Transcranial focused ultrasound stimulation of cortical and thalamic somatosensory areas in human

Bioadhesive hydrogel-coupled and miniaturized ultrasound transducer system for long-term, wearable neuromodulation

Transcranial focused ultrasound is a promising non-invasive method for neuromodulation, particularly for neurodegenerative and psychiatric conditions. However, its use in wearable systems has been limited due to bulky devices and reliance on ultrasound gel, which dehydrates and lacks stable adhesion for long-term use. Here, we present a miniaturized wearable ultrasound device, comparable in size to standard electrophysiological electrodes, integrated with a bioadhesive hydrogel for stable, long-term somatosensory cortical stimulation. Our air-cavity Fresnel lens based self-focusing acoustic transducer was fabricated via a lithography-free microfabrication process, achieving 30.7 W/cm² (1.92 MPa) acoustic intensity and 10 mm focal depth. The hydrogel couplant exhibited less than 13% acoustic attenuation and maintained a stable adhesion force of 0.961 N/cm for 35 days. Using this system, we successfully suppressed somatosensory evoked potentials elicited by functional electrical stimulation over 28 days, demonstrating the device's potential for long-term, wearable neuromodulation applications.

Probing phased-array focused ultrasound transducers using realistic 3D in-silico trabecular skull models: a numerical study

Transcranial focused ultrasound (tFUS) is an emerging neuromodulation technology with transformative potential for brain disease therapies. This study explores how the trabecular structure of the human skull affects the performance of multi-element tFUS transducers. Numerical simulations were conducted using realistic 3D skull models with varying porosities (0 %, 50 %, and 60 %), comparing the pressure fields generated by two geometrically distinct 96-elements phased-array transducers (f-number = 0.8 -transducer 1- and f-number = 1.1 -transducer 2-). Pressure distribution maps and -6dB isosurfaces were analyzed to quantify focal and scattered volumes, as well as focus shifts. Results demonstrate that porous skull models significantly impact the pressure field, introducing scattering and hotspots outside the target area, that are undetectable with non-porous models. Both transducers exhibit focus shifts along the propagation axis, with transducer 2 showing lower selectivity and nearly 450 % and 1000 % increased scattering compared to transducer 1 in the porous models. These findings emphasize the necessity of incorporating such models in tFUS simulations to improve the accuracy of pressure predictions and device performance. Our results highlight the critical importance of accurately modelling skull porosity in tFUS simulations. Using simplified non-porous models can obscure scattering effects and lead to distorted predictions of transducer performance. This work also demonstrates how generating in-silico porous models with varying porosity allows for testing the reliability and robustness of a numerically designed transducer. It also provides valuable insights into optimizing transducer design ultimately improving target precision while mitigating unintended sonication, laying the groundwork for safer and more effective tFUS therapies.

Neuromodulation with Ultrasound: Hypotheses on the Directionality of Effects and Community Resource

Low-intensity Transcranial Ultrasound Stimulation is a promising non-invasive technique for brain stimulation and focal neuromodulation. Research with humans and animal models has raised the possibility that TUS can be biased towards enhancing or suppressing neural function. Here, we first collate a set of hypotheses on the directionality of TUS effects and conduct an initial meta-analysis on the available healthy human participant TUS studies reporting stimulation parameters and outcomes (n = 47 studies, 52 experiments). In these initial exploratory analyses, we find that parameters such as the intensity and continuity of stimulation (duty cycle) with univariate tests show only statistical trends towards likely enhancement or suppressed of function with TUS. Multivariate machine learning analyses are currently limited by the small sample size. Given that human TUS sample sizes will continue to increase, predictability on the directionality of TUS effects could improve if this database can continue to grow as TUS studies more systematically explore the TUS stimulation parameter space and report outcomes. Therefore, we establish an inTUS database and resource for the systematic reporting of TUS parameters and outcomes to assist in greater precision in TUS use for brain stimulation and neuromodulation. The paper concludes with a selective review of human clinical TUS studies illustrating how hypotheses on the directionality of TUS effects could be developed for empirical testing in the intended clinical application, not limited to the examples provided.

Label free, capillary-scale blood flow mapping in vivo reveals that low-intensity focused ultrasound evokes persistent dilation in cortical microvasculature

Non-invasive, low intensity focused ultrasound is an emerging neuromodulation technique that offers the potential for precision, personalized therapy. An increasing body of research has identified mechanosensitive ion channels that can be modulated by FUS and support acute electrical activity in neurons. However, neuromodulatory effects that persist from hours to days have also been reported. The brain's ability to provide blood flow to electrically active regions involves a multitude of non-neuronal cell types and signaling pathways in the cerebral vasculature; an open question is whether persistent effects can be attributed, at least partly, to vascular mechanisms. Using an in vivo optical approach, we found that microvasculature, and not larger vessels, exhibit significant persistent dilation following sonication without the use of microbubbles. This finding reveals a heretofore unseen aspect of the effects of FUS in vivo and indicates that concurrent changes in neurovascular function may partially underly persistent neuromodulatory effects.

Impact of Developmental Alcohol Exposure on the Thalamus

This chapter comprehensively explores the impact of prenatal alcohol (ethanol) exposure (PAE) on the thalamus, integrating findings from animal models and human studies spanning various developmental stages. Animal model investigations, encompassing first and second trimester-equivalent exposures and the critical third trimester, where the brain growth spurt starts, reveal specific alterations in thalamic structures and circuits, emphasizing the specificity of damage to corticothalamic loops. The ventrobasal thalamic nucleus exhibits a unique response to PAE, involving intricate interactions with postnatal neurogenesis and neurotrophin responsiveness. Third trimester-equivalent exposure consistently induces apoptotic neurodegeneration in various thalamic nuclei, highlighting the heightened susceptibility of the visual thalamus, particularly the lateral geniculate nucleus, during critical developmental periods. The nucleus reuniens, vital for cognitive processes, was shown to be significantly affected by alcohol exposure during this period. Investigations into the trigeminal/somatosensory system activity revealed disruptions in glucose utilization and increased neuronal activity in the thalamus. Research on binge-like alcohol exposure during the brain growth spurt demonstrates lasting modifications in action-potential properties and synaptic currents in thalamic neurons projecting to the retrosplenial cortex. Human studies, employing advanced techniques like super-resolution fetal MRI and functional MRI, underscore the PAE-induced structural and functional consequences in the thalamus and its connections, spanning from fetal development to adulthood. The complex effects of PAE on thalamic structure and function vary across developmental stages, emphasizing the importance of considering factors such as age and concurrent exposures. The development of higher-resolution imaging tools is essential for assessing the impact of PAE on the structure and function of individual thalamic nuclei in humans.

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.

Transcranial Focused Ultrasound Neuromodulation in Psychiatry: Main Characteristics, Current Evidence, and Future Directions

Non-invasive brain stimulation (NIBS) techniques are designed to precisely and selectively target specific brain regions, thus enabling focused modulation of neural activity. Among NIBS technologies, low-intensity transcranial focused ultrasound (tFUS) has emerged as a promising new modality. The application of tFUS can safely and non-invasively stimulate deep brain structures with millimetric precision, offering distinct advantages in terms of accessibility to non-cortical regions over other NIBS methods. However, to date, several tFUS aspects still need to be characterized; furthermore, there are only a handful of studies that have utilized tFUS in psychiatric populations. This narrative review provides an up-to-date overview of key aspects of this NIBS technique, including the main components of a tFUS system, the neuronavigational tools used to precisely target deep brain regions, the simulations utilized to optimize the stimulation parameters and delivery of tFUS, and the experimental protocols employed to evaluate the efficacy of tFUS in psychiatric disorders. The main findings from studies in psychiatric populations are presented and discussed, and future directions are highlighted.

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.

Label free, capillary-scale blood flow mapping in vivo reveals that low intensity focused ultrasound evokes persistent dilation in cortical microvasculature

Non-invasive, low intensity focused ultrasound (FUS) is an emerging neuromodulation technique that offers the potential for precision, personalized therapy. An increasing body of research has identified mechanosensitive ion channels that can be modulated by FUS and support acute electrical activity in neurons. However, neuromodulatory effects that persist from hours to days have also been reported. The brain's ability to provide targeted blood flow to electrically active regions involve a multitude of non-neuronal cell types and signaling pathways in the cerebral vasculature; an open question is whether persistent effects can be attributed, at least partly, to vascular mechanisms. Using a novel in vivo optical approach, we found that microvascular responses, unlike larger vessels which prior investigations have explored, exhibit persistent dilation following sonication without the use of microbubbles. This finding and approach offers a heretofore unseen aspect of the effects of FUS in vivo and indicate that concurrent changes in neurovascular function may partially underly persistent neuromodulatory effects.

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.

A review of functional neuromodulation in humans using low-intensity transcranial focused ultrasound

Transcranial ultrasonic neuromodulation is a rapidly burgeoning field where low-intensity transcranial focused ultrasound (tFUS), with exquisite spatial resolution and deep tissue penetration, is used to non-invasively activate or suppress neural activity in specific brain regions. Over the past decade, there has been a rapid increase of tFUS neuromodulation studies in healthy humans and subjects with central nervous system (CNS) disease conditions, including a recent surge of clinical investigations in patients. This narrative review summarized the findings of human neuromodulation studies using either tFUS or unfocused transcranial ultrasound (TUS) reported from 2013 to 2023. The studies were categorized into two separate sections: healthy human research and clinical studies. A total of 42 healthy human investigations were reviewed as grouped by targeted brain regions, including various cortical, subcortical, and deep brain areas including the thalamus. For clinical research, a total of 22 articles were reviewed for each studied CNS disease condition, including chronic pain, disorder of consciousness, Alzheimer's disease, Parkinson's disease, depression, schizophrenia, anxiety disorders, substance use disorder, drug-resistant epilepsy, and stroke. Detailed information on subjects/cohorts, target brain regions, sonication parameters, outcome readouts, and stimulatory efficacies were tabulated for each study. In later sections, considerations for planning tFUS neuromodulation in humans were also concisely discussed. With an excellent safety profile to date, the rapid growth of human tFUS research underscores the increasing interest and recognition of its significant potential in the field of non-invasive brain stimulation (NIBS), offering theranostic potential for neurological and psychiatric disease conditions and neuroscientific tools for functional brain mapping.