Focused ultrasound neuromodulation of cortical and subcortical brain structures using 1.9 MHz

A systematic review of preclinical and clinical transcranial ultrasound neuromodulation and opportunities for functional connectomics

BACKGROUND

Low-intensity transcranial ultrasound has surged forward as a non-invasive and disruptive tool for neuromodulation with applications in basic neuroscience research and the treatment of neurological and psychiatric conditions.

OBJECTIVE

To provide a comprehensive overview and update of preclinical and clinical transcranial low intensity ultrasound for neuromodulation and emphasize the emerging role of functional brain mapping to guide, better understand, and predict responses.

METHODS

A systematic review was conducted by searching the Web of Science and Scopus databases for studies on transcranial ultrasound neuromodulation, both in humans and animals.

RESULTS

187 relevant studies were identified and reviewed, including 116 preclinical and 71 clinical reports with subjects belonging to diverse cohorts. Milestones of ultrasound neuromodulation are described within an overview of the broader landscape. General neural readouts and outcome measures are discussed, potential confounds are noted, and the emerging use of functional magnetic resonance imaging is highlighted.

CONCLUSION

Ultrasound neuromodulation has emerged as a powerful tool to study and treat a range of conditions and its combination with various neural readouts has significantly advanced this platform. In particular, the use of functional magnetic resonance imaging has yielded exciting inferences into ultrasound neuromodulation and has the potential to advance our understanding of brain function, neuromodulatory mechanisms, and ultimately clinical outcomes. It is anticipated that these preclinical and clinical trials are the first of many; that transcranial low intensity focused ultrasound, particularly in combination with functional magnetic resonance imaging, has the potential to enhance treatment for a spectrum of neurological conditions.

Sustained reduction of essential tremor with low-power non-thermal transcranial focused ultrasound stimulations in humans

BACKGROUND

Transcranial ultrasound stimulation (TUS) is a non-invasive brain stimulation technique; when skull aberrations are compensated for, this technique allows, with millimetric accuracy, circumvention of the invasive surgical procedure associated with deep brain stimulation (DBS) and the limited spatial specificity of transcranial magnetic stimulation.

OBJECTIVE

/hypothesis: We hypothesize that MR-guided low-power TUS can induce a sustained decrease of tremor power in patients suffering from medically refractive essential tremor.

METHODS

The dominant hand only was targeted, and two anatomical sites were sonicated in this exploratory study: the ventral intermediate nucleus of the thalamus (VIM) and the dentato-rubro-thalamic tract (DRT). Patients (N = 9) were equipped with MR-compatible accelerometers attached to their hands to monitor their tremor in real-time during TUS.

RESULTS

VIM neurostimulations followed by a low-duty cycle (5 %) DRT stimulation induced a substantial decrease in the tremor power in four patients, with a minimum of 89.9 % reduction when compared with the baseline power a few minutes after the DRT stimulation. The only patient stimulated in the VIM only and with a low duty cycle (5 %) also experienced a sustained reduction of the tremor (up to 93.4 %). Four patients (N = 4) did not respond. The temperature at target was 37.2 ± 1.4 °C compared to 36.8 ± 1.4 °C for a 3 cm away control point.

CONCLUSIONS

MR-guided low power TUS can induce a substantial and sustained decrease of tremor power. Follow-up studies need to be conducted to reproduce the effect and better to understand the variability of the response amongst patients. MR thermometry during neurostimulations showed no significant thermal rise, supporting a mechanical effect.

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.

Transcranial Functional Ultrasound Imaging Detects Focused Ultrasound Neuromodulation Induced Hemodynamic Changes in Mouse and Nonhuman Primate Brains In Vivo

Focused ultrasound (FUS) is an emerging noinvasive technique for neuromodulation in the central nervous system (CNS). To evaluate the effects of FUS-induced neuromodulation, many studies used behavioral changes, functional magnetic resonance imaging (fMRI) or electroencephalography (EEG). However, behavioral readouts are often not easily mapped to specific brain activity, EEG has low spatial resolution limited to the surface of the brain and fMRI requires a large importable scanner that limits additional readouts and manipulations. In this context, functional ultrasound imaging (fUSI) holds promise to directly monitor the effects of FUS neuromodulation with high spatiotemporal resolution in a large field of view, with a comparatively simple and flexible setup. fUSI uses ultrafast Power Doppler Imaging (PDI) to measure changes in cerebral blood volume, which correlates well with neuronal activity and local field potentials. We designed a setup that aligns a FUS transducer with a linear array to allow immediate subsequent monitoring of the hemodynamic response with fUSI during and after FUS neuromodulation. We established a positive correlation between FUS pressure and the size of the activated area, as well as changes in cerebral blood volume (CBV) and found that unilateral sonications produce bilateral hemodynamic changes with ipsilateral accentuation in mice. We further demonstrated the ability to perform fully noninvasive, transcranial FUS-fUSI in nonhuman primates for the first time by using a lower-frequency transducer configuration.

Ultrasound Modulates Calcium Activity in Cultured Neurons, Glial Cells, Endothelial Cells and Pericytes

OBJECTIVE

Ultrasound is being researched as a method to modulate the brain. Studies of the interaction of sound with neurons support the hypothesis that mechanosensitive ion channels play an important role in ultrasound neuromodulation. The response of cells other than neurons (e.g., astrocytes, pericytes and endothelial cells) have not been fully characterized, despite playing an important role in brain function.

METHODS

To address this gap in knowledge, we examined cultured murine primary cortical neurons, astrocytes, endothelial cells and pericytes in an in vitro widefield microscopy setup during application of a 500 ms burst of 250 kHz focused ultrasound over a pressure range known to elicit neuromodulation. We examined cell membrane health in response to a range of pulses and used optical calcium indicators in conjunction with pharmacological antagonists to selectively block different groups of thermo- and mechanosensitive ion channels known to be responsive to ultrasound.

RESULTS

All cell types experienced an increase in calcium fluorescence in response to ultrasound. Gadolinium (Gad), 2-aminoethoxydiphenyl borate (2-APB) and ruthenium red (RR) reduced the percentage of responding neurons and magnitude of response. The percentage of astrocytes responding was significantly lowered only by Gad, whereas both 2-APB and Gad decreased the amplitude of the fluorescence response. 2-APB decreased the percentage of responding endothelial cells, whereas only Gad reduced the magnitude of responses. Pericytes exposed to RR or Gad were less likely to respond to stimulation. RR had no detectable effect on the magnitude of the pericyte responses while 2-APB and Gad significantly decreased the fluorescence intensity, despite not affecting the percentage responding.

CONCLUSION

Our study highlights the role of non-neuronal cells during FUS neuromodulation. All of the investigated cell types are sensitive to mechanical ultrasound stimulation and rely on mechanosensitive ion channels to undergo ultrasound neuromodulation.

Modulation of cardio-respiratory activity in mice via transcranial focused ultrasound

OBJECTIVE

The objective of this study was to investigate the effect of FUS on autonomic nervous system activity, including heart and respiratory rates, and to separate the thermal modulation from combined thermal and mechanical FUS effects.

METHODS

The thalamus and hypothalamus of wild-type mice were sonicated with a continuous-wave, 2 MHz FUS transducer at pressures of 425 and 850 kPa for 60 seconds. Cardiac and respiratory rates were monitored as signs of autonomic nervous activity. FUS-induced changes in autonomic activity were compared to FUS targeted to a spatially-distant motor region and to laser-induced heating.

RESULTS

FUS delivered to the primary target over the thalamus and hypothalamus at 850 kPa reversibly increased the respiratory rate by 6.5±3.2 breaths per minute and decreased the heart rate by 3.2±1.8 beats per minute. No significant changes occurred in this region at 425 kPa or when targeting the motor regions at 850 kPa. Laser heating with the same temperature rise profile produced by 850 kPa sonication resulted in cardiorespiratory modulation similar to that of FUS.

CONCLUSIONS

FUS is capable of reversibly and non-invasively modulating cardiorespiratory activity in mice. Localized changes in temperature may constitute the main cause for this activity, though further investigation is warranted into the distinct and complementary mechanisms of mechanically- and thermally-induced FUS neuromodulation. Close monitoring of vital signs during FUS neuromodulation may be warranted to monitor systemic responses to stimulation.

Small volume blood-brain barrier opening in macaques with a 1 MHz ultrasound phased array

The use of focused ultrasound to open the blood-brain barrier (BBB) has the potential to deliver drugs to specific regions of the brain. The size of the BBB opening and ability to localize the opening determines the spatial extent and is a limiting factor in many applications of BBB opening where targeting a small brain region is desired. Here we evaluate the performance of a system designed for small opening volumes and highlight the unique challenges associated with pushing the spatial precision of this technique. To achieve small volume openings in cortical regions of the macaque brain, we tested a custom 1 MHz array transducer integrated into a magnetic resonance image-guided focused ultrasound system. Using real-time cavitation monitoring, we demonstrated twelve instances of single sonication, small volume BBB opening with average volumes of 59 ± 37 mm3 and 184 ± 2 mm3 in cortical and subcortical targets, respectively. We found high correlation between subject-specific acoustic simulations and observed openings when incorporating grey matter segmentation (R2 = 0.8577), and the threshold for BBB opening based on simulations was 0.53 MPa. Analysis of MRI-based safety assessment and cavitation signals indicate a safe pressure range for 1 MHz BBB opening and suggest that our system can be used to deliver drugs and gene therapy to small brain regions.

Development and validation of a computational method to predict unintended auditory brainstem response during transcranial ultrasound neuromodulation in mice

BACKGROUND

Transcranial ultrasound stimulation (TUS) is a promising noninvasive neuromodulation modality. The inadvertent and unpredictable activation of the auditory system in response to TUS obfuscates the interpretation of non-auditory neuromodulatory responses.

OBJECTIVE

The objective was to develop and validate a computational metric to quantify the susceptibility to unintended auditory brainstem response (ABR) in mice premised on time frequency analyses of TUS signals and auditory sensitivity.

METHODS

Ultrasound pulses with varying amplitudes, pulse repetition frequencies (PRFs), envelope smoothing profiles, and sinusoidal modulation frequencies were selected. Each pulse's time-varying frequency spectrum was differentiated across time, weighted by the mouse hearing sensitivity, then summed across frequencies. The resulting time-varying function, computationally predicting the ABR, was validated against experimental ABR in mice during TUS with the corresponding pulse.

RESULTS

There was a significant correlation between experimental ABRs and the computational predictions for 19 TUS signals (R2 = 0.97).

CONCLUSIONS

To reduce ABR in mice during in vivo TUS studies, 1) reduce the amplitude of a rectangular continuous wave envelope, 2) increase the rise/fall times of a smoothed continuous wave envelope, and/or 3) change the PRF and/or duty cycle of a rectangular or sinusoidal pulsed wave to reduce the gap between pulses and increase the rise/fall time of the overall envelope. This metric can aid researchers performing in vivo mouse studies in selecting TUS signal parameters that minimize unintended ABR. The methods for developing this metric can be adapted to other animal models.

A phased array ultrasound system with a robotic arm for neuromodulation

BACKGROUND

Ultrasonic neuromodulation (UNMOD) provides a non-invasive brain stimulation. However, the high-resolution region-specificity of UNMOD with a single element transducer combined with a mechanical positioning system could have limits due to the intrinsic positioning error from mechanical systems.

OBJECTIVE/HYPOTHESIS

A phased array system could lead to highly selective neuromodulation with electronic control.

METHODS

A specialized phased-array system with a robotic arm is implemented for a rhesus monkey model. Various primary motor cortex areas related to tail, hand, and mouth were stimulated with a 200 μm step size. The ultrasonic parameters were ISPTA of 840 mW/cm2, pulse repetition frequency of 100 Hz, and a 5% duty factor at 600 kHz. The induced movement were recorded and analyzed.

RESULTS

Separate digits, mouth, and tongue motions were successfully induced by electronically controlling the focus. The identical body part movement could be induced when the focus was moved back to the identical primary motor cortex with electronic control. Accordingly, the reproducibility of UNMOD could be partially validated with rhesus monkey model.

CONCLUSION

A phased-array system appears to have a potential for the non-invasive and region-selective neuromodulation method.

Acoustic radiation force for analyzing the mechanical stress in ultrasound neuromodulation

Objective. Although recent studies have shown that mechanical stress plays an important role in ultrasound neuromodulation, the magnitude and distribution of the mechanical stress generated in tissues by focused ultrasound transducers have not been adequately examined. Various acoustic radiation force (ARF) equations used in previous studies have been evaluated based on the tissue displacement results and are suitable for estimating the displacement. However, it is unclear whether mechanical stress can be accurately determined. This study evaluates the mechanical stress predicted by various AFR equations and suggests the optimal equation for estimating the mechanical stress in the brain tissue.Approach. In this paper, brain tissue responses are compared through numerical finite element simulations by applying the three most used ARF equations-Reynolds stress force ((RSF)), momentum flux density tensor force, and attenuation force. Three ARF fields obtained from the same pressure field were applied to the linear elastic model to calculate the displacement, mechanical stress, and mean pressure generated inside the tissue. Both the simple pressure field using a single transducer and the complex standing wave pressure field using two transducers were simulated.Main results. For the case using a single transducer, all three ARFs showed similar displacement. However, when comparing the mechanical stress results, only the results using the RSF showed a strong stress tensor at the focal point. For the case of using two transducers, the displacement and stress tensor field of the pattern related to the standing wave were calculated only from the results using the RSF.Significance. The model using RSF equation allows accurate analysis on stress tensor inside the tissue for ultrasound neuromodulation.

Evaluation of a Novel Acoustic Coupling Medium for Human Low-Intensity Focused Ultrasound Neuromodulation Applications

OBJECTIVE

Single-element low-intensity focused ultrasound (LIFU) is an emerging form of human neuromodulation. Current coupling methods are impractical for clinical bedside use. Here, we evaluate commercially available high-viscosity gel polymer matrices as couplants for human LIFU neuromodulation applications.

METHODS

We first empirically tested the acoustic transmission of three densities at 500 kHz and then subjected the gel with the least acoustic attenuation to further tests of the effect of thickness, frequency, de-gassing and production variability.

RESULTS

The highest-density gel had the lowest acoustic attenuation (3.3%) with low lateral (<0.5 mm) and axial (<2 mm) beam distortion. Different thicknesses of the gel up to 10 mm did not appreciably affect results. The gel polymers exhibited frequency-dependent attenuation at 1 and 3 MHz up to 86.6%, as well as significant beam distortion >4 mm. Poor de-gassing methods also increased pressure attenuation at 500 kHz up to 59.6%. Standardized methods of making these gels should be established to reduce variability.

CONCLUSION

Commercially available de-gassed, high-density gel matrices are a low-cost, easily malleable, low-attenuation and distortion medium for the coupling of single-element LIFU transducers for human neuromodulation applications at 500 kHz.

Higher harmonics dynamic focalization in single-element ring transducers using biaxial driving

Biaxial driving is a new driving technique that allows the steering of the ultrasound field generated by a single-element piezoceramic transducer. Because of their natural axisymmetric geometry, ultrasound generation with ring transducers can take advantage of the biaxial driving to change the focus of the beam generated by this type of transducer using only two driving signals. In this study, we applied the biaxial driving technique into a single-element PZT ring transducer operating at 500 kHz to produce a change in size and position of the focal spot while using the 1st (482 kHz), 3rd (1.362 MHz) and 5th (2.62 MHz) harmonic excitation. The transducer had a thickness of 2.85 mm, an inner diameter of 9.75 mm and a ring width of 2.0 mm, and two pairs of electrodes as required for biaxial driving. Simulation and experimental results showed that both the focal area and the distance at which the focal area centre was located changed as a function of the phase and power difference between the two driving signals. Experimental results showed that the focal area could be reduced from 31.6 mm² (conventional driving) to 3.4 mm² (89 % reduction) when using the first harmonic excitation. For the third harmonic, the focal area could be reduced from 4.0 mm² (conventional driving) to 3.3 mm² (17.5 % reduction). For the fifth harmonic, the focal area could be reduced from 1.7 mm² (conventional driving) to 1 mm² (41.7 % reduction). Results also demonstrated the centre of the focus could be displaced between 3.0 mm and 9.3 mm from the surface of the transducer when using the first harmonic, between 7.3 mm and 8.4 mm at the third harmonic, and between 4.9 mm and 8.2 mm at the fifth harmonic. The reduction in the focus area, as well as the possibility to displace the focus dynamically will be advantageous for preclinical applications of focused ultrasound, especially on drug delivery and neuromodulation studies in small rodents.

Focused ultrasound neuromodulation of the spleen activates an anti-inflammatory response in humans

Focused ultrasound stimulation (FUS) activates mechanosensitive ion channels and is emerging as a method of noninvasive neuromodulation. In preclinical studies, FUS of the spleen (sFUS) activates an anti-inflammatory neural pathway which suppresses acute and chronic inflammation. However, the relevance of sFUS for regulating inflammatory responses in humans is unknown. Here, we used a modified diagnostic ultrasound imaging system to target the spleen of healthy human subjects with 3 min of continuously swept or stationary focused pulsed ultrasound, delivered at three different energy levels within allowable safety exposure limits. Potential anti-inflammatory effects of sFUS were assessed by measuring sFUS-elicited changes in endotoxin-induced tumor necrosis factor (TNF) production in whole blood samples from insonified subjects. We found that stimulation with either continuously swept or focused pulsed ultrasound has an anti-inflammatory effect: sFUS lowers TNF production for >2 h, with TNF returning to baseline by 24 h following sFUS. This response is independent of anatomical target (i.e., spleen hilum or parenchyma) or ultrasound energy level. No clinical, biochemical, or hematological parameters are adversely impacted. This is the first demonstration that sFUS suppresses the normal inflammatory response in humans, with potential implications for noninvasive bioelectronic therapy of inflammatory disorders.

Small volume blood-brain barrier opening in macaques with a 1 MHz ultrasound phased array

Focused ultrasound blood-brain barrier (BBB) opening is a promising tool for targeted delivery of therapeutic agents into the brain. The volume of opening determines the extent of therapeutic administration and sets a lower bound on the size of targets which can be selectively treated. We tested a custom 1 MHz array transducer optimized for cortical regions in the macaque brain with the goal of achieving small volume openings. We integrated this device into a magnetic resonance image guided focused ultrasound system and demonstrated twelve instances of small volume BBB opening with average opening volumes of 59 ± 37 mm ³ and 184 ± 2 mm ³ in cortical and subcortical targets, respectively. We developed real-time cavitation monitoring using a passive cavitation detector embedded in the array and characterized its performance on a bench-top flow phantom mimicking transcranial BBB opening procedures. We monitored cavitation during in-vivo procedures and compared cavitation metrics against opening volumes and safety outcomes measured with FLAIR and susceptibility weighted MR imaging. Our findings show small BBB opening at cortical targets in macaques and characterize the safe pressure range for 1 MHz BBB opening. Additionally, we used subject-specific simulations to investigate variance in measured opening volumes and found high correlation (R ² = 0.8577) between simulation predictions and observed measurements. Simulations suggest the threshold for 1 MHz BBB opening was 0.53 MPa. This system enables BBB opening for drug delivery and gene therapy to be targeted to more specific brain regions.

Evaluation of Non-invasive Optogenetic Stimulation with Transcranial Functional Ultrasound Imaging

Optogenetics employs engineered viruses to genetically modify cells to express specific light-sensitive ion channels. The standard method for gene delivery in the brain involves invasive craniotomies that expose the brain and direct injections of viruses that invariably damage neural tissue along the syringe tract. A recently proposed alternative in which non-invasive optogenetics is performed with focused ultrasound (FUS)-mediated blood-brain barrier (BBB) openings has been found to non-invasively facilitate gene delivery for optogenetics in mice. Although gene delivery can be performed non-invasively, validating successful viral transduction and expression of encoded ion channels in target tissue typically involves similar invasive techniques, such as craniotomies in longitudinal studies and/or postmortem histology. Functional ultrasound imaging (fUSi) is an emerging neuroimaging technique that can be used to transcranially detect changes in cerebral blood volume following introduction of a stimulus. In this study, we implemented a fully non-invasive combined FUS-fUSi technique for performing optogenetics in mice. FUS successfully delivered viruses encoding the red-shifted channelrhodopsin variant ChrimsonR in all treated subjects. fUSi successfully identified stimulus-evoked cerebral blood volume changes preferentially in brain regions expressing the light-sensitive ion channels. Improvements in cell-specific targeting of viral vectors and transcranial ultrasound imaging will make the combined technique a useful tool for neuroscience research in small animals.

C. elegans: Sensing the low-frequency profile of amplitude-modulated ultrasound

Several research groups have demonstrated that C. elegans can respond to pulsed ultrasound stimuli, and elucidating the underlying mechanisms is necessary to develop ultrasound neuromodulation. Here, amplitude-modulated (AM) ultrasound is applied to C. elegans, and its behavioral responses are investigated in detail. By loading surface acoustic waves (SAWs) onto free-moving worms on an agar surface, a carrier wave with a frequency of 8.80 MHz is selected. The signal is modulated by a rectangular or sinusoidal profile. It is demonstrated that sinusoidal modulation can produce similar responses in worms to rectangular modulation, with the strongest responses occurring at modulation frequencies of around 1.00 kHz. Meanwhile, the behavioral response is relatively weak when the ultrasonic signal is unmodulated, that is, when only the carrier wave is applied. At modulation frequencies other than 100.00 Hz to 10.00 kHz, the worms respond weakly, but when a second modulation frequency of 1.00 kHz is introduced, an improvement in response can be observed. These results suggest that C. elegans may sense the low-frequency envelope and respond to amplitude-modulated ultrasonic stimuli like an amplitude demodulator. MEC-4, an ion channel for touch sensing, is involved in the behavioral response of C. elegans to ultrasound in the present setup.

Effects of focused ultrasound pulse duration on stimulating cortical and subcortical motor circuits in awake sheep

Low-intensity transcranial focused ultrasound (tFUS) offers new functional neuromodulation opportunities, enabling stimulation of cortical as well as deep brain areas with high spatial resolution. Brain stimulation of awake sheep, in the absence of the confounding effects of anesthesia on brain function, provides translational insight into potential human applications with safety information supplemented by histological analyses. We examined the effects of tFUS pulsing parameters, particularly regarding pulse durations (PDs), on stimulating the cortical motor area (M1) and its thalamic projection in unanesthetized, awake sheep (n = 8). A wearable tFUS headgear, custom-made for individual sheep, enabled experiments to be conducted without using anesthesia. FUS stimuli, each 200 ms long, were delivered to the M1 and the thalamus using three different PDs (0.5, 1, and 2 ms) with the pulse repetition frequency (PRF) adjusted to maintain a 70% duty cycle at a derated in situ spatial-peak temporal-average intensity (Ispta) of 3.6 W/cm2. Efferent electromyography (EMG) responses to stimulation were quantified from both hind limbs. Group-averaged EMG responses from each of the hind limbs across the experimental conditions revealed selective responses from the hind limb contralateral to sonication. The use of 0.5 and 1 ms PDs generated higher EMG signal amplitudes compared to those obtained using a 2 ms PD. Faster efferent response was also observed from thalamic stimulation than that from stimulating the M1. Post-sonication behavioral observation and histological assessment performed 24 h and 1 month after sonication were not indicative of any abnormalities. The results suggest the presence of pulsing scheme-dependent effects of tFUS on brain stimulation and attest its safety in awake large animals.

A review of the bioeffects of low-intensity focused ultrasound and the benefits of a cellular approach

This review article highlights the historical developments and current state of knowledge of an important neuromodulation technology: low-intensity focused ultrasound. Because compelling studies have shown that focused ultrasound can modulate neuronal activity non-invasively, especially in deep brain structures with high spatial specificity, there has been a renewed interest in attempting to understand the specific bioeffects of focused ultrasound at the cellular level. Such information is needed to facilitate the safe and effective use of focused ultrasound to treat a number of brain and nervous system disorders in humans. Unfortunately, to date, there appears to be no singular biological mechanism to account for the actions of focused ultrasound, and it is becoming increasingly clear that different types of nerve cells will respond to focused ultrasound differentially based on the complement of their ion channels, other membrane biophysical properties, and arrangement of synaptic connections. Furthermore, neurons are apparently not equally susceptible to the mechanical, thermal and cavitation-related consequences of focused ultrasound application-to complicate matters further, many studies often use distinctly different focused ultrasound stimulus parameters to achieve a reliable response in neural activity. In this review, we consider the benefits of studying more experimentally tractable invertebrate preparations, with an emphasis on the medicinal leech, where neurons can be studied as unique individual cells and be synaptically isolated from the indirect effects of focused ultrasound stimulation on mechanosensitive afferents. In the leech, we have concluded that heat is the primary effector of focused ultrasound neuromodulation, especially on motoneurons in which we observed a focused ultrasound-mediated blockade of action potentials. We discuss that the mechanical bioeffects of focused ultrasound, which are frequently described in the literature, are less reliably achieved as compared to thermal ones, and that observations ascribed to mechanical responses may be confounded by activation of synaptically-coupled sensory structures or artifacts associated with electrode resonance. Ultimately, both the mechanical and thermal components of focused ultrasound have significant potential to contribute to the sculpting of specific neural outcomes. Because focused ultrasound can generate significant modulation at a temperature <5°C, which is believed to be safe for moderate durations, we support the idea that focused ultrasound should be considered as a thermal neuromodulation technology for clinical use, especially targeting neural pathways in the peripheral nervous system.

Improving Transcranial Acoustic Targeting: The Limits of CT-Based Velocity Estimates and the Role of MR

Transcranial magnetic resonance-guided focused ultrasound (tcMRgFUS) enables the noninvasive treatment of the deep brain. This capacity relies on the ability to focus acoustic energy through the in-tact skull, a feat that requires accurate estimates of the acoustic velocity in individual patient skulls. In current practice, these estimates are generated using a pretreatment computed tomography (CT) scan and then registered to a magnetic resonance (MR) dataset on the day of the treatment. Treatment safety and efficacy can be improved by eliminating the need to register the CT data to the MR images and by improving the accuracy of acoustic velocity measurements. In this study, we examine the capacity of MR to supplement or replace CT as a means of estimating velocity in the skull. We find that MR can predict velocity with less but comparable accuracy to CT. We then use micro-CT imaging to better understand the limitations of Hounsfield unit (HU)-based estimates of velocity, demonstrating that the macrostructure of pores in the skull contributes to the acoustic velocity of the bone. We find evidence that detailed T2 measurements provide information about pore macrostructure similar to the information obtained with micro-CT, offering a potential clinical mechanism for improving patient-specific estimates of acoustic velocity in the human skull.

The thermodynamic soliton theory of the nervous impulse and possible medical implications

The textbook picture of nerve activity is that of a propagating voltage pulse driven by electrical currents through ion channel proteins, which are gated by changes in voltage, temperature, pressure or by drugs. All function is directly attributed to single molecules. We show that this leaves out many important thermodynamic couplings between different variables. A more recent alternative picture for the nerve pulse is of thermodynamic nature. It considers the nerve pulse as a soliton, i.e., a macroscopic excited region with properties that are influenced by thermodynamic variables including voltage, temperature, pressure and chemical potentials of membrane components. All thermodynamic variables are strictly coupled. We discuss the consequences for medical treatment in a view where one can compensate a maladjustment of one variable by adjusting another variable. For instance, one can explain why anesthesia can be counteracted by hydrostatic pressure and decrease in pH, suggest reasons why lithium over-dose may lead to tremor, and how tremor is related to alcohol intoxication. Lithium action as well as the effect of ethanol and the anesthetic ketamine in bipolar patients may fall in similar thermodynamic patterns. Such couplings remain obscure in a purely molecular picture. Other fields of application are the response of nerve activity to muscle stretching and the possibility of neural stimulation by ultrasound.

Guiding and monitoring focused ultrasound mediated blood-brain barrier opening in rats using power Doppler imaging and passive acoustic mapping

The blood-brain barrier (BBB) prevents harmful toxins from entering brain but can also inhibit therapeutic molecules designed to treat neurodegenerative diseases. Focused ultrasound (FUS) combined with microbubbles can enhance permeability of BBB and is often performed under MRI guidance. We present an all-ultrasound system capable of targeting desired regions to open BBB with millimeter-scale accuracy in two dimensions based on Doppler images. We registered imaging coordinates to FUS coordinates with target registration error of 0.6 ± 0.3 mm and used the system to target microbubbles flowing in cellulose tube in two in vitro scenarios (agarose-embedded and through a rat skull), while receiving echoes on imaging transducer. We created passive acoustic maps from received echoes and found error between intended location in imaging plane and location of pixel with maximum intensity after passive acoustic maps reconstruction to be within 2 mm in 5/6 cases. We validated ultrasound-guided procedure in three in vivo rat brains by delivering MRI contrast agent to cortical regions of rat brains after BBB opening. Landmark-based registration of vascular maps created with MRI and Doppler ultrasound revealed BBB opening inside the intended focus with targeting accuracy within 1.5 mm. Combined use of power Doppler imaging with passive acoustic mapping demonstrates an ultrasound-based solution to guide focused ultrasound with high precision in rodents.

Numerical investigation of the energy distribution of Low-intensity transcranial focused ultrasound neuromodulation for hippocampus

OBJECTIVE

Ultrasonic neuromodulation as a safe and non-invasive brain stimulation method that delivers a low-intensity, focused ultrasound to nervous system tissue in a targeted area of the brain. The objective of this study is to numerically investigate the ultrasound wave propagation and the energy distribution within the brain tissues using customized single element focused ultrasound transducers (SEFT), targeting the hippocampus.

METHODS

A high resolution detailed human head model with seven tissue types was constructed from magnetic resonance imaging (MRI). A full-wave finite-difference time-domain simulation platform, Sim4life, was then used to simulate a 3D non-linear ultrasound wave equation to the specific region of interest, the hippocampus. Three customized SEFT were used to test the effect of transducer positions, and another customized transducer was used to compare the sensitivity effect on heterogeneous and homogeneous brain models. Finally, the sensitivity and performance of low intensity focusing ultrasound stimulation were evaluated.

RESULTS

An optimized application of SEFT was customized to deliver 100 W/m² intensity of energy deposition at the hippocampus region. About 85.65% of the generated volume beam was delivered to the targeted hippocampus region and the beam overlap parameter was affected by different transducer positions. Deflection angle changes of SEFT at the range of ± 5% did not have a significant effect on energy delivery and position displacement. Only 0.5% of peak pressure change was observed between heterogeneous and homogeneous brain models. The sensitivity analysis also showed that the sound speed is the most influential acoustic parameter.

SIGNIFICANCE

This study demonstrated that ultrasound neuromodulation targeting the depth brain tissue of the hippocampus could be a potential and promising alternative method to some non-acoustic brain stimulation modalities. In the numerical study of ultrasound brain stimulations, ultrasound parameters and the brain model need to be properly determined to simulate the ultrasonic neuromodulations.

Multiscale approach for tFUS neurocomputational modelling

Among the non-invasive methods employed for brain stimulation, trans cranial Focused Ultrasound Stimulation (tFUS) is the technique with the best penetration into the tissues and spatial resolution. The development of computational models of US propagation in brain tissue can be useful for estimating the behaviour of neural cells subjected to mechanical stimulus due to US. This paper aims at studying the neural cell response of a cortical Regular Spiking point neuron model, for different values of stimulus Duty Cycle (DC). The main goal is to use a multiscale approach to couple the results obtained from a macroscale simulation on wave propagation in tissue, with neuron model described by Hodgkin-Huxley equations to study latency and firing rate of the RS model. The obtained results showed that latency and firing rate have slight variations along the propagation direction of the US beam, in the focal region under the skull model, for different stimulus DC.

Mechanistic insights into ultrasonic neurostimulation of disconnected neurons using single short pulses

Ultrasonic neurostimulation is a potentially potent noninvasive therapy, whose mechanism has yet to be elucidated. We designed a system capable of applying ultrasound with minimal reflections to neuronal cultures. Synaptic transmission was pharmacologically controlled, eliminating network effects, enabling examination of single-cell processes. Short single pulses of low-intensity ultrasound were applied, and time-locked responses were examined using calcium imaging. Low-pressure (0.35MPa) ultrasound directly stimulated ∼20% of pharmacologically disconnected neurons, regardless of membrane poration. Stimulation was resistant to the blockade of several purinergic receptor and mechanosensitive ion channel types. Stimulation was blocked, however, by suppression of action potentials. Surprisingly, even extremely short (4μs) pulses were effective, stimulating ∼8% of the neurons. Lower-pressure pulses (0.35MPa) were less effective than higher-pressure ones (0.65MPa). Attrition effects dominated, with no indication of compromised viability. Our results detract from theories implicating cavitation, heating, non-transient membrane pores >1.5nm, pre-synaptic release, or gradual effects. They implicate a post-synaptic mechanism upstream of the action potential, and narrow down the list of possible targets involved.

Patterned Interference Radiation Force for Transcranial Neuromodulation

Compared with the conventional method of transcranial focused ultrasound stimulation using a single transducer or a focused beam, the compression and tensile forces are generated from the high-pressure gradient of a standing wave that can generate increased stimulation. We experimentally verified a neuromodulation system using patterned interference radiation force (PIRF) and propose a method for obtaining the magnitude of the radiation force, which is considered the main factor influencing ultrasound neuromodulation. The radiation forces generated using a single focused transducer and a standing wave created via two focused transducers were compared using simulations. Radiation force was calculated based on the relationship between the acoustic pressure, radiation force and time-averaged second-order pressure obtained using an acoustic streaming simulation. The presence of the radiation force was verified by measuring the time-averaged second-order pressure generated due to the radiation force, by using a glass tube.

Focused Ultrasound Stimulation as a Neuromodulatory Tool for Parkinson's Disease: A Scoping Review

Non-invasive focused ultrasound stimulation (FUS) is a non-ionising neuromodulatory technique that employs acoustic energy to acutely and reversibly modulate brain activity of deep-brain structures. It is currently being investigated as a potential novel treatment for Parkinson’s disease (PD). This scoping review was carried out to map available evidence pertaining to the provision of FUS as a PD neuromodulatory tool. In accordance with the Preferred Reporting Items for Systematic Reviews and Meta-Analysis Extension for Scoping Reviews, a search was applied to Ovid MEDLINE, Embase, Web of Science and Cochrane Central Register of Controlled Trials on 13 January 2022, with no limits applied. In total, 11 studies were included: 8 were from China and 1 each from Belgium, South Korea and Taiwan. All 11 studies were preclinical (6 in vivo, 2 in vitro, 2 mix of in vivo and in vitro and 1 in silico). The preclinical evidence indicates that FUS is safe and has beneficial neuromodulatory effects on motor behaviour in PD. FUS appears to have a therapeutic role in influencing the disease processes of PD, and therefore holds great promise as an attractive and powerful neuromodulatory tool for PD. Though these initial studies are encouraging, further study to understand the underlying cellular and molecular mechanisms is required before FUS can be routinely used in PD.

Transcranial Neuromodulation Array With Imaging Aperture for Simultaneous Multifocus Stimulation in Nonhuman Primates

Even simple behaviors arise from the simultaneous activation of multiple regions in the brain. Thus, the ability to simultaneously stimulate multiple regions within a brain circuit should allow for better modulation of function. However, performing simultaneous multifocus ultrasound neuromodulation introduces challenges to transducer design. Using 3-D Fullwave simulations, we have designed an ultrasound neuromodulation array for nonhuman primates that: 1) can simultaneously focus on multiple targets and 2) include an imaging aperture for additional functional imaging. This design is based on a spherical array, with 128 15-mm elements distributed in a spherical helix pattern. It is shown that clustering the elements tightly around the 65-mm imaging aperture located at the top of the array improves targeting at shallow depths, near the skull surface. Spherical arrays have good focusing capabilities through the skull at the center of the array, but focusing on off-center locations is more challenging due to the natural geometric configuration and the angle of incidence with the skull. In order to mitigate this, the 64 elements closest to the aperture were rotated toward and focusing on a shallow target, and the 64 elements farthest from the aperture were rotated toward and focusing on a deeper target. Data illustrated that this array produced focusing on the somatosensory cortex with a gain of 4.38 and to the thalamus with a gain of 3.82. To improve upon this, the array placement was optimized based on phase aberration simulations, allowing for the elements with the largest impact on the gain at each focal point to be found. This optimization resulted in an array design that can focus on the somatosensory cortex with a gain of 5.19 and the thalamus with a gain of 4.45. Simulations were also performed to evaluate the ability of the array to focus on 28 additional brain regions, showing that off-center target regions can be stimulated, but those closer to the skull will require corrective steps to deliver the same amount of energy to those locations. This simulation and design process can be adapted to an individual monkey or human skull morphologies and specific target locations within individuals by using orientable 3-D printing of the transducer case and by electronic phase aberration correction.

High resolution ultrasonic neural modulation observed via in vivo two-photon calcium imaging

Neural modulation plays a major role in delineating the circuit mechanisms and serves as the cornerstone of neural interface technologies. Among the various modulation mechanisms, ultrasound enables noninvasive label-free deep access to mammalian brain tissue. To date, most if not all ultrasonic neural modulation implementations are based on ∼1 MHz carrier frequency. The long acoustic wavelength results in a spatially coarse modulation zone, often spanning over multiple function regions. The modulation of one function region is inevitably linked with the modulation of its neighboring regions. Moreover, the lack of in vivo cellular resolution cell-type-specific recording capabilities in most studies prevents the revealing of the genuine cellular response to ultrasound. To significantly increase the spatial resolution, we explored the application of high-frequency ultrasound. To investigate the neuronal response at cellular resolutions, we developed a dual-modality system combining in vivo two-photon calcium imaging and focused ultrasound modulation. The studies show that the ∼30 MHz ultrasound can suppress the neuronal activity in awake mice at 100-μm scale spatial resolutions, paving the way for high-resolution ultrasonic neural modulation. The dual-modality in vivo system validated through this study will serve as a general platform for studying the dynamics of various cell types in response to ultrasound.

Transcranial focused ultrasound modulates cortical and thalamic motor activity in awake sheep

Transcranial application of pulsed low-intensity focused ultrasound (FUS) modulates the excitability of region-specific brain areas, and anesthetic confounders on brain activity warrant the evaluation of the technique in awake animals. We examined the neuromodulatory effects of FUS in unanesthetized sheep by developing a custom-fit headgear capable of reproducibly placing an acoustic focus on the unilateral motor cortex (M1) and corresponding thalamic area. The efferent responses to sonication, based on the acoustic parameters previously identified in anesthetized sheep, were measured using electromyography (EMG) from both hind limbs across three experimental conditions: on-target sonication, off-target sonication, and without sonication. Excitatory sonication yielded greater amplitude of EMG signals obtained from the hind limb contralateral to sonication than that from the ipsilateral limb. Spurious appearance of motion-related EMG signals limited the amount of analyzed data (~ 10% selection of acquired data) during excitatory sonication, and the averaged EMG response rates elicited by the M1 and thalamic stimulations were 7.5 ± 1.4% and 6.7 ± 1.5%, respectively. Suppressive sonication, while sheep walked on the treadmill, temporarily reduced the EMG amplitude from the limb contralateral to sonication. No significant change was found in the EMG amplitudes during the off-target sonication. Behavioral observation throughout the study and histological analysis showed no sign of brain tissue damage caused by the acoustic stimulation. Marginal response rates observed during excitatory sonication call for technical refinement to reduce motion artifacts during EMG acquisitions as well as acoustic aberration correction schemes to improve spatial accuracy of sonication. Yet, our results indicate that low-intensity FUS modulated the excitability of regional brain tissues reversibly and safely in awake sheep, supporting its potential in theragnostic applications.

A novel matrix-array-based MR-conditional ultrasound system for local hyperthermia of small animals

OBJECTIVE

The goal of this work was to develop a novel modular focused ultrasound hyperthermia (FUS-HT) system for preclinical applications with the following characteristics: MR-compatible, compact probe for integration into a PET/MR small animal scanner, 3D-beam steering capabilities, high resolution focusing for generation of spatially confined FUS-HT effects.

METHODS

For 3D-beam steering capabilities, a matrix array approach with 11 11 elements was chosen. For reaching the required level of integration, the array was mounted with a conductive backing directly on the interconnection PCB. The array is driven by a modified version of our 128 channel ultrasound research platform DiPhAS. The system was characterized using sound field measurements and validated using tissue-mimicking phantoms. Preliminary MR-compatibility tests were performed using a 7T Bruker MRI scanner.

RESULTS

Four 11 11 arrays between 0.5 and 2 MHz were developed and characterized with respect to sound field properties and HT generation. Focus sizes between 1 and 4 mm were reached depending on depth and frequency. We showed heating by 4C within 60 s in phantoms. The integration concept allows a probe thickness of less than 12 mm.

CONCLUSION

We demonstrated FUS-HT capabilities of our modular system based on matrix arrays and a 128 channel electronics system within a 3D-steering range of up to 30. The suitability for integration into a small animal MR could be demonstrated in basic MR-compatibility tests.

SIGNIFICANCE

The developed system presents a new generation of FUS-HT for preclinical and translational work providing safe, reversible, localized, and controlled HT.

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.

Safety evaluation of a clinical focused ultrasound system for neuronavigation guided blood-brain barrier opening in non-human primates

An emerging approach with potential in improving the treatment of neurodegenerative diseases and brain tumors is the use of focused ultrasound (FUS) to bypass the blood-brain barrier (BBB) in a non-invasive and localized manner. A large body of pre-clinical work has paved the way for the gradual clinical implementation of FUS-induced BBB opening. Even though the safety profile of FUS treatments in rodents has been extensively studied, the histological and behavioral effects of clinically relevant BBB opening in large animals are relatively understudied. Here, we examine the histological and behavioral safety profile following localized BBB opening in non-human primates (NHPs), using a neuronavigation-guided clinical system prototype. We show that FUS treatment triggers a short-lived immune response within the targeted region without exacerbating the touch accuracy or reaction time in visual-motor cognitive tasks. Our experiments were designed using a multiple-case-study approach, in order to maximize the acquired data and support translation of the FUS system into human studies. Four NHPs underwent a single session of FUS-mediated BBB opening in the prefrontal cortex. Two NHPs were treated bilaterally at different pressures, sacrificed on day 2 and 18 post-FUS, respectively, and their brains were histologically processed. In separate experiments, two NHPs that were earlier trained in a behavioral task were exposed to FUS unilaterally, and their performance was tracked for at least 3 weeks after BBB opening. An increased microglia density around blood vessels was detected on day 2, but was resolved by day 18. We also detected signs of enhanced immature neuron presence within areas that underwent BBB opening, compared to regions with an intact BBB, confirming previous rodent studies. Logistic regression analysis showed that the NHP cognitive performance did not deteriorate following BBB opening. These preliminary results demonstrate that neuronavigation-guided FUS with a single-element transducer is a non-invasive method capable of reversibly opening the BBB, without substantial histological or behavioral impact in an animal model closely resembling humans. Future work should confirm the observations of this multiple-case-study work across animals, species and tasks.

Mechanical and mechanothermal effects of focused ultrasound elicited distinct electromyographic responses in mice

The objective of this study was to compare focused ultrasound (FUS) neuromodulation-induced motor responses under two physical mechanisms: mechanical and mechanothermal effects. Mice were divided into two groups. One group was subjected to short-duration FUS stimulation (0.3 s) that induced mechanical effects (mechanical group). The other group underwent long-duration FUS stimulation (15 s) that produced not only mechanical but also thermal effects (mechanothermal group). FUS was targeted at the deep cerebellar nucleus in the cerebellum to induce motor responses, which were evaluated by recording the evoked electromyographic (EMG) signals and tail movements. Brain tissue temperature rise associated with the FUS stimulation was quantified by noninvasive magnetic resonance thermometryin vivo. Temperature rise was negligible for the mechanical group (0.2 °C ± 0.1 °C) but did rise within the range of 0.6 °C ± 0.2 °C-3.3 °C ± 0.9 °C for the mechanothermal group. The elongated FUS beam also induced heating in the dorsal brain (below the top skull) and ventral brain (above the bottom skull) along the beam path for the mechanothermal group. Both mechanical and mechanothermal groups achieved successful FUS neuromodulation. EMG response latencies were within the range of 0.03-0.1 s at different intensity levels for the mechanical group. The mechanothermal effect of FUS could induce both short-latency EMG (0.2-1.4 s) and long-latency EMG (8.7-13.0 s) under the same intensity levels as the mechanical group. The different temporal dynamics of evoked EMG suggested that FUS-induced mechanical and mechanothermal effects could evoke different responses in the brain.

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.

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.

Displacement Imaging During Focused Ultrasound Median Nerve Modulation: A Preliminary Study in Human Pain Sensation Mitigation

Focused ultrasound (FUS)-based viscoelastic imaging techniques using high frame rate (HFR) ultrasound to track tissue displacement can be used for mechanistic monitoring of FUS neuromodulation. However, a majority of techniques avoid imaging during the active push transmit (interleaved or postpush acquisitions) to mitigate ultrasound interference, which leads to missing temporal information of ultrasound effects when FUS is being applied. Furthermore, critical for clinical translation, use of both axial steering and real-time (<1 s) capabilities for optimizing acoustic parameters for tissue engagement are largely missing. In this study, we describe a method of noninterleaved, single Vantage imaging displacement within an active FUS push with simultaneous axial steering and real-time capabilities using a single ultrasound acquisition machine. Results show that the pulse sequence can track micron-sized displacements using frame rates determined by the calculated time-of-flight (TOF), without interleaving the FUS pulses and imaging acquisition. Decimation by 3-7 frames increases signal-to-noise ratio (SNR) by 15.09±7.03 dB. Benchmarking tests of CUDA-optimized code show increase in processing speed of 35- and 300-fold in comparison with MATLAB parallel processing GPU and CPU functions, respectively, and we can estimate displacement from steered push beams ±10 mm from the geometric focus. Preliminary validation of displacement imaging in humans shows that the same driving pressures led to variable nerve engagement, demonstrating important feedback to improve transducer coupling, FUS incident angle, and targeting. Regarding the use of our technique for neuromodulation, we found that FUS altered thermal perception of thermal pain by 0.9643 units of pain ratings in a single trial. Additionally, 5 [Formula: see text] of nerve displacement was shown in on-target versus off-target sonications. The initial feasibility in healthy volunteers warrants further study for potential clinical translation of FUS for pain suppression.

Ultrasound activates mechanosensitive TRAAK K+ channels through the lipid membrane

Ultrasound modulates the electrical activity of excitable cells and offers advantages over other neuromodulatory techniques; for example, it can be noninvasively transmitted through the skull and focused to deep brain regions. However, the fundamental cellular, molecular, and mechanistic bases of ultrasonic neuromodulation are largely unknown. Here, we demonstrate ultrasound activation of the mechanosensitive K⁺ channel TRAAK with submillisecond kinetics to an extent comparable to canonical mechanical activation. Single-channel recordings reveal a common basis for ultrasonic and mechanical activation with stimulus-graded destabilization of long-duration closures and promotion of full conductance openings. Ultrasonic energy is transduced to TRAAK through the membrane in the absence of other cellular components, likely increasing membrane tension to promote channel opening. We further demonstrate ultrasonic modulation of neuronally expressed TRAAK. These results suggest mechanosensitive channels underlie physiological responses to ultrasound and could serve as sonogenetic actuators for acoustic neuromodulation of genetically targeted cells.

Tissue Characterization by Low-Frequency Acoustic Waves Generated by a Single High-Frequency Focused Ultrasound Beam

The mechanical properties of biological tissues are fingerprints of certain pathologic processes. Ultrasound systems have been used as a non-invasive technique to both induce kilohertz-frequency mechanical vibrations and detect waves resulting from interactions with biological structures. However, existing methodologies to produce kilohertz-frequency mechanical vibrations using ultrasound require the use of variable-frequency, dual-frequency and high-power systems. Here, we propose and demonstrate the use of bursts of megahertz- frequency acoustic radiation to observe kilohertz-frequency mechanical responses in biological tissues. Femoral bones were obtained from 10 healthy mice and 10 mice in which osteoporosis had been induced. The bones' porosity, trabecular number, trabecular spacing, connectivity and connectivity density were determined using micro-computed tomography (μCT). The samples were irradiated with short, focused acoustic radiation pulses (f = 3.1 MHz, t = 15 μs), and the low-frequency acoustic response (1-100 kHz) was acquired using a dedicated hydrophone. A strong correlation between the spectral maps of the acquired signals and the μCT data was found. In a subsequent evaluation, soft tissue stiffness measurements were performed with a gel wax-based tissue-mimicking phantom containing three spherical inclusions of the same type of gel but different densities and Young's moduli, yet with approximately the same echogenicity. Conventional B-mode ultrasound was unable to image the inclusions, while the novel technique proposed here showed good image contrast.

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.

Synchronous temperature variation monitoring during ultrasound imaging and/or treatment pulse application: a phantom study

Ultrasound attenuation through soft tissues can produce an acoustic radiation force (ARF) and heating. The ARF-induced displacements and temperature evaluations can reveal tissue properties and provide insights into focused ultrasound (FUS) bio-effects. In this study, we describe an interleaving pulse sequence tested in a tissue-mimicking phantom that alternates FUS and plane-wave imaging pulses at a 1 kHz frame rate. The FUS is amplitude modulated, enabling the simultaneous evaluation of tissue-mimicking phantom displacement using harmonic motion imaging (HMI) and temperature rise using thermal strain imaging (TSI). The parameters were varied with a spatial peak temporal average acoustic intensity (I _spta ) ranging from 1.5 to 311 W.cm^-2, mechanical index (MI) from 0.43 to 4.0, and total energy (E) from 0.24 to 83 J.cm^-2. The HMI and TSI processing could estimate displacement and temperature independently for temperatures below 1.80°C and displacements up to ~117 μm (I _spta <311 W.cm^-2, MI<4.0, and E<83 J.cm^-2) indicated by a steady-state tissue-mimicking phantom displacement throughout the sonication and a comparable temperature estimation with simulations in the absence of tissue-mimicking phantom motion. The TSI estimations presented a mean error of ±0.03°C versus thermocouple estimations with a mean error of ±0.24°C. The results presented herein indicate that HMI can operate at diagnostic-temperature levels (i.e., <1°C) even when exceeding diagnostic acoustic intensity levels (720 mW.cm^-2 < I _spta < 207 W.cm^-2). In addition, the combined HMI and TSI can potentially be used for simultaneous evaluation of safety during tissue elasticity imaging as well as FUS mechanism involved in novel ultrasound applications such as ultrasound neuromodulation and tumor ablation.

Transcranial Low-Intensity Pulsed Ultrasound Stimulation Induces Neuronal Autophagy

Autophagy, or cellular self-digestion, is an essential process for eliminating abnormal protein in mammalian cells. Accumulating evidence indicates that increased neuronal autophagy has a protective effect on neurodegenerative disorders. It has been reported that low-intensity pulsed ultrasound (LIPUS) can noninvasively modulate neural activity in the brain. Yet, the effect of LIPUS on neuronal autophagy is still unclear. The objective of this study was to examine whether LIPUS stimulation could induce neuronal autophagy. Primary neurons were treated by LIPUS with a frequency of 0.68 MHz, a pulse repetition frequency (PRF) of 500 Hz, a spatial peak temporal-average intensities ( [Formula: see text]) of 70 and 165 mW/cm². Then, the immunofluorescent analysis of LC3B was carried out for evaluating neuronal autophagy. Furthermore, 0.5-MHz LIPUS was noninvasively delivered to the cortex and hippocampus of adult mice ( n = 16 ) with PRF of 500 Hz and [Formula: see text] of 235 mW/cm². The LC3BII/LC3BI ratio and p62 (autophagic markers) were measured by western blot analysis. In the in vitro study, the expression of LC3B in primary neurons was statistically improved after LIPUS stimulation was implemented for 4 h ( ). With the increase in the irradiation duration or acoustic intensity of LIPUS stimulation, the expression of LC3B in primary neurons was increased. Furthermore, transcranial LIPUS stimulation increased the LC3BII/LC3BI ratio ( ) and decreased the expression of p62 ( ) in the cortex and hippocampus. We concluded that LIPUS provides a safe and capable tool for activating neuronal autophagy in vitro and in vivo.

Systematic examination of low-intensity ultrasound parameters on human motor cortex excitability and behavior

Low-intensity transcranial ultrasound (TUS) can non-invasively modulate human neural activity. We investigated how different fundamental sonication parameters influence the effects of TUS on the motor cortex (M1) of 16 healthy subjects by probing cortico-cortical excitability and behavior. A low-intensity 500 kHz TUS transducer was coupled to a transcranial magnetic stimulation (TMS) coil. TMS was delivered 10 ms before the end of TUS to the left M1 hotspot of the first dorsal interosseous muscle. Varying acoustic parameters (pulse repetition frequency, duty cycle, and sonication duration) on motor-evoked potential amplitude were examined. Paired-pulse measures of cortical inhibition and facilitation, and performance on a visuomotor task was also assessed. TUS safely suppressed TMS-elicited motor cortical activity, with longer sonication durations and shorter duty cycles when delivered in a blocked paradigm. TUS increased GABAA-mediated short-interval intracortical inhibition and decreased reaction time on visuomotor task but not when controlled with TUS at near-somatosensory threshold intensity.

Ultrasound neuromodulation depends on pulse repetition frequency and can modulate inhibitory effects of TTX

Ultrasound is gaining traction as a neuromodulation method due to its ability to remotely and non-invasively modulate neuronal activity with millimeter precision. However, there is little consensus about optimal ultrasound parameters required to elicit neuromodulation and how specific parameters drive mechanisms that underlie ultrasound neuromodulation. We address these questions in this work by performing a study to determine effective ultrasound parameters in a transgenic mouse brain slice model that enables calcium imaging as a quantitative readout of neuronal activity for ultrasound neuromodulation. We report that (1) calcium signaling increases with the application of ultrasound; (2) the neuronal response rate to ultrasound is dependent on pulse repetition frequency (PRF); and (3) ultrasound can reversibly alter the inhibitory effects of tetrodotoxin (TTX) in pharmacological studies. This study offers mechanistic insight into the PRF dependence of ultrasound neuromodulation and the nature of ultrasound/ion channel interaction.