Abstract
Transcranial magnetic stimulation has been gaining popularity in the therapy for several neurological disorders. A time-varying magnetic field is used to generate electric field in the brain. As the development of TMS methods takes place, emphasis on the coil design increases in order to improve focal stimulation. Ideally reduction of stimulation of neighboring regions of the target area is desired. This study, focused on the improvement of the focality of the Quadruple Butterfly Coil (QBC) with supplemental use of different passive shields. Parameters such as shape, position and permeability of the shields have been explored to improve the focus of stimulation. Results have been obtained with the help of computer modelling of a MRI derived heterogeneous head model over the vertex position and the dorsolateral prefrontal cortex position using a finite element tool. Variables such as maximum electric field induced on the grey matter and scalp, volume and area of stimulation above half of the maximum value of electric field on the grey matter, and ratio of the maximum electric field in the brain versus the scalp have been investigated.
Highlights
Transcranial Magnetic Stimulation (TMS) is a FDA approved non-invasive intervention for Treatment Resistant Depression (TRD) and migraine.[1,2] TMS has beneficial effects on other neurological disorders including: Parkinson’s Disease (PD), Obsessive Compulsive Disorder (OCD), Post-Traumatic Stress Disorder (PTSD), Schizophrenia and Traumatic Brain Injury (TBI).[3,4,5,6] TMS works on the principal of Faraday’s law of induction in which neural circuits are excited due to a time varying magnetic field
The computer modelling tool,[10] simulation settings, post processing method, and parameters of interest including (a) E-Max brain, (b) A-Half, (c) V-Half, and (d) E-Max head were discussed in Rastogi et al.[6]. These parameters refer to the (a) maximum electric field intensity in the grey matter (GM) and white matter (WM), (b) the surface area of the brain exposed to electric field intensities at least one half of maximum electric field (E-Max), (c) the volume of the brain exposed to electric field intensities at least one half of E-Max, and (d) the maximum electric field intensity in the entire head respectively
Permeability of shielding material has been explored for the shield used in Rastogi et al[6] along with the Quadruple Butterfly Coil (QBC) on the vertex position of the head
Summary
Transcranial Magnetic Stimulation (TMS) is a FDA approved non-invasive intervention for Treatment Resistant Depression (TRD) and migraine.[1,2] TMS has beneficial effects on other neurological disorders including: Parkinson’s Disease (PD), Obsessive Compulsive Disorder (OCD), Post-Traumatic Stress Disorder (PTSD), Schizophrenia and Traumatic Brain Injury (TBI).[3,4,5,6] TMS works on the principal of Faraday’s law of induction in which neural circuits are excited due to a time varying magnetic field. Researchers and scholars are interested in fabricating TMS coils which have higher focus, yet still provide a strong enough electric field to depolarize neurons and initiate neuronal firing. As TMS research progresses, it is important for new coils to be developed that can reduce the spread of the electric field to non-targeted brain regions. The main considerations discussed here include the stimulation output of the coil for a given current intensity, and the ratio of stimulation received in the brain as compared to the scalp
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