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 Table of Contents  
ORIGINAL ARTICLE
Year : 2022  |  Volume : 6  |  Issue : 3  |  Page : 382-386

Designing a circular coil of repetitive transcranial magnetic stimulation at frequencies of 0.5 and 1 Hz using CST studio suite software and comparison of results with theoretical calculations


1 Radiation Injuries Research Center, Baqiyatallah University of Medical Sciences; Department of Physiology and Medical Physics, Faculty of Medicine, Baqiyatallah University of Medical Sciences, Tehran, Iran
2 Department of Biomedical Engineering, Faculty of Electrical Engineering, South Tehran Branch, Islamic Azad University, Tehran, Iran

Date of Submission01-Jul-2022
Date of Decision24-Jul-2022
Date of Acceptance08-Jul-2022
Date of Web Publication17-Sep-2022

Correspondence Address:
Hassan Tavakoli
Radiation Injuries Research Center, Baqiyatallah University of Medical Sciences, Tehran
Iran
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Source of Support: None, Conflict of Interest: None


DOI: 10.4103/bbrj.bbrj_174_22

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  Abstract 


Context: Since the 1990s, repetitive transcranial magnetic stimulation (rTMS) has been used as a noninvasive method to diagnose and manage the treatment of many neurological disorders. The circular coil is one of the most common coils in rTMS devices. This study presents the design documentation of a circular coil for use in the rTMS device and examines the consistency of the magnetic field intensity (H) and the magnetic flux density (B) obtained from the simulation of its use with the theoretical values. Aims: This study aimed to design a rTMS circular coil at frequencies of 0.5 and 1 Hz using CST Studio Suite software and compare the intensity of the B- and H-fields with theoretical calculations. Materials and Methods: After designing a circular coil, we measured B and H by simulation with CST Studio Suite software and compared the results with theoretical calculations. Results: The magnetic field intensity and the magnetic flux density obtained from the circular coil simulation we designed had an acceptable consistency with the theoretical values. Conclusions: The B- and H-field intensity whether on the body or in the air, or on the antenna, is fairishly consistent with theoretical calculations.

Keywords: B-field, circular coil, H-field, repetitive transcranial magnetic stimulation, simulation


How to cite this article:
Tavakoli H, Heidarpanah A. Designing a circular coil of repetitive transcranial magnetic stimulation at frequencies of 0.5 and 1 Hz using CST studio suite software and comparison of results with theoretical calculations. Biomed Biotechnol Res J 2022;6:382-6

How to cite this URL:
Tavakoli H, Heidarpanah A. Designing a circular coil of repetitive transcranial magnetic stimulation at frequencies of 0.5 and 1 Hz using CST studio suite software and comparison of results with theoretical calculations. Biomed Biotechnol Res J [serial online] 2022 [cited 2022 Oct 5];6:382-6. Available from: https://www.bmbtrj.org/text.asp?2022/6/3/382/356151




  Introduction Top


The technique of transcranial magnetic stimulation (TMS), as a noninvasive treatment method, has received much attention in recent decades. In this technique, an electrical pulse generator (a stimulator) is connected to a coil that is located close to the brain. The stimulator creates a variable electric current inside the coil that induces a magnetic field. This field then, in turn, induces a second inverted electric charge in the brain. One type of TMS is repetitive TMS (rTMS), in which trains of magnetic pulses are delivered. rTMS can cause a lasting change in neural activity. This long-lasting effect causes a change in cerebral cortex excitability that decreases with low-frequency rTMS (<1 Hz) and increases with high-frequency rTMS (>10 Hz).[1] It has been shown that rTMS with low frequencies (0.5 Hz or less) can be more effective in treating epilepsy.[2] The specific geometry of each coil used in TMS devices determines the shape, strength, and overall focus of the induced electric field, and thus the type of stimulation of the brain. The circular coils, as well as figure-of-8 coils, are among the most commonly used coils. Circular coil is the oldest and simplest TMS coil design that has been used since its introduction by Barker et al. in 1985.[3],[4] A single coil in the center creates a spherical magnetic field perpendicular to the coil itself (thus creating a magnetic sink in the middle). The electric field from the circular coils is induced directly below the coils, meaning that the circular coils do not produce just one area with the maximum field but provide the ability to simultaneously excite both hemispheres by placing the coil at the top of the skull. In contrast, figure-of-8 coil, also known as butterfly coil, was introduced in 1988 by experiments using frog nerve–muscle preparations by Ueno et al., as a way to achieve local stimulation by placing two coils in next to each other. The induced electric fields from these two coils are combined to allow centralized excitation.[5] Although figure-of-8 coils are now more widely used and have advantages such as targeted brain excitation,[6] the decay of electric field within a homogeneous volume conductor has been shown to occur more rapidly for a figure-8 coil compared to a circular coil, which reduces the ability to stimulate the brain more deeply than a round coil.[7],[8] Functionally, except for the duration of the cortical silent period, there is no significant difference when using a round coil with a figure-8 coil. As some studies have suggested, circular coils can also be used reliably in TMS studies.[9],[10] The electric field created by placing the rTMS device near the brain has resulted from the phenomenon of electromagnetic induction, which was discovered in 1831 by the English scientist Michael Faraday. He found that an electric field would be created by changing the magnetic field over time.[11] It should be noted that studies have shown that the resulting electric field strength varies under the influence of the coil design used.[12] In electromagnetism, the term “magnetic field” is used for two separate but close vector fields denoted by the symbols B and H, where B denotes the magnetic flux density and H denotes the magnetic field intensity, and their units in SI are Tesla (T) and ampere per meter, respectively.[13] In this study, we provide documentation for constructing a circular coil for use in rTMS devices that the experimental results obtained from its simulation using CST Studio Suite on the voxel of the child's brain are significantly consistent with the theory estimates of B and H.


  Materials and Methods Top


We used a circular coil of 1 turn with a radius of 5 cm, a length of 3 cm, a diameter of 2.5 mm, a current of 1200 amps, and a voltage of 1000 volts. The image of the single circular coil used for the simulation is shown in [Figure 1].
Figure 1: A circular coil with a frequency of 0.5 and 1 Hz projected into the temporal lobe

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Since, as a general rule, rTMS using low frequency (≤1 Hz) can reduce the cortical excitability at the stimulation site,[14] we applied two frequencies of 0.5 and 1 Hz in this study. We used CST Studio Suite software to simulate the use of coils and measure the intensity of magnetic fields. CST Studio Suite is a software product for simulating electromagnetic systems used for designing and optimizing operating devices in a wide frequency-static range in the visual field. This software can design, model, and assemble devices and circuits and electromagnetic systems in a three-dimensional environment and numerical simulations in electromagnetic fields. According to its creators, CST simulates the platform to observe the real world better.[15] To introduce magnetic radiation, a normal baby brain voxel sample with a density of 1045 kg/m3, electrical conductivity of 0.442 S/m, thermal conductivity of 0.5 W/mK, specific heat capacity of 3700 J/kg-K, diffusivity of 1.29316 × 10−7 m2/s, blood flow of 4000 W/K/mm3, and metabolic rate of 7000 W/mm3 were examined. The related image of which is shown in [Figure 2].
Figure 2: A sample of the brain voxel that has been studied in the introduction of magnetic radiation

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  Results Top


We designed the proposed circular coil in the software and performed the simulation in two separate parts in H- and B-field, each with frequencies of 0.5 and 1 Hz, on the abovementioned child's brain voxel. The results obtained from the simulation are presented in this section in [Figure 3], [Figure 4], [Figure 5], [Figure 6], and the data is included in [Table 1], [Table 2], [Table 3], [Table 4].
Figure 3: Simulation of H-field with the proposed circular coil at the frequency of 0.5 Hz

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Figure 4: Simulation of H-field with the proposed circular coil at the frequency of 1 Hz

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Figure 5: Simulation of B-field with the proposed circular coil at the frequency of 0.5 Hz

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Figure 6: Simulation of B-field with the proposed circular coil at the frequency of 1 Hz

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Table 1: Results of H-field simulation with the proposed circular coil at the frequency of 0.5 Hz

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Table 2: Results of H-field simulation with the proposed circular coil at the frequency of 1 Hz

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Table 3: Results of B-field simulation with the proposed circular coil at the frequency of 0.5 Hz

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Table 4: Results of B-field simulation with the proposed circular coil at the frequency of 1 Hz

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We designed that to calculate magnetic flux density (B), the inductance must first be calculated; inductance (L) is the tendency of an electrical conductor to resist the change in electric current that passes through it. An electric current creates a magnetic field around a conductor, and the intensity of the field depends on the magnitude of the current. The equation shows the inductance of a coil:



where L is the inductance in Henry, μ0 is the vacuum permeability constant (4×π×10-7), N is the number of turns of the coil, A is the cross-sectional area of the core in square meters, and l is the length of the core in meters. The magnetic flux density (B) is related to inductance through Ampere's law:



where N is the number of turns of the coil, L is the inductance in terms of Henry, A is the cross-sectional area, and I is the intensity of the current in amperes.

Finally, the following equation shows the relationship between the magnetic flux density (B) and the magnetic field intensity (H) in the vacuum:

H = B/μ.

Based on the coil information used, the inductance is equal to:





As a result, the magnetic flux density (B) can be calculated from Ampere's law as follows:



and so the magnetic field intensity (H) can also be calculated:




  Discussion Top


Today, noninvasive therapies play an undeniable role in managing a wide range of neurological disorders. The TMS technique, especially its more widely used form, rTMS, is also crucial as one of the most promising of these treatment protocols. Our proposed coil, which can be used as an antenna in rTMS devices, was able to provide acceptable results in accordance with theoretical calculations in experimental simulation with CST Studio Suite software. In short, the general simulation results are presented in [Table 5]. The magnetic flux density (B) and the magnetic field intensity (H), whether on the body or in the air, or on the antenna, are fairishly consistent with theoretical calculations as well as results of previous studies.
Table 5: Overall results of B- and H-field simulations with the proposed circular coil at frequencies of 0.5 and 1 Hz

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Recommendations for further research can be included similar simulations with the use of other types of coils, especially the figure-of-8 coils as one of the most widely used.

Limitation of the study

The most important limitation of this research was the lack of available and reliable data in which simulation data of similar circular coils were presented on human head voxels so that we could enrich it by comparing their values with the values of the current research.

Ethical statement

Ethics approval was not required for this study.

Financial support and sponsorship

Nil.

Conflicts of interest

There are no conflicts of interest.



 
  References Top

1.
Pazzaglia C, Vollono C, Testani E, Coraci D, Granata G, Padua L, et al. Low-frequency rTMS of the primary motor area does not modify the response of the cerebral cortex to phasic nociceptive stimuli. Front Neurosci 2018;12:878.  Back to cited text no. 1
    
2.
Cooper YA, Pianka ST, Alotaibi NM, Babayan D, Salavati B, Weil AG, et al. Repetitive transcranial magnetic stimulation for the treatment of drug-resistant epilepsy: A systematic review and individual participant data meta-analysis of real-world evidence. Epilepsia Open 2018;3:55-65.  Back to cited text no. 2
    
3.
Roth BJ, Maccabee PJ, Eberle LP, Amassian VE, Hallett M, Cadwell J, et al. In vitro evaluation of a 4-leaf coil design for magnetic stimulation of peripheral nerve. Electroencephalogr Clin Neurophysiol 1994;93:68-74.  Back to cited text no. 3
    
4.
Barker AT, Jalinous R, Freeston IL. Non-invasive magnetic stimulation of human motor cortex. Lancet 1985;1:1106-7.  Back to cited text no. 4
    
5.
Ueno S, Tashiro T, Harada K. Localized stimulation of neural tissues in the brain by means of a paired configuration of time-varying magnetic fields. J Appl Phys 1988;64:5862-4.  Back to cited text no. 5
    
6.
Ueno S, Sekino M. Figure-eight coils for magnetic stimulation: From focal stimulation to deep stimulation. Front Hum Neurosci 2021;15:805971.  Back to cited text no. 6
    
7.
Cohen LG, Roth BJ, Nilsson J, Dang N, Panizza M, Bandinelli S, et al. Effects of coil design on delivery of focal magnetic stimulation. Technical considerations. Electroencephalogr Clin Neurophysiol 1990;75:350-7.  Back to cited text no. 7
    
8.
Maccabee PJ, Eberle L, Amassian VE, Cracco RQ, Rudell A, Jayachandra M. Spatial distribution of the electric field induced in volume by round and figure '8' magnetic coils: Relevance to activation of sensory nerve fibers. Electroencephalogr Clin Neurophysiol 1990;76:131-41.  Back to cited text no. 8
    
9.
Badawy RA, Tarletti R, Mula M, Varrasi C, Cantello R. The routine circular coil is reliable in paired-TMS studies. Clin Neurophysiol 2011;122:784-8.  Back to cited text no. 9
    
10.
Chae JH, Kim W, Bahk WM, Jun TY, Kim KS. Repeated transcranial magnetic stimulation (rTMS) with circular coil for patients with treatment resistant depression: An open trial. Psychiatry Investig 2005;2:61-5.  Back to cited text no. 10
    
11.
Mishra A, Maiti R, Mishra BR, Jena M, Srinivasan A. Effect of repetitive transcranial magnetic stimulation on seizure frequency and epileptiform discharges in drug-resistant epilepsy: A meta-analysis. J Clin Neurol 2020;16:9-18.  Back to cited text no. 11
    
12.
Gomez LJ, Goetz SM, Peterchev AV. Design of transcranial magnetic stimulation coils with optimal trade-off between depth, focality, and energy. J Neural Eng 2018;15:046033.  Back to cited text no. 12
    
13.
Newell DB, Tiesinga E. The international system of units (SI). NIST Special Publication 2019;330:1-38.  Back to cited text no. 13
    
14.
Cash RF, Weigand A, Zalesky A, Siddiqi SH, Downar J, Fitzgerald PB, et al. Using brain imaging to improve spatial targeting of transcranial magnetic stimulation for depression. Biol Psychiatry 2021;90:689-700.  Back to cited text no. 14
    
15.
Studio Suite 3D EM simulation and analysis software [Internet]. 3ds.com. 2022 [cited 1 July 2022]. Available from: https://www.3ds.com/products-services/simulia/products/cst-studio-suite/.  Back to cited text no. 15
    


    Figures

  [Figure 1], [Figure 2], [Figure 3], [Figure 4], [Figure 5], [Figure 6]
 
 
    Tables

  [Table 1], [Table 2], [Table 3], [Table 4], [Table 5]



 

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