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.
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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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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.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 Objective. The objective of this study was to investigate the effects of micromagnetic stimuli strength and frequency from theMag neticPen (MagPen) on the rat right sciatic nerve. The nerve’s response was measured by recording muscle activity and movement of the right hind limb.Approach. The MagPen was custom-built to be stably held over the sciatic nerve. Rat leg muscle twitches were captured on video, and movements were extracted using image processing algorithms. EMG recordings were also used to measure muscle activity.Main results. The MagPen prototype, when driven by an alternating current, generates a time-varying magnetic field, which, according to Faraday’s law of electromagnetic induction, induces an electric field for neuromodulation. The orientation-dependent spatial contour maps of the induced electric field from the MagPen prototype have been numerically simulated. Furthermore, in thisin vivo work onµ MS, a dose-response relationship has been reported by experimentally studying how varying the amplitude (Range: 25 mV p-pthrough 6V p-p) and frequency (range: 100 Hz through 5 kHz) of the MagPen stimuli alters hind limb movement. The primary highlight of this dose-response relationship (repeated overn rats, wheren = 7) is that for aµ MS stimuli of higher frequency, significantly smaller amplitudes can trigger hind limb muscle twitch. This frequency-dependent activation can be justified by Faraday’s Law, which states that the magnitude of the induced electric field is directly proportional to the frequency.Significance. This work reports thatµ MS can successfully activate the sciatic nerve in a dose-dependent manner. The impact of this dose-response curve addresses the controversy in this research community about whether the stimulation from theseμ coils arise from a thermal effect or micromagnetic stimulation. MagPen probes do not have a direct electrochemical interface with tissue and therefore do not experience electrode degradation, biofouling, and irreversible redox reactions like traditional direct contact electrodes. Magnetic fields from theμ coils create more precise activation than electrodes because they apply more focused and localized stimulation. Finally, unique features ofµ MS, such as the orientation dependence, directionality, and spatial specificity, have been discussed.
- Free Publicly Accessible Full Text
-
Accepted Manuscript1.0
- Journal Article:
- https://doi.org/DOI: 10.1109/TMAG.2018.2796098
