A computational model for real-time calculation of electric field due to transcranial magnetic stimulation in clinics

The aim of this paper is to propose an approach for an accurate and fast (real-time) computation of the electric field induced inside the whole brain volume during a transcranial magnetic stimulation (TMS) procedure. The numerical solution implements the admittance method for a discretized realistic brain model derived from Magnetic Resonance Imaging (MRI). Results are in a good agreement with those obtained using commercial codes and require much less computational time. An integration of the developed codewith neuronavigation toolswill permit real-time evaluation of the stimulated brain regions during the TMSdelivery, thus improving the efficacy of clinical applications

Setting reference level in the human safety guidelines via nerve activation intercomparison at IF

International guidelines/standards have been published for human protection from electromagnetic field exposure. The research in the intermediate frequencies (IF: 300 Hz-10 MHz) is scattered unlike for other frequencies, and thus the limit prescribed in the guidelines/standards are different by a factor of 10. The IEEE International Committee on Electromagnetic Safety has published a research agenda for exploring the electrostimulation thresholds. However, the consistency of the excitation models for specific target tissue needs to be revised. For this purpose, we present the first intercomparison study using multiphysics modelling to investigate stimulation thresholds during transcranial magnetic stimulation (TMS). To define the stimulation threshold, a noninvasive technique for brain stimulation has been used. In this study, by incorporating individual neurons into electromagnetic computation in realistic head models, stimulation thresholds can be determined. The study case of one subject showed that the allowable external magnetic field strength in the current guidelines/standard is conservative

Transcranial Electrical Neuromodulation Based on the Reciprocity Principle

A key challenge in multi-electrode transcranial electrical stimulation (TES) or transcranial direct current stimulation (tDCS) is to find a current injection pattern that delivers the necessary current density at a target and minimizes it in the rest of the head, which is mathematically modeled as an optimization problem. Such an optimization with the Least Squares (LS) or Linearly Constrained Minimum Variance (LCMV) algorithms is generally computationally expensive and requires multiple independent current sources. Based on the reciprocity principle in electroencephalography (EEG) and TES, it could be possible to find the optimal TES patterns quickly whenever the solution of the forward EEG problem is available for a brain region of interest. Here, we investigate the reciprocity principle as a guideline for finding optimal current injection patterns in TES that comply with safety constraints. We define four different trial cortical targets in a detailed seventissue finite element head model, and analyze the performance of the reciprocity family of TES methods in terms of electrode density, targeting error, focality, intensity, and directionality using the LS and LCMV solutions as the reference standards. It is found that the reciprocity algorithms show good performance comparable to the LCMV and LS solutions. Comparing the 128 and 256 electrode cases, we found that use of greater electrode density improves focality, directionality, and intensity parameters. The results show that reciprocity principle can be used to quickly determine optimal current injection patterns in TES and help to simplify TES protocols that are consistent with hardware and software availability and with safety constraints.Laboratorio de Electrónica Industrial, Control e Instrumentación (LEICI

Brain cortical stimulation thresholds to different magnetic field sources exposures at intermediate frequencies

Permissible field strengths in the international guidelines/standard for human protection are derived from peripheral nerve system stimulation at the intermediate frequencies where electrostimulation (attributable to axon activation) is more dominant than thermal effect. Recently, multiscale computation has been used to investigate neuron stimulation thresholds by incorporating individual neurons into realistic head models. However, the consistency of excitation models and permissible levels to specific target tissues (central nervous system) needs to be clarified. This article aims to investigate brain cortical stimulation thresholds using a multiscale computational approach for different scenarios of magnetic field exposures. The magnetic exposures include transcranial magnetic stimulation, uniform exposure, and wireless power transfer systems. Our results confirmed the consistency of the multiscale computations of the cortical thresholds between two independent groups for electromagnetic exposure of transcranial magnetic stimulation (thresholds in the range of motor cortex activation). We also quantified the conservativeness of permissible field strengths of international guidelines/standards at intermediate frequencies. Finally, with the multiscale approach, we confirmed that 10 000 kW of transmitting power of wireless power transfer (WPT) in an electric vehicle charging system may not induce an adverse effect for cortical activation

OpenMEEG: opensource software for quasistatic bioelectromagnetics

Background: Interpreting and controlling bioelectromagnetic phenomena require realistic physiological models and accurate numerical solvers. A semi-realistic model often used in practise is the piecewise constant conductivity model, for which only the interfaces have to be meshed. This simplified model makes it possible to use Boundary Element Methods. Unfortunately, most Boundary Element solutions are confronted with accuracy issues when the conductivity ratio between neighboring tissues is high, as for instance the scalp/skull conductivity ratio in electro-encephalography. To overcome this difficulty, we proposed a new method called the symmetric BEM, which is implemented in the OpenMEEG software. The aim of this paper is to presen

FDTD-based Transcranial Magnetic Stimulation model applied to specific neurodegenerative disorders

Non-invasive treatment of neurodegenerative diseases is particularly challenging in Western countries, where the population age is increasing. In this work, magnetic propagation in human head is modelled by Finite-Difference Time-Domain (FDTD) method, taking into account specific characteristics of Transcranial Magnetic Stimulation (TMS) in neurodegenerative diseases. It uses a realistic high-resolution three-dimensional human head mesh. The numerical method is applied to the analysis of magnetic radiation distribution in the brain using two realistic magnetic source models: a circular coil and a figure-8 coil commonly employed in TMS. The complete model was applied to the study of magnetic stimulation in Alzheimer and Parkinson Diseases (AD, PD). The results show the electrical field distribution when magnetic stimulation is supplied to those brain areas of specific interest for each particular disease. Thereby the current approach entails a high potential for the establishment of the current underdeveloped TMS dosimetry in its emerging application to AD and PD

Suurikokoiset päällekkäiset kelat, uusi lähestymistapa monikanavaisen transkraniaalisen magneettistimulaatiolaitteen rakentamiseen

Transcranial magnetic stimulation (TMS) allows for studying the functionality of the brain. Present TMS devices have one or two separate stimulation coils. More stimulation coils would allow new types of stimulation sequences, and thus they could be used to reveal more about brain functionality. However, due to the dimensions of the existing TMS coils, having multiple separate coils is a very limited approach. Rather, the coils should be combined into a single multichannel (mTMS) device. The purpose of this Thesis is to make mTMS more feasible. In order to realize this purpose, a new coil design paradigm is introduced which employs large thin overlapping coils. This paradigm requires a new coil design method and a new coil-former design method, which are developed and tested in this Thesis. This Thesis solves two problems that appear with existing mTMS designs and is a significant step towards successful mTMS.Transkraniaalinen magneettistimulaatio (TMS) mahdollistaa aivotoiminnan tutkimisen. Nykyisissä TMS-laitteissa on yleensä yksi tai joissain tapauksissa kaksi erillistä stimulaatiokelaa. Suurempi kelamäärä mahdollistaisi uudentyyppisiä stimulaatiosekvenssejä, jotka mahdollistaisivat monipuolisemman aivotoiminnan tutkimisen. Koska TMS-kelat ovat verrattain suurikokoisia, ei tätä tavoitetta kuitenkaan pystytä saavuttamaan yhdistämällä monta erillistä TMS-kelaa. Sen sijaan tarvittaisiin yksi monikanavainen (mTMS) laite, jossa eri kanavien kelat on yhdistetty yhdeksi suuremmaksi kokonaisuudeksi. Tämän diplomityön tarkoitus on edistää osaltaan mTMS-laitteen suunnittelua. Tätä varten esitellään uusi mTMS-rakenne, jossa mTMS-kela koostuu suurikokoisista ohuista päällekkäisistä keloista. Tässä diplomityössä kehitetään ja testataan yksittäisten kelojen suunnittelumenetelmä tämäntyyppistä mTMS-kelaa varten. Diplomityössä esiteltävä rakenne ratkaisee kaksi nykyisissä mTMS-kelarakennesuunnitelmissa esiintyvää ongelmaa

Optimisation of a Wearable Neuromodulator for Migraine Using Computational Methods

Migraine is the third most common neurological disorder and the sixth cause of disability. It may be characterized by a headache, nausea, vomiting, photo- phobia and phonophobia. Available pharmaceutical treatments of migraine are not completely effective and have troublesome side-effects. Thus, there is a need for alternative treatments such as neuromodulation. Neuromodulation may be delivered invasively; however, this exposes the patients to the associated risks. Transcutaneous electrical nerve stimulation is a non-invasive technique that is widely used to relieve pain. A significant number of migraine sufferers complaint the symptoms of pain originating in the frontal region of the head. Thus, mi- graine may be associated with the supraorbital nerve and supratrochlear nerve which passes below the frontal bone exits from the orbital rim and penetrates the corrugator and frontalis muscles. Transcutaneous frontal nerve stimulation has been applied on a large group of patients who have episodic migraine us- ing a device called Cefaly. This study produced mixed results (50% response rate). A post–marketing survey led to 53% satisfaction while the most limiting factor is reported to be paraesthesia and painful sensation. The possible causes of these inconclusive results may be associated with neuroanatomical variations, patient compliance and neurophysiological effects. The most plausible cause may be related to the neuroanatomical variations across different subjects. The neu- roanatomical variations may lead to excessively high current levels being required. Since this solution is patient–operated, these relatively high required levels are not applied. In addition, as the electrodes are positioned near pain–sensitive structures, pain may be induced even at low current levels, further limiting the efficacy of the solution. There has been no robust investigation identifying the underlying causes of ineffi- cacy. This is partly due to the physical limitations of studying the neuroanatomy of each subject and different settings of electrode arrangements. Computational models may enable researchers to estimate current stimulation thresholds in neu- romodulation therapy and investigate the effects of various parameters. Such computational models are composed of a volume conductor model and an ad- vanced Hodgkin–Huxley–type model of neural tissue referred to as a hybrid model. Once the human head anatomy, the human nervous system and available solu- tions for migraine are detailed, the computational model of the human head is generated. A highly detailed human head model based on magnetic resonance imaging (MRI) studies, microscopic structure of the skin(including sweat ducts, keratinocytes and lipid) and those of a simplified head model (which built from geometric shapes) are compared based on neural excitation to assess the usabil- ity of geometrically realistic(simplified) human head models in the subsequent studies to save computations cost. The induced electric field due to an electrode setting is simulated in the volume conductor model and the resulting electric potential values along the nerve are passed on to the neural model to simulate nerve’s response. It is shown that a simplified model may be used with a marginal error (≈2%) in the subsequent work when assessing the effect of neuroanatomical variations on the efficacy of the target solution and possible ensuing optimiza- tions. The first step is to identify if neuroanatomical variations had any effect on the required stimulus current levels using state of the art computational bio–models. Ten realistic human head models are developed by varying thirteen neuroanatom- ical features including human head size, thicknesses of the tissue layers and vari- ations in the courses of the nerve by considering their respective statistical distributions as reported in the literature. A novel algorithm is developed to account for the variations of the nerve in different individuals and mimic statistically relevant large population. In each case, the required stimulus current levels are simulated. The findings show that the combined neuroanatomical variations have a significant effect on the neural response for the electrode setting used in Cefaly device. Therefore, a potential improvement is to align the axis of electrodes with the target nerve, so that the electrical potential along the trajectory of the nerve changes polarity. This may lead to lower required stimulus current levels. Align- ing electrodes with the nerve, the required current may be reduced by at least 60%. This new orientation reduces current density near pain– sensitive struc- tures by diverting the current away from them, which may lead to a higher level of patient compliance, further improving the efficacy of the solution. Using an electrodes array arrangement, the required current levels is further reduced due to incorporating multiple electrodes array elements to maximise the variations of the electrical field in the simulation of the fibres in one phase. The findings of this thesis indicate that the highly detailed human head model can be simplified while minimally affecting the outcome. Additionally, it is shown that neuroanatomical variations have a significant impact on the stimulus current thresholds but it is not possible to conclude if these thresholds solely depend on a specific neuroanatomical variation. The relatively high required levels of the stimulus currents are beyond the current capabilities of existing device and pos- sible pain thresholds. Furthermore, the proposed new electrode arrangement has multiple benefits including the reduction of the stimulus current levels and diver- sion of current spread from possible pain–sensitive structures. This improvement, based on modelling, can potentially improve the clinical outcome of the neuro- modulator substantially if confirmed in the subsequent clinical studies