Neuronavigated Focalized Transcranial Direct Current Stimulation Administered During Functional Magnetic Resonance Imaging

Summary

This protocol describes the method of neuronavigated electrode placement for focal, transcranial direct current stimulation (tDCS) administered during functional magnetic resonance imaging (fMRI).

Abstract

Transcranial direct current stimulation (tDCS) is a noninvasive brain stimulation technique that allows the modulation of the excitability and plasticity of the human brain. Focalized tDCS setups use specific electrode arrangements to constrain the current flow to circumscribed brain regions. However, the effectiveness of focalized tDCS can be compromised by electrode positioning errors on the scalp, resulting in significant reductions of the current dose reaching the target brain regions for tDCS. Electrode placement guided by neuronavigation based on the individual's head and brain anatomy derived from structural magnetic resonance imaging (MRI) data may be suited to improve positioning accuracy.

This protocol describes the method of neuronavigated electrode placement for a focalized tDCS setup, which is suitable for concurrent administration during functional MRI (fMRI). We also quantify the accuracy of electrode placement and investigate electrode drift in a concurrent tDCS-fMRI experiment. Critical steps involve the optimization of electrode positions based on current modeling that considers the individual's head and brain anatomy, the implementation of neuronavigated electrode placement on the scalp, and the administration of optimized and focal tDCS during fMRI.

The regional precision of electrode placement is quantified using the Euclidean norm (L² Norm) to determine deviations of the actual from the intended electrode positions during a concurrent tDCS-fMRI study. Any potential displacement of electrodes (drift) during the experiment is investigated by comparing actual electrode positions before and after the fMRI acquisition. In addition, we directly compare the placement accuracy of neuronavigated tDCS to that achieved by a scalp-based targeting approach (a 10-20 Electroencephalography (EEG) system). These analyses demonstrate superior placement accuracy for neuronavigation compared to scalp-based electrode placement and negligible electrode drift across a 20 min scanning period.

Introduction

Transcranial direct current stimulation (tDCS) is a noninvasive brain stimulation technique that allows the modification of cognition and physiological brain functions in experimental and clinical contexts^1,2,3. Acute administration of tDCS can have transient changes in neuronal excitability, with the aftereffects lasting from minutes to hours after the stimulation^4,5. The applied current does not induce action potentials but rather transiently shifts the resting membrane potential of the neuron toward either de- or h....

Protocol

All experimental procedures presented in this protocol have been reviewed and approved by the ethics committee of the University Medicine Greifswald. All participants provided informed consent prior to study inclusion and granted permission for their data to be published anonymously.

1. Screening of contraindications and general considerations

1. Prior to study enrollment, carefully screen participants for MRI²⁸ and tDCS²⁹

Discussion

Critical steps, potential modifications, and troubleshooting of the method
Accurate positioning of electrodes is a crucial technical factor in tDCS experiments, and deviations from intended scalp positions or electrode drift can affect current flow to the intended target brain regions^42,43. This is particularly relevant for focalized tDCS, as the regional specificity of the administered current makes these setups particularly susceptible to.......

Materials

Name	Company	Catalog Number	Comments
Brainsight neuronavigation system	Brainsight; Rogue Research Inc., Montréal, Canada
CR-5 Pro high temp 3D printer	CREALITY, Shenzhen, China
DC-STIMULATOR MC	NeuroConn GmbH, Ilmenau, Germany	https://www.neurocaregroup.com/technology/dc-stimulator-mc
EMLA Cream 5%	Aspen, Dublin, Ireland
MAGNETOM Vida 3T, syngo_MR_XA50 software	Siemens Healthineers AG, Forchheim, Germany
Polaris camera	Polaris Vicra; Northern Digital Inc., Waterloo, Canada
Ten20 conductive EEG paste	Weaver and Company, Aurora, USA
TPU 3D printer filament	SUNLU International, Hong-Kong, China
Example of alternatives
Ingenia 3.0T (MR-scanner)	Phillips, Amsterdam, Netherlands
Localite TMS Navigator (Neuronavigation equipment)	Localite, Bonn, Germany
Neural Navigator (Neuronavigation equipment)	Soterix, New Jersey, USA
PEBA 3D printer filament	Kimya, Nantes, France
PLA 3D printer filament	Filamentworld, Neu-Ulm, Deutschland
StarStim (Stimulator)	Neuroelectrics, Barcelona, Spain