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 resting-state 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.
Highlights
Transcranial Direct Current Stimulation is an emerging neuromodulation technique that has demonstrated therapeutic potential in a range of neurological and psychiatric disorders (Alonso-Alonso et al, 2007; Brunoni et al, 2012; Allman et al, 2016; Kuo et al, 2017; Pontillo et al, 2018), and may improve cognition in healthy subjects (Coffman et al, 2014; Current-Mapping and blood oxygenation level dependent (BOLD) During transcranial direct current stimulation (tDCS)Berryhill and Martin, 2018)
The key innovation of the proposed technique is the capability to simultaneously image markers of tDCS cause, as well as ensuing neural effects; i.e., markers of the tDCS electric current, and the physiological state of the brain over time. One of these markers can be measured by existing imaging techniques, including BOLD fMRI, arterial spin labeling (ASL), EEG, MEG (Polania et al, 2011; Krishnamurthy et al, 2015; HeinrichsGraham et al, 2017; Donaldson et al, 2019), NEMO [Neuroelectromagnetic oscillations (Bodurka and Bandettini, 2002; Chai et al, 2017; Truong et al, 2019)] and Magnetic Resonance Electrical Impedance Tomography (MREIT)
Our results indicate that tDCS induced electromagnetic fields are stronger than those predicted by computational modeling, which is consistent with other studies (Goksu et al, 2018)
Summary
Transcranial Direct Current Stimulation (tDCS) is an emerging neuromodulation technique that has demonstrated therapeutic potential in a range of neurological and psychiatric disorders (Alonso-Alonso et al, 2007; Brunoni et al, 2012; Allman et al, 2016; Kuo et al, 2017; Pontillo et al, 2018), and may improve cognition in healthy subjects (Coffman et al, 2014; Current-Mapping and BOLD During tDCSBerryhill and Martin, 2018). Computational models have shown that focal current flow distributions can be achieved by employing relatively complex montages (compared to the conventional two-electrode montage), the resultant current-flow patterns are more susceptible to individual differences in anatomy (Mikkonen et al, 2020). Even as these models are validated to a certain degree (Edwards et al, 2013; Huang et al, 2017), clarifying the role of individual anatomy in shaping complex current flow patterns remains a challenge. It is critical to map these fields in individual subjects as modeling studies suggest that the current distribution depends on the detailed geometry of individual brains (Opitz et al, 2015)
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