Concurrent Imaging of Markers of Current Flow and Neurophysiological Changes During tDCS

Abstract

Despite being a popular neuromodulation technique, clinical translation of transcranial direct current stimulation (tDCS) is hampered by variable responses observed within treatment cohorts. Addressing this challenge has been difficult due to the lack of an effective means of mapping the neuromodulatory electromagnetic fields together with the brain's response. In this study, we present a novel imaging technique that provides the capability of concurrently mapping markers of tDCS currents, as well as the brain's response to tDCS. A dual-echo echo-planar imaging (DE-EPI) sequence is used, wherein the phase of the acquired MRI-signal encodes the tDCS current induced magnetic field, while the magnitude encodes the blood oxygenation level dependent (BOLD) contrast. The proposed technique was first validated in a custom designed phantom. Subsequent test-retest experiments in human participants showed that tDCS-induced magnetic fields can be detected reliably in vivo. The concurrently acquired BOLD data revealed large-scale networks characteristic of a brain in restingstate as well as a 'cathodal' and an 'anodal' resting-state component under each electrode. Moreover, 'cathodal's BOLD-signal was observed to significantly decrease with the applied current at the group level in all datasets. With its ability to image markers of electromagnetic cause as well as neurophysiological changes, the proposed technique may provide an effective means to visualize neural engagement in tDCS at the group level. Our technique also contributes to addressing confounding factors in applying BOLD fMRI concurrently with tDCS.

In-vivo imaging of targeting and modulation of depression-relevant circuitry by transcranial direct current stimulation: a randomized clinical trial

“…In our experiments, DE-EPI MRI revealed significant tDCS current-induced magnetic fields for both Active-stimulation montages in regions encompassing the left DLPFC (BA 46 and BA 9). Recent studies have compared magnetic field measurements and simulations and found good agreement 53 , 57 , 61 , 71 . However, there are still concerns regarding simulations (e.g.…”

“…Note that the current-induced magnetic fields used in the analysis were corrected for stray-fields following the approach of Goksu et al 57 , 58 (described in Supplementary material S6 ). A confound-analysis was also performed for the BOLD data (following the approach of Jog et al 53 , shown in Supplementary Fig. S12 ), and indicated that the measured tDCS-induced BOLD-signal change was likely neurophysiological in origin.…”

“…The approach used here is based on Ampere’s law which states that electric currents induce linearly proportional magnetic fields. These induced fields can be detected in vivo using recently developed MRI techniques 53 , 57 , 61 , 70 , 71 . In our experiments, DE-EPI MRI revealed significant tDCS current-induced magnetic fields for both Active-stimulation montages in regions encompassing the left DLPFC (BA 46 and BA 9).…”

“…The BOLD data from DE-EPI was modeled similarly with the applied current convolved with the hemodynamic response function as a predictor. These predictor estimates can be interpreted as the tDCS current-induced magnetic fields and BOLD-signal changes, respectively (in nT/mA tDCS, and a.u., respectively) 53 . The pCASL data was modeled using the general kinetic model for perfusion 55 to quantify CBF (in ml/100 g/min).…”

“…At visit 0, a T1-MPRAGE structural MRI was acquired to guide electrode placement. At visits 1 and 12 (baseline and post-treatment), data from a T1-MPRAGE, 3D GRASE pseudo-continuous arterial spin labeling (pCASL) scan 52 , and a dual-echo echo planar imaging (DE-EPI) scan 53 were acquired. Note that tDCS was applied in real-time during the DE-EPI scan.…”