TMS Coil Design

Transcranial Magnet Stimulator (TMS) is a tool for the study of the human brain as well as a medical agent in psychiatry and neurology. TMS stimulate the brain by sending a pulsed current through a coil which produces a magnetic field that induces electric field in the brain and cause neurons to fire

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

A Compact 1200 V, 700 A, IGBT-Based Pulse Generator for Repetitive Transcranial Magnetic Stimulation in Vivo Laboratory Experiments on Small Animals

An insulated-gate bipolar transistor (IGBT) pulse generator for repetitive transcranial magnetic stimulation used for in vivo laboratory experiments on small animals, such as mice, is reported. The pulse generator is based upon an IGBT that can switch 700 A of current for 1 ms and that has a DC breakdown voltage of 1200 V. The duration of the design’s output pulse is controlled by, and follows, an input trigger pulse. The voltage amplitude of the output pulses is determined by an external high-voltage power supply and the energy stored in a 330 µF capacitor bank. The approach enables the amplitude of the voltage applied across the coil, the length of time the voltage is applied, and the number of times the voltage pulses are applied all to be controlled and adjusted to facilitate a wide range of experimental options. This paper provides a detailed schematic of the design, design discussions, and some representative experimental results. Additionally, the reported design can be scaled to higher currents by using an IGBT with a higher current rating

Doctor of Philosophy

dissertationInterfacing with the peripheral nervous system via stimulating neurotechnologies has allowed for therapies which can restore sensorimotor and autonomic function previously lost to injury or disease. Magnetic stimulation (MS) is one such technology that m

The role of pulse shape in motor cortex transcranial magnetic stimulation using full-sine stimuli

A full-sine (biphasic) pulse waveform is most commonly used for repetitive transcranial magnetic stimulation (TMS), but little is known about how variations in duration or amplitude of distinct pulse segments influence the effectiveness of a single TMS pulse to elicit a corticomotor response. Using a novel TMS device, we systematically varied the configuration of full-sine pulses to assess the impact of configuration changes on resting motor threshold (RMT) as measure of stimulation effectiveness with single-pulse TMS of the non-dominant motor hand area (M1). In young healthy volunteers, we (i) compared monophasic, half-sine, and full-sine pulses, (ii) applied two-segment pulses consisting of two identical half-sines, and (iii) manipulated amplitude, duration, and current direction of the first or second full-sine pulse half-segments. RMT was significantly higher using half-sine or monophasic pulses compared with full-sine. Pulses combining two half-sines of identical polarity and duration were also characterized by higher RMT than fullsine stimuli resulting. For full-sine stimuli, decreasing the amplitude of the halfsegment inducing posterior-anterior oriented current in M1 resulted in considerably higher RMT, whereas varying the amplitude of the half-segment inducing anterior-posterior current had a smaller effect. These findings provide direct experimental evidence that the pulse segment inducing a posterior anterior directed current in M1 contributes most to corticospinal pathway excitation. Preferential excitation of neuronal target cells in the posterior-anterior segment or targeting of different neuronal structures by the two half-segments can explain this result. Thus, our findings help understanding the mechanisms of neural stimulation by full-sine TMS

TMS with fast and accurate electronic control : Measuring the orientation sensitivity of corticomotor pathways

Background: Transcranial magnetic stimulation (TMS) coils allow only a slow, mechanical adjustment of the stimulating electric field (E-field) orientation in the cerebral tissue. Fast E-field control is needed to synchronize the stimulation with the ongoing brain activity. Also, empirical models that fully describe the relationship between evoked responses and the stimulus orientation and intensity are still missing. Objective: We aimed to (1) develop a TMS transducer for manipulating the E-field orientation electronically with high accuracy at the neuronally meaningful millisecond-level time scale and (2) devise and validate a physiologically based model describing the orientation selectivity of neuronal excitability. Methods: We designed and manufactured a two-coil TMS transducer. The coil windings were computed with a minimum-energy optimization procedure, and the transducer was controlled with our custommade electronics. The electronic E-field control was verified with a TMS characterizer. The motor evoked potential amplitude and latency of a hand muscle were mapped in 3 degrees steps of the stimulus orientation in 16 healthy subjects for three stimulation intensities. We fitted a logistic model to the motor response amplitude. Results: The two-coil TMS transducer allows one to manipulate the pulse orientation accurately without manual coil movement. The motor response amplitude followed a logistic function of the stimulus orientation; this dependency was strongly affected by the stimulus intensity. Conclusion: The developed electronic control of the E-field orientation allows exploring new stimulation paradigms and probing neuronal mechanisms. The presented model helps to disentangle the neuronal mechanisms of brain function and guide future non-invasive stimulation protocols. (C) 2022 The Authors. Published by Elsevier Inc.Peer reviewe

Transcranial brain stimulation: Past and future

This article provides a brief summary of the history of transcranial methods for stimulating the human brain in conscious volunteers and reviews the methodology and physiology of transcranial magnetic stimulation and transcranial direct current stimulation. The former stimulates neural axons and generates action potentials and synaptic activity, whereas the latter polarises the membrane potential of neurones and changes their sensitivity to ongoing synaptic inputs. When coupled with brain imaging methods such as functional magnetic resonance imaging or electroencephalography, transcranial magnetic stimulation can be used to chart connectivity within the brain. In addition, because it induces artificial patterns of activity that interfere with ongoing information processing within a cortical area, it is frequently used in cognitive psychology to produce a short-lasting ‘virtual lesion’. Both transcranial magnetic stimulation and transcranial direct current stimulation can produce short-lasting changes in synaptic excitability and associated changes in behaviour that are presently the source of much research for their therapeutic potential

Aaltomuodon hallinta transkraniaalisessa magneettistimulaatiossa

Transcranial magnetic stimulation (TMS) is a non-invasive tool for stimulating the brain via induced electric fields generated by driving a strong current pulse through a stimulation coil. The TMS group at Aalto University has developed a multi-locus TMS (mTMS) device which exploits linear superposition of electric fields. This allows for the rotation or movement of the stimulated area to be controlled electrically by having a set of overlapping coils instead of physically moving a single coil. The device utilizes an H-bridge topology which can be used to carefully control the current through a coil by changing the path of current through the stimulator circuitry with insulated gate bipolar transistors (IGBTs), and the aim of this Thesis was to develop a method for controlling these currents in such a way that a given reference current pulse (i.e., waveform) could be approximated. For better IGBT control, new controller cards had recently been designed, and one objective of this Thesis was to test them. Additionally, in preparation for more coils to be added to the system, a coil identification system utilizing digital temperature sensors and a microcontroller was prototyped. The bulk of this Thesis, however, consists of the algorithm that was developed for IGBT control. The idea is to calculate a timing sequence for the IGBTs in such a way that a waveform generated by a lower initial voltage reference pulse is effectively simulated by periodically driving current to the coil from a high-voltage source. The non-idealities present in the circuit, however, pose a problem for approximation accuracy, and this was compensated for by further developing the model by adding a back-prediction module that tries to predict a better input sequence for the system based on previous measurements. The controller cards for the IGBTs were found to be satisfactory, and the prototyped coil identification system seems like a feasible solution even in the presence of strong magnetic fields. The waveform approximation was found to give rising-phase predictions with 0.3—7.7% relative difference in maximum amplitude compared to actual output for the sequences tested, depending on the chosen correction parameters. The falling-phase predictions varied significantly due to lack of parameter data. The tools developed in this Thesis give a good starting point for further development of waveform control in TMS.Transkraniaalinen magneettistimulaatio (engl. transcranial magnetic stimulation, TMS) on ei-invasiivinen menetelmä aivojen stimulaatioon. Menetelmä perustuu indusoituihin sähkökenttiin, jotka luodaan ajamalla suuri virtapulssi stimulaatiokäämin läpi. Aalto-yliopiston TMS-ryhmä on kehittänyt uuden sukupolven mTMS-laitetta (engl. multi-locus TMS), jossa sähkökenttien superpositioon perustuen stimulaatioaluetta voidaan siirtää ja kääntää sähköisesti hyödyntäen useita keloja. Tyypillisesti stimulaatioalueen siirto tai kääntö toteutetaan fyysisesti kelaa liikuttamalla. Yliopistolla kehitetty laite perustuu sähköiseen H-siltakytkentään, jossa sähkövirran kulkureittiä voidaan hallita kytkemällä IGBT-transistoreita (engl. insulated gate bipolar transistor) päälle tai pois päältä. Tämän diplomityön tavoite oli kehittää menetelmä piirin virrankulun hallintaan siten, että haluttu referenssipulssi (eli aaltomuoto) voidaan mallintaa. IGBT-transistoreiden parempaa hallittavuutta varten ryhmässä oli aiemmin kehitetty uudet ohjainkortit, joiden testaaminen oli yksi tämän diplomityön osa-alueista, ja lisäksi uusien kelojen lisäämistä silmälläpitäen järjestelmälle valmistettiin prototyyppi kelojen tunnistusta varten. Pääpaino työssä oli kuitenkin kehittää IGBT-transistoreiden hallintaan algoritmi, jolla kelan läpi kulkevaa virtaa voidaan tarkasti hallita. Perusidea tässä algoritmissa on, että suurella alkujännitteellä ajetaan stimulaatiokelaan virtaa vain hetkittäin, jolloin voidaan efektiivisesti simuloida tilannetta, jossa alemmalla alkujännitteellä ajetaan virtaa kelan läpi jatkuvasti. Laitteen epäideaalisuudet johtivat hyvin epätarkkaan approksimaatioon, mitä varten kehitettiin ennustusmoduuli, joka pyrkii aiempiin mittauksiin perustuen antamaan paremman ennusteen aaltomuodon käyttäytymisestä. IGBT-ohjainkortit toimivat mittausten perusteella hyvin, ja kelojen tunnistusjärjestelmä vaikuttaa ainakin ensiarvioiden perusteella hyvältä, vahvoista magneettikentistä huolimatta. Approksimointialgoritmi testatuille aaltomuodoille antoi 0.3—7.7% suhteellisen eron maksimiamplitudien välille riippuen korjausparametreista. Työssä esitetyt työkalut antavat hyvän lähtökohdan aaltomuotojen hallinnan jatkokehitykseen TMS:ssä

Development of Human Body CAD Models and Related Mesh Processing Algorithms with Applications in Bioelectromagnetics

Simulation of the electromagnetic response of the human body relies heavily upon efficient computational CAD models or phantoms. The Visible Human Project (VHP)-Female v. 3.1 - a new platform-independent full-body electromagnetic computational model is revealed. This is a part of a significant international initiative to develop powerful computational models representing the human body. This model’s unique feature is full compatibility both with MATLAB and specialized FEM computational software packages such as ANSYS HFSS/Maxwell 3D and CST MWS. Various mesh processing algorithms such as automatic intersection resolver, Boolean operation on meshes, etc. used for the development of the Visible Human Project (VHP)-Female are presented. The VHP - Female CAD Model is applied to two specific low frequency applications: Transcranial Magnetic Stimulation (TMS) and Transcranial Direct Current Stimulation (tDCS). TMS and tDCS are increasingly used as diagnostic and therapeutic tools for numerous neuropsychiatric disorders. The development of a CAD model based on an existing voxel model of a Japanese pregnant woman is also presented. TMS for treatment of depression is an appealing alternative to drugs which are teratogenic for pregnant women. This CAD model was used to study fetal wellbeing during induced peak currents by TMS in two possible scenarios: (i) pregnant woman as a patient; and (ii) pregnant woman as an operator. An insight into future work and potential areas of research such as a deformable phantom, implants, and RF applications will be presented