kR. Here, standard results for Φ are extended as a power series in kR using prior Ys results. Also, new terms are found for fluid spheres and applied to the kR dependence of Φ for strongly responsive and weakly responsive examples. Partial-wave phase shifts are used in the derivation.

Noninvasive neuromodulation of subregions of the human insula differentially affect pain processing and heart-rate variability: a within-subjects pseudo-randomized trial

ABSTRACT

The insula is an intriguing target for pain modulation. Unfortunately, it lies deep to the cortex making spatially specific noninvasive access difficult. Here, we leverage the high spatial resolution and deep penetration depth of low-intensity focused ultrasound (LIFU) to nonsurgically modulate the anterior insula (AI) or posterior insula (PI) in humans for effect on subjective pain ratings, electroencephalographic (EEG) contact heat-evoked potentials, as well as autonomic measures including heart-rate variability (HRV). In a within-subjects, repeated-measures, pseudo-randomized trial design, 23 healthy volunteers received brief noxious heat pain stimuli to the dorsum of their right hand during continuous heart-rate, electrodermal, electrocardiography and EEG recording. Low-intensity focused ultrasound was delivered to the AI (anterior short gyrus), PI (posterior longus gyrus), or under an inert Sham condition. The primary outcome measure was pain rating. Low-intensity focused ultrasound to both AI and PI similarly reduced pain ratings but had differential effects on EEG activity. Low-intensity focused ultrasound to PI affected earlier EEG amplitudes, whereas LIFU to AI affected later EEG amplitudes. Only LIFU to the AI affected HRV as indexed by an increase in SD of N-N intervals and mean HRV low-frequency power. Taken together, LIFU is an effective noninvasive method to individually target subregions of the insula in humans for site-specific effects on brain biomarkers of pain processing and autonomic reactivity that translates to reduced perceived pain to a transient heat stimulus.

Transcranial focused ultrasound activates feedforward and feedback cortico-thalamo-cortical pathways by selectively activating excitatory neurons

Transcranial focused ultrasound stimulation (tFUS) has been proven capable of altering focal neuronal activities and neural circuits non-invasively in both animals and humans. The abilities of tFUS for cell-type selection within the targeted area like somatosensory cortex have been shown to be parameter related. However, how neuronal subpopulations across neural pathways are affected, for example how tFUS affected neuronal connections between brain areas remains unclear. In this study, multi-site intracranial recordings were used to quantify the neuronal responses to tFUS stimulation at somatosensory cortex (S1), motor cortex (M1) and posterior medial thalamic nucleus (POm) of cortico-thalamo-cortical (CTC) pathway. We found that when targeting at S1 or POm, only regular spiking units (RSUs, putative excitatory neurons) responded to specific tFUS parameters (duty cycle: 6%-60% and pulse repetition frequency: 1500 and 3000 Hz ) during sonication. RSUs from the directly connected area (POm or S1) showed a synchronized response, which changed the directional correlation between RSUs from POm and S1. The tFUS induced excitation of RSUs activated the feedforward and feedback loops between cortex and thalamus, eliciting delayed neuronal responses of RSUs and delayed activities of fast spiking units (FSUs) by affecting local network. Our findings indicated that tFUS can modulate the CTC pathway through both feedforward and feedback loops, which could influence larger cortical areas including motor cortex.

An Investigation of Parameter-Dependent Cell-Type Specific Effects of Transcranial Focused Ultrasound Stimulation Using an Awake Head-Fixed Rodent Model

Transcranial focused ultrasound (tFUS) is a promising neuromodulation technique able to target shallow and deep brain structures with high precision. Previous studies have demonstrated that tFUS stimulation responses are both cell-type specific and controllable through altering stimulation parameters. Specifically, tFUS can elicit time-locked neural activity in regular spiking units (RSUs) that is sensitive to increases in pulse repetition frequency (PRF), while time-locked responses are not seen in fast spiking units (FSUs). These findings suggest a unique capability of tFUS to alter circuit network dynamics with cell-type specificity; however, these results could be biased by the use of anesthesia, which significantly modulates neural activities. In this study, we develop an awake head-fixed rat model specifically designed for tFUS study, and address a key question if tFUS still has cell-type specificity under awake conditions. Using this novel animal model, we examined a series of PRFs and burst duty cycles (DCs) to determine their effects on neuronal subpopulations without anesthesia. We conclude that cell-type specific time-locked and delayed responses to tFUS as well as PRF and DC sensitivity are present in the awake animal model and that despite some differences in response, isoflurane anesthesia is not a major confound in studying the cell-type specificity of ultrasound neuromodulation. We further determine that, in an awake, head-fixed setting, the preferred PRF and DC for inducing time-locked excitation with our pulsed tFUS paradigm are 1500 Hz and 60%, respectively.

Controlled ultrasonic interventions through the human skull

Transcranial focused ultrasound enables precise and non-invasive manipulations of deep brain circuits in humans, promising to provide safe and effective treatments of various neurological and mental health conditions. Ultrasound focused to deep brain targets can be used to modulate neural activity directly or localize the release of psychoactive drugs. However, these applications have been impeded by a key barrier-the human skull, which attenuates ultrasound strongly and unpredictably. To address this issue, we have developed an ultrasound-based approach that directly measures and compensates for the ultrasound attenuation by the skull. No additional skull imaging, simulations, assumptions, or free parameters are necessary; the method measures the attenuation directly by emitting a pulse of ultrasound from an array on one side of the head and measuring with an array on the opposite side. Here, we apply this emerging method to two primary future uses-neuromodulation and local drug release. Specifically, we show that the correction enables effective stimulation of peripheral nerves and effective release of propofol from nanoparticle carriers through an ex vivo human skull. Neither application was effective without the correction. Moreover, the effects show the expected dose-response relationship and targeting specificity. This article highlights the need for precise control of ultrasound intensity within the skull and provides a direct and practical approach for addressing this lingering barrier.

Recent advances in stimuli-responsive controlled release systems for neuromodulation

Neuromodulation aims to modulate the signaling activity of neurons or neural networks by the precise delivery of electrical stimuli or chemical agents and is crucial for understanding brain function and treating brain disorders. Conventional approaches, such as direct physical stimulation through electrical or acoustic methods, confront challenges stemming from their invasive nature, dependency on wired power sources, and unstable therapeutic outcomes. The emergence of stimulus-responsive delivery systems harbors the potential to revolutionize neuromodulation strategies through the precise and controlled release of neurochemicals in a specific brain region. This review comprehensively examines the biological barriers controlled release systems may encounter in vivo and the recent advances and applications of these systems in neuromodulation. We elucidate the intricate interplay between the molecular structure of delivery systems and response mechanisms to furnish insights for material selection and design. Additionally, the review contemplates the prospects and challenges associated with these systems in neuromodulation. The overarching objective is to propel the application of neuromodulation technology in analyzing brain functions, treating brain disorders, and providing insightful perspectives for exploiting new systems for biomedical applications.

Transcranial focused ultrasound to V5 enhances human visual motion brain-computer interface by modulating feature-based attention

A brain-computer interface (BCI) enables users to control devices with their minds. Despite advancements, non-invasive BCIs still exhibit high error rates, prompting investigation into the potential reduction through concurrent targeted neuromodulation. Transcranial focused ultrasound (tFUS) is an emerging non-invasive neuromodulation technology with high spatiotemporal precision. This study examines whether tFUS neuromodulation can improve BCI outcomes, and explores the underlying mechanism of action using high-density electroencephalography (EEG) source imaging (ESI). As a result, V5-targeted tFUS significantly reduced the error in a BCI speller task. Source analyses revealed a significantly increase in theta and alpha activities in the tFUS condition at both V5 and downstream in the dorsal visual processing pathway. Correlation analysis indicated that the connection within the dorsal processing pathway was preserved during tFUS stimulation, while the ventral connection was weakened. These findings suggest that V5-targeted tFUS enhances feature-based attention to visual motion.

Agarose as a Tissue Mimic for the Porcine Heart, Kidney, and Liver: Measurements and a Springpot Model

Agarose gels are often used as a tissue mimic. The goal of this work was to determine the appropriate agarose concentrations that result in mechanical properties that match three different porcine organs. Strain tests were carried out with an amplitude varying from 0.01% to 10% at a frequency of 1 Hz on a range of agarose concentrations and porcine organs. Frequency sweep tests were performed from 0.1 Hz to a maximum of 9.5 Hz at a shear strain amplitude of 0.1% for agarose and porcine organs. In agarose samples, the effect of pre-compression of the samples up to 10% axial strain was considered during frequency sweep tests. The experimental measurements from agarose samples were fit to a fractional order viscoelastic (springpot) model. The model was then used to predict stress relaxation in response to a step strain of 0.1%. The prediction was compared to experimental relaxation data, and the results agreed within 12%. The agarose concentrations (by mass) that gave the best fit were 0.25% for the liver, 0.3% for the kidney, and 0.4% for the heart. At a frequency of 0.1 Hz and a shear strain of 0.1%, the agarose concentrations that best matched the shear storage modulus of the porcine organs were 0.4% agarose for the heart, 0.3% agarose for the kidney, and 0.25% agarose for the liver.

Transcranial focused ultrasound to the posterior cingulate cortex modulates default mode network and subjective experience: an fMRI pilot study

Background

Transcranial focused ultrasound (TFUS) is an emerging neuromodulation tool for temporarily altering brain activity and probing network functioning. The effects of TFUS on the default mode network (DMN) are unknown.

Objective

The study examined the effects of transcranial focused ultrasound (TFUS) on the functional connectivity of the default mode network (DMN), specifically by targeting the posterior cingulate cortex (PCC). Additionally, we investigated the subjective effects of TFUS on mood, mindfulness, and self-related processing.

Methods

The study employed a randomized, single-blind design involving 30 healthy subjects. Participants were randomly assigned to either the active TFUS group or the sham TFUS group. Resting-state functional magnetic resonance imaging (rs-fMRI) scans were conducted before and after the TFUS application. To measure subjective effects, the Toronto Mindfulness Scale, the Visual Analog Mood Scale, and the Amsterdam Resting State Questionnaire were administered at baseline and 30 min after sonication. The Self Scale and an unstructured interview were also administered 30 min after sonication.

Results

The active TFUS group exhibited significant reductions in functional connectivity along the midline of the DMN, while the sham TFUS group showed no changes. The active TFUS group demonstrated increased state mindfulness, reduced Global Vigor, and temporary alterations in the sense of ego, sense of time, and recollection of memories. The sham TFUS group showed an increase in state mindfulness, too, with no other subjective effects.

Conclusions

TFUS targeted at the PCC can alter DMN connectivity and cause changes in subjective experience. These findings support the potential of TFUS to serve both as a research tool and as a potential therapeutic intervention.

An implantable piezoelectric ultrasound stimulator (ImPULS) for deep brain activation

Precise neurostimulation can revolutionize therapies for neurological disorders. Electrode-based stimulation devices face challenges in achieving precise and consistent targeting due to the immune response and the limited penetration of electrical fields. Ultrasound can aid in energy propagation, but transcranial ultrasound stimulation in the deep brain has limited spatial resolution caused by bone and tissue scattering. Here, we report an implantable piezoelectric ultrasound stimulator (ImPULS) that generates an ultrasonic focal pressure of 100 kPa to modulate the activity of neurons. ImPULS is a fully-encapsulated, flexible piezoelectric micromachined ultrasound transducer that incorporates a biocompatible piezoceramic, potassium sodium niobate [(K,Na)NbO3]. The absence of electrochemically active elements poses a new strategy for achieving long-term stability. We demonstrated that ImPULS can i) excite neurons in a mouse hippocampal slice ex vivo, ii) activate cells in the hippocampus of an anesthetized mouse to induce expression of activity-dependent gene c-Fos, and iii) stimulate dopaminergic neurons in the substantia nigra pars compacta to elicit time-locked modulation of nigrostriatal dopamine release. This work introduces a non-genetic ultrasound platform for spatially-localized neural stimulation and exploration of basic functions in the deep brain.

Photoacoustic: A Versatile Nongenetic Method for High-Precision Neuromodulation

High-precision neuromodulation plays a pivotal role in elucidating fundamental principles of neuroscience and treating specific neurological disorders. Optical neuromodulation, enabled by spatial resolution defined by the diffraction limit at the submicrometer scale, is a general strategy to achieve such precision. Optogenetics offers single-neuron spatial resolution with cellular specificity, whereas the requirement of genetic transfection hinders its clinical application. Direct photothermal modulation, an alternative nongenetic optical approach, often associates a large temperature increase with the risk of thermal damage to surrounding tissues.Photoacoustic (also called optoacoustic) neural stimulation is an emerging technology for neural stimulation with the following key features demonstrated. First, the photoacoustic approach demonstrated high efficacy without the need for genetic modification. The generated pulsed ultrasound upon ns laser pulses with energy ranging from a few μJ to tens of μJ is sufficient to activate wild-type neurons. Second, the photoacoustic approach provides sub-100-μm spatial precision. It overcomes the fundamental wave diffraction limit of ultrasound by harnessing the localized ultrasound field generated through light absorption. A spatial precision of 400 μm has been achieved in rodent brains using a fiber-based photoacoustic emitter. Single-cell stimulation in neuronal cultures in vitro and in brain slices ex vivo is achieved using tapered fiber-based photoacoustic emitters. This precision is 10 to 100 times better than that for piezo-based low-frequency ultrasound and is essential to pinpoint a specific region or cell population in a living brain. Third, compared to direct photothermal stimulation via temperature increase, photoacoustic stimulation requires 40 times less laser energy dose to evoke neuron activities and is associated with a minimal temperature increase of less than 1 °C, preventing potential thermal damage to neurons. Fourth, photoacoustics is a versatile approach and can be designed in various platforms aiming at specific applications. Our team has shown the design of fiber-based photoacoustic emitters, photoacoustic nanotransducers, soft biocompatible photoacoustic films, and soft photoacoustic lenses. Since they interact with neurons through ultrasound without the need for direct contact, photoacoustic enables noninvasive transcranial and dura-penetrating brain stimulation without compromising high precision.In this Account, we will first review the basic principles of photoacoustic and discuss the key design elements of PA transducers for neural modulation guided by the principle. We will also highlight how these design goals were achieved from a materials chemistry perspective. The design of different PA interfaces, their unique capability, and their applications in neural systems will be reviewed. In the end, we will discuss the remaining challenges and future perspectives for this technology.

A biodegradable and flexible neural interface for transdermal optoelectronic modulation and regeneration of peripheral nerves

Optoelectronic neural interfaces can leverage the photovoltaic effect to convert light into electrical current, inducing charge redistribution and enabling nerve stimulation. This method offers a non-genetic and remote approach for neuromodulation. Developing biodegradable and efficient optoelectronic neural interfaces is important for achieving transdermal stimulation while minimizing infection risks associated with device retrieval, thereby maximizing therapeutic outcomes. We propose a biodegradable, flexible, and miniaturized silicon-based neural interface capable of transdermal optoelectronic stimulation for neural modulation and nerve regeneration. Enhancing the device interface with thin-film molybdenum significantly improves the efficacy of neural stimulation. Our study demonstrates successful activation of the sciatic nerve in rodents and the facial nerve in rabbits. Moreover, transdermal optoelectronic stimulation accelerates the functional recovery of injured facial nerves.

Mechanobiological insight into brain diseases based on mechanosensitive channels: Common mechanisms and clinical potential

BACKGROUND

As physical signals, mechanical cues regulate the neural cells in the brain. The mechanosensitive channels (MSCs) perceive the mechanical cues and transduce them by permeating specific ions or molecules across the plasma membrane, and finally trigger a series of intracellular bioelectrical and biochemical signals. Emerging evidence supports that wide-distributed, high-expressed MSCs like Piezo1 play important roles in several neurophysiological processes and neurological disorders.

AIMS

To systematically conclude the functions of MSCs in the brain and provide a novel mechanobiological perspective for brain diseases.

METHOD

We summarized the mechanical cues and MSCs detected in the brain and the research progress on the functional roles of MSCs in physiological conditions. We then concluded the pathological activation and downstream pathways triggered by MSCs in two categories of brain diseases, neurodegenerative diseases and place-occupying damages. Finally, we outlined the methods for manipulating MSCs and discussed their medical potential with some crucial outstanding issues.

RESULTS

The MSCs present underlying common mechanisms in different brain diseases by acting as the "transportation hubs" to transduce the distinct signal patterns: the upstream mechanical cues and the downstream intracellular pathways. Manipulating the MSCs is feasible to alter the complicated downstream processes, providing them promising targets for clinical treatment.

CONCLUSIONS

Recent research on MSCs provides a novel insight into brain diseases. The common mechanisms mediated by MSCs inspire a wide range of therapeutic potentials targeted on MSCs in different brain diseases.