Methodology for tDCS integration with fMRI

Abstract

Integration of tDCS with fMRI holds promise for investigation the underlying mechanism of stimulation effect. There are 118 published tDCS studies (up to 1st Oct 2018) that used fMRI as a proxy measure of neural activation to answer mechanistic, predictive, and localization questions about how brain activity is modulated by tDCS. FMRI can potentially contribute as: a measure of cognitive state-level variance in baseline brain activation before tDCS; inform the design of stimulation montages that aim to target functional networks during specific tasks; and act as an outcome measure of functional response to tDCS. In this systematic review we explore methodological parameter space of tDCS integration with fMRI. Existing tDCS-fMRI literature shows little replication across these permutations; few studies used comparable study designs. Here, we use a case study with both task and resting state fMRI before and after tDCS in a cross-over design to discuss methodological confounds. We further outline how computational models of current flow should be combined with imaging data to understand sources of variability in responsiveness. Through the case study, we demonstrate how modeling and imaging methodology can be integrated for individualized analysis. Finally, we discuss the importance of conducting tDCS-fMRI with stimulation equipment certified as safe to use inside the MR scanner, and of correcting for image artifacts caused by tDCS. tDCS-fMRI can address important questions on the functional mechanisms of tDCS action (e.g. target engagement) and has the potential to support enhancement of behavioral interventions, provided studies are designed rationally.

Network-level mechanisms underlying effects of transcranial direct current stimulation (tDCS) on visuomotor learning

“…The stimulation pad wires were each equipped with 5.6 kΩ resistors to avoid temperature increases that could result from induction voltages. For the MRI-tDCS setup we followed the precautions detailed in ( Esmaeilpour et al, 2019 ) and closely replicated the set up in ( Antal et al, 2011b ). The stimulator device was connected to the subject via a waveguide with a length:width ratio of 4:1 with RF filters on either side.…”

“…Scanning was conducted on a 3T Siemens TiM Trio system (Erlangen, Germany), using a standard twelve-channel circularly polarized head coil, at the Center for Biomedical Imaging and Neuromodulation at NKI using standard measures ( Sehatpour et al, 2010 ; Sehatpour et al, 2006 ) and considerations for concurrent MRI-tDCS implementation outlined in ( Esmaeilpour et al, 2019 ). Briefly, we acquired two resting state scans, one during sham and one during active tDCS, in axial orientation aligned to the AC-PC plane (TR = 2500 ms, TE = 30 ms, FOV = 216 mm, matrix = 72 × 72, 38 3.3 mm slices, 0.7 mm gap, acceleration factor = 2) with a time series duration of 240 TRs and task-based (same sequence) scans with a time series duration of 209 TRs during task + tDCS stimulation.…”

Efficacy of transcranial direct current stimulation in people with multiple sclerosis: a review

“…Similar methods could be used to individualize the localization of tDCS target sites. In this line, several studies [42] have attempted to provide a methodology for integration of fMRI with tDCS. Besides functional connectivity, knowledge regarding the effective connectivity, which is the direction of flow of information between two activated regions, may help inform the choice of novel stimulation sites for tDCS beyond the common targets.…”

Transcranial Direct Current Stimulation (tDCS) for Memory Enhancement

“…For these reasons, DLPFC and PPC are most frequently used as stimulation targets to enhance memory. Positioning of the electrodes can be further refined based on current flow modeling 23 and validated in studies that combine tDCS with neuroimaging techniques 24 .…”