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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High-sensitivity and spatial resolution transient magnetic and electric field probes for transcranial magnetic stimulator characterizations
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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- Award ID(s):
- 1631820
- NSF-PAR ID:
- 10063425
- Date Published:
- Journal Name:
- Instrumentation science & technology
- ISSN:
- 1525-6030
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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Abstract Full‐wave electromagnetic simulations have been employed to analyze the design approaches that allow for solving the problems preventing inductive performance of micro‐solenoidal RF probes and formation of strong and homogeneous magnetic fields inside probes at the studies of millimeter‐range sized objects in 14 T MRI scanners. In particular, the effects of non‐uniform coil wrapping on field homogeneity inside extended coils and of partitioning the coils by dielectric separators on coil self‐resonances have been investigated. The possibility to utilize tunable C‐C matching circuits with the coils and to mitigate the effects of sample insertion on the probe resonance frequency has been demonstrated. Challenges of coil fabrication have been also addressed.
- Free Publicly Accessible Full Text
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Accepted Manuscript1.0
- Journal Article:
- https://doi.org/DOI: 10.1080/10739149.2017.1401547
