The invention relates to methods and a system of using ultrasound for neuromodulation. More specifically, the invention relates to methods and a system of using transcranial focused ultrasound (“tFUS”) to stimulate different neuron types and modulate synaptic connectivity in living subjects and to induce sustained neural effects after the cessation of tFUS.
Neuromodulation is a technique to intervene with the nervous system in an attempt to improve the quality of life of subjects suffering from neurological disorders. For decades, a myriad of brain neuromodulatory approaches, such as deep brain stimulation, transcranial magnetic stimulation, transcranial current stimulation, transcranial focused ultrasound, transcranial static magnetic field stimulation, optogenetics, and designer receptors exclusively activated by designer drugs, have been developed in order to modulate and study the brain. Among these methods, optogenetics receives considerable attention for its capacity to selectively stimulate distinct cell-types with high spatial and temporal resolution. However, optogenetics heavily relies on methods such as transgenic approaches, viral vector transfection, or nanoparticle injection for deep brain application, which pose practical challenges for translation in human clinical utility. In contrast, non-invasive methods such as transcranial magnetic stimulation and transcranial current stimulation are readily translated to clinical utility, but are challenged to achieve highly spatial focus and deep penetration.
Unlike other noninvasive neuromodulation technologies such as transcranial magnetic stimulation and transcranial current stimulation, low-intensity tFUS can be applied in many neuromodulation applications due to its high spatial focality and its non-invasive nature. During tFUS neuromodulation, pulsed mechanical energy is transmitted through the skull with high spatial selectivity, which can be steered and utilized to elicit activation or inhibition through parameter tuning. Prior studies have investigated the neural effects of ultrasound parameters, such as ultrasound fundamental frequencies (UFF), intensities (UI), durations (UD), duty cycles (UDC), pulse repetition frequencies (UPRF), and other parameters. tFUS has been observed to induce behavioral changes, e.g. motor responses, electrophysiological responses, e.g. electromyography (EMG), electroencephalography (EEG), local field potentials (LFPs), and multi-unit activities (MUAs), with high in-vivo temporal/spatial measurement fidelity, or neurovascular activities, e.g. blood-oxygenation-level-dependent (BOLD) signal. To further achieve selectivity in stimulating brain circuits or even among cellular populations, focused ultrasound has been employed in combination with specific neuromodulatory drug-laden nanoparticles, cell-specific expression of ultrasound sensitizing ion channels, or acoustically distinct reporter genes in microorganisms. So far, prior studies have not explored the intrinsic effects of the wide range of ultrasound parameters on specific neuron subpopulations, such as regular-spiking and fast-spiking units.
The ability to selectively stimulate neural subpopulations non-invasively can provide a powerful scientific or clinical tool. For example, tFUS may be used to modulate atrophied brain regions in patients with Alzheimer disease to prevent disease progression or improve cognitive function. Studies have shown the Papez circuit in the anterior nucleus of the thalamus projects to multiple areas of the brain involving memory such as the dentate gyrus, anterior cingulate cortex, and frontal and temporal regions. Deep brain stimulation of these regions has been explored to help improve memory. Therefore, the innovation of embodiments of the present invention is methods and a system for application of transcranial focused ultrasound to target specific neuron populations and to elicit long-term changes in synaptic connectivity in the deep brain, allowing the delivery of long lasting therapy for clinical utility.
According to one embodiment, specific neuron types are targeted non-invasively for neuromodulation by altering the tFUS pulse repetition frequency, where a dynamic acoustic radiation force is induced by the tFUS at the ultrasound pulse repetition frequency. In an alternative embodiment, the parameters of tFUS are chosen to encode temporally dependent stimulation paradigms into neural circuits and non-invasively elicit long-term changes in synaptic connectivity. In certain embodiments, a collimator is used to focus and direct ultrasound energy to specific areas of the brain.
In one embodiment, a method of stimulating a response in specific neural populations comprises generating pulsed tFUS using a single-element transducer 101, guided to a scalp location over the cortex through a mounted 3-D printed collimator 102 filled with aqueous ultrasound gel at an incidence angle of 40°, as shown in
Cell-Type Selective Effects of tFUS
The fundamental unit for constructing the above ultrasound wave is the tone burst period which includes 100 cycles sinusoidal wave per pulse. The UPRF determines the durations between two consecutive ultrasound pulses. The sonication duration, tone burst duration (i.e. cycle per pulse number), the fundamental frequency, and ultrasound pressure magnitude, etc. can be used as shown in
All recorded action potentials from a 32-channel electrode array can be sorted based on the spike waveforms and inter-spike intervals (ISpI). The extracted features are the durations of initial phase (IP) of the action potential, i.e. from onset to the re-crossing of baseline, and afterhyperpolarization period (AHP), i.e. from the end of the IP to its re-crossing of baseline, shown in
The neural effects of the administered pulsed tFUS can be confirmed through intracranial MUA recordings. Using peri-stimulus time histograms (PSTH), a significant increase of spike rate (6.23±1.10 spikes/sec) in a possible regular-spiking somatosensory cortical neuron (mean spike waveform IP: 0.85 ms, AHP: 1.8 ms) when stimulated with a tFUS condition (UPRF=300 Hz, Ispta=3.0 mW/cm2) is observed, with a further increased spiking rate (14.35±1.65 spikes/sec) in response to the increase sonication (UPRF=3000 Hz, Ispta=30.4 mW/cm2). For a more intuitive comparison, increased spike rate as a function of time along 478 consecutive trials are demonstrated with the raster plot (
In contrast, a fast-spiking cortical neuron (
In a population level, the RSUs significantly increase their firing rates in response to UPRFs at 3000 and 4500 Hz when both comparing to that induced by a low UPRF at 30 Hz (UPRFx1 vs. UPRFx100: p=0.003; UPRFx1 vs. UPRFx150: p=0.0004). Whereas in the FSU group, no significant difference between tFUS conditions could be found. This implies that the spike rates of the FSUs are not significantly altered by the levels of UPRF.
The contrast between the responses observed in these two different neuron types suggests a cell-type selective mechanism by tFUS. The RSU group did not show significant differences among the five levels of sham ultrasound conditions.
Since the length of the refractory period determines the minimum time between neuronal firings, it follows that FSU spikes faster than the RSU did (the pre-stimulus firing rate as illustrated in
Long-Term Effects of tFUS
Beyond investigations on the short-term intrinsic effects of UPRF on neuron subtypes, the following method uses tFUS parameters for encoding frequency specific information into the brain for long-term effects. In this method, a specific tFUS temporal sequence and ultrasound pressure is delivered to the deep brain, e.g. the synaptic connections in hippocampus, to induce more than 30-minute sustained neural effects after the cessation of tFUS with minimal temperature rise at skull-brain interface and at the brain target.
According to the method of this embodiment, the ultrasound spatial-peak pressure and UPRF are increased to 99 kPa and 3-10 kHz, and the inter-sonication interval is largely decreased to 20 msec, i.e. the inter-sonication frequency is increased to 50 Hz.
For a deeper brain target, another collimator 102 is used to allow normal incidence of tFUS at the scalp as shown in
The ultrasound collimator 102, as shown in
To test whether tFUS can induce frequency encoded potentiation in the synapse, the induction of long-term potentiation (LTP) using pulsed tFUS in naïve rats was attempted using the method of the present invention. In the application of this example embodiment, pulsed tFUS stimulation was applied with various UPRFs at 50-100 Hz sonication frequency (
It can be expected to observe LTP after tFUS stimulation since tFUS was applied at the same frequency as the high frequency tetanic stimulation used in certain prior art. However, the observed results did not show LTP, suggesting that the temporal encoding using tFUS does not share the same efficiency and/or mechanism as electrical tetanus stimulation. As such, the demonstrated long-term effect is a promising new feature of tFUS stimulation to be employed as a potential non-invasive therapeutic neuromodulation technique. The results suggest that tFUS can be used to encode time dependent stimulation paradigms into neural networks and non-invasively elicit long-term changes in the strength of synaptic connections.
In order to determine whether tFUS UPRF has an effect on strength of LTD induction, a range of UPRFs from 3 to 10 kHz were examined. As shown in
In the methods described above, single element focused transducers 101 were used for tFUS stimulation. In one embodiment, the transducer diameter is 28.5 mm with an ultrasound fundamental frequency (UFF) of 0.5 MHz, a −6 dB bandwidth at 300-690 kHz, and a nominal focal distance of 38 mm. For example, transducer model V391-SU-F1.5IN-PTF manufactured by Olympus Scientific Solutions Americas, Inc., USA can be used. Collimators 102 were 3D printed with VeroClear™ material to match the focal length of the transducer 101 and the animal model, the outlet or aperture 104 of the angled collimator 102 for the rat model has an elliptical area of 25.6 mm2 (major axis length: 6.8 mm, minor axis length: 5 mm), and the one for the ultrasound normal incidence has a circular area of 19.64 mm2. The size of collimators' outlet 104 was set to be no less than or at least commensurate with one ultrasound wavelength (i.e. 3 mm in soft tissue when using UPRF=500 kHz). One single-channel waveform generator can be used in connection with another double-channel generator to control the timing of each sonication, synchronize the ultrasound transmission with neural recording, and form the initial ultrasound waveform to be amplified, thus driving the transducer. A 50-watt wide-band radio-frequency (RF) power amplifier can be employed to amplify the low-voltage ultrasound waveform signal. The employed ultrasound intensity levels and duty cycles are described in
A single-element focused transducer 101 delivers tFUS stimulation at the dentate gyrus through the rat skull. The transducer 101 interfaces with the skull via a collimator 102 filled with ultrasound gel, with a tip diameter of 5 mm. 10-20 min of baseline fEPSP recorded before tFUS stimulation. Pulsed tFUS stimulation was delivered for 5 minutes at various UPRFs.
Referring again to
In another embodiment, the said-EEG recordings over the scalp are used to localize and image brain electrical sources that are induced by ultrasound stimulation. This can be done by minimizing the difference between a source-model predicted scalp EEG and the recorded EEG over the scalp. The source distributions within such a brain are used to inform targets of focused ultrasound stimulation.
In another embodiment, the cell-type selectiveness and the long-term effects of tFUS can be delivered by multi-element (i.e. more than one element) tFUS stimulation at the cortical brain and deep brain areas, respectively. The multi-element transducer array shown in
Overall, as shown in
While the disclosure has been described in detail and with reference to specific embodiments thereof, it will be apparent to one skilled in the art that various changes and modification can be made therein without departing from the spirit and scope of the embodiments. Thus, it is intended that the present disclosure cover the modifications and variations of this disclosure provided they come within the scope of the appended claims and their equivalents.
This application claims the benefit under 35 U.S.C. § 119 of Provisional Application Ser. No. 62/766,306, filed Oct. 11, 2018, which is incorporated herein by reference.
This invention was made with government support under NIH-7RF1MH114233 awarded by the National Institutes of Health. The government has certain rights in this invention.
Filing Document | Filing Date | Country | Kind |
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PCT/US2019/055955 | 10/11/2019 | WO | 00 |
Number | Date | Country | |
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62766306 | Oct 2018 | US |