Brain simulation techniques have demonstrated undisputable therapeutic effects on neural
diseases. Invasive stimulation techniques like deep brain stimulation (DBS) and noninvasive
techniques like transcranial magnetic stimulation (TMS) have been approved by FDA as
treatments for many drug resist neural disorders and diseases. Developing noninvasive, deep, and
targeted brain stimulation techniques is currently one of the important tasks in brain researches.
Transcranial direct current stimulation (tDCS) and transcranial alternative current stimulation
(tACS) techniques have the advantages of low cost and portability. However, neither of them can
produce targeted stimulation due to lacking of electrical field focusing mechanism. Recently,
Grossman et al. reported using the down beating signals of two tACS signals to accomplish
focused stimulation. By sending two sine waves running at slightly different high frequencies
(~2kHz), they demonstrated that they can modulate a “localized” neuron group at the difference
frequency of the two sine waves and at the same time avoid excitation of neurons at other locations.
As a result, equivalent focusing effect was accomplished by such beating mechanism. In this work,
we show neither theoretically nor experimentally the beating mechanism can produce “focusing
effect” and the beating signal spread globally across the full brain. The localized modulation effect
likely happened right at the electrode contact sites when the electrode contact area is small and the
current is concentrated. We conclude that to accomplish noninvasive and focused stimulation at
current stage the only available tool is the focused TMS system we recently demonstrated.
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Development of Focused Transcranial Magnetic Stimulation for Rodents by Copper-Array Shields
Transcranial magnetic stimulation (TMS) is one of the most widely used noninvasive brain stimulation methods. It has been
utilized for both treatment and diagnosis of many neural diseases, such as neuropathic pain and loss of function caused by stroke.
Existing TMS tools cannot deliver focused electric field to targeted penetration depth even though many important neurological
disorders are originated from there. A breakthrough is needed to achieve noninvasive, focused brain stimulation. We demonstrated
using magnetic shield to achieve magnetic focusing without sacrificing significant amount of throughput. The shield is composed of
multiple layers of copper ring arrays, which utilize induced current to generate counter magnetic fields. We experimentally set up
a two-pole stimulator system to verify device simulation. A transient magnetic field probe was used for field measurements. The
focusing effect highly depends on the geometric design of shield. A tight focal spot with a diameter of smaller than 5 mm (plotted in
MATLAB contour map) can be achieved by using copper ring arrays. With properly designed array structures and ring locations,
the combined original and induced counter fields can produce a tightly focused field distribution with enhanced field strength at a
depth of 7.5 mm beyond the shield plane, which is sufficient to reach many deep and critical parts of a mouse brain.
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- Award ID(s):
- 1631820
- NSF-PAR ID:
- 10063432
- Date Published:
- Journal Name:
- IEEE transactions on magnetics
- ISSN:
- 1941-0069
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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In recent years, there is an increasing interest in noninvasive treatments for neurological disorders like Alzheimer and Depression. Transcranial magnetic stimulation (TMS) is one of the most effective methods used for this purpose. The performance of TMS highly depends on the coils used for the generation of magnetic field and induced electric field particularly their designs affecting depth and focality tradeoff characteristics. Among a variety of proposed and used TMS coil designs, circular coils are commonly used both in research and medical and clinical applications. In current study, we focus on changing the outer and inner sizes (diameter) and winding turns of ring coils and try to reach deeper brain regions without significant field strength decay. The induced electric field and the decay rate of the generated field with depth were studied with finite element method calculations. The results of the performed simulations indicate that larger diameter coils have a larger equivalent field emission aperture and produce larger footprint of induced electric field initially. However, their emission solid angles are smaller and, as a result, the field divergence or the decay rates of the generated field with depth are smaller as well, which give them a good potential to perform better for deep brain stimulation compared with that of smaller coil.more » « less
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Transcranial magnetic stimulation (TMS) is widely used for noninvasive brain stimulation. However, existing TMS tools cannot deliver targeted neural stimulation to deep brain regions, even though many important neurological disorders originate from there. To design TMS tools capable of delivering deep and focused stimulation, we have developed both electric and magnetic field probes to evaluate and improve new designs and calibrate products. Previous works related to magnetic field measurement had no detailed description of probe design or optimization. In this work, we demonstrated a magnetic field probe made of a cylindrical inductor and an electrical field probe modified from Rogowski coil structure. Both have much smaller size and higher directivity than commercial dipole probes. Using probe, we can calibrate and monitor any new types of TMS coil or array design and verify measured results with the other probe. We mathematically analyze their characteristics and performance and obtained a two-dimensional vector plot of the induced electric field, which matched the measured results from the second type of probe. A commercial circular coil and a figure-8 coil, with relatively complex vector field distribution, were used as examples to demonstrate the high-resolution and accurate measurement capability of our probes.more » « less
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Abstract Objective. Transcranial magnetic stimulation (TMS) is a non-invasive brain stimulation method that is used to study brain function and conduct neuropsychiatric therapy. Computational methods that are commonly used for electric field (E-field) dosimetry of TMS are limited in accuracy and precision because of possible geometric errors introduced in the generation of head models by segmenting medical images into tissue types. This paper studies E-field prediction fidelity as a function of segmentation accuracy. Approach. The errors in the segmentation of medical images into tissue types are modeled as geometric uncertainty in the shape of the boundary between tissue types. For each tissue boundary realization, we then use an in-house boundary element method to perform a forward propagation analysis and quantify the impact of tissue boundary uncertainties on the induced cortical E-field. Main results. Our results indicate that predictions of E-field induced in the brain are negligibly sensitive to segmentation errors in scalp, skull and white matter (WM), compartments. In contrast, E-field predictions are highly sensitive to possible cerebrospinal fluid (CSF) segmentation errors. Specifically, the segmentation errors on the CSF and gray matter interface lead to higher E-field uncertainties in the gyral crowns, and the segmentation errors on CSF and WM interface lead to higher uncertainties in the sulci. Furthermore, the uncertainty of the average cortical E-fields over a region exhibits lower uncertainty relative to point-wise estimates. Significance. The accuracy of current cortical E-field simulations is limited by the accuracy of CSF segmentation accuracy. Other quantities of interest like the average of the E-field over a cortical region could provide a dose quantity that is robust to possible segmentation errors.more » « less
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Abstract The emergence of distributed patterns of neural activity supporting brain functions and behavior can be understood by study of the brain’s low-dimensional topology. Functional neuroimaging demonstrates that brain activity linked to adaptive behavior is constrained to low-dimensional manifolds. In human participants, we tested whether these low-dimensional constraints preserve working memory performance following local neuronal perturbations. We combined multi-session functional magnetic resonance imaging, non-invasive transcranial magnetic stimulation (TMS), and methods translated from the fields of complex systems and computational biology to assess the functional link between changes in local neural activity and the reshaping of task-related low dimensional trajectories of brain activity. We show that specific reconfigurations of low-dimensional trajectories of brain activity sustain effective working memory performance following TMS manipulation of local activity on, but not off, the space traversed by these trajectories. We highlight an association between the multi-scale changes in brain activity underpinning cognitive function.
- Free Publicly Accessible Full Text
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Accepted Manuscript1.0
- Journal Article:
- https://doi.org/DOI: 10.1109/TMAG.2018.2796098
