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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Effect of coil size on transcranial magnetic stimulation (TMS) focality
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.
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- Award ID(s):
- 1631820
- NSF-PAR ID:
- 10109660
- Date Published:
- Journal Name:
- SPIE Proceedings Smart Biomedical and Physiological Sensor Technology XV; 110200Z (2019)
- Volume:
- 11020
- Page Range / eLocation ID:
- 38
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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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.more » « less
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Purpose In this work, we investigated how the position of the radiofrequency (RF) shield can affect the signal‐to‐noise ratio (SNR) of a receive RF coil. Our aim was to obtain physical insight for the design of a 10.5T 32‐channel head coil, subject to the constraints on the diameter of the RF shield imposed by the head gradient coil geometry.
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Results Our simulations showed that there is an SNR‐optimal RF shield size at UHF strength, whereas at low field the SNR monotonically increases with the shield diameter. For a 32‐channel head coil at 10.5T, an optimally sized RF shield could act as a cylindrical waveguide and increase the SNR in the brain by 27% compared to moving the shield as far as possible from the coil. Our results also showed that a separate transmit array between the RF shield and the receive array could considerably reduce SNR even if they are decoupled.
Conclusion At sufficiently high magnetic field strength, the design of local RF coils should be optimized together with the design of the RF shield to benefit from both near field and resonant modes.
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Magnetic Field Assisted Additive Manufacturing (MFAAM) enables 3D printing of magnetic materials of various shapes which exhibit a complex anisotropy energy surface containing contributions generated from different origins such as sample, particle, and agglomerate shape anisotropy, flow and field induced anisotropy, and particle crystal anisotropy. These novel magnet shapes require the need to measure the x, y, and z components of the magnetic dipole moment simultaneously to fully understand the magnetic reversal mechanism and unravel the complex magnetic anisotropy energy surface of 3D printed magnetic composites. This work aims to develop a triaxial vibrating sample magnetometer (VSM) by adding a z-coil set to a pre-existing biaxial VSM employing a modified Mallison coil set. The optimum size and location of the sensing coils were determined by modeling the sensitivity matrix of the z-coil set. The designed coil set was implemented using 3D printed spools, a manual coil winder, and gauge 38 copper wire. A 3D printed strontium ferrite nylon composite sample was used to estimate the sensitivity of the z-coils (50 mV/emu). The results herein are applicable for any VSM using a modified Mallison biaxial coil configuration allowing for a quick implementation on pre-existing systems.
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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
- Free Publicly Accessible Full Text
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Accepted Manuscript1.0
- Conference Paper:
- https://doi.org/10.1117/12.2524503
