Imaging artifacts induced by electrical stimulation during conventional fMRI of the brain

Abstract

Functional magnetic resonance imaging (fMRI) of brain activation during transcranial electrical stimulation is used to provide insight into the mechanisms of neuromodulation and targeting of particular brain structures. However, the passage of current through the body may interfere with the concurrent detection of blood oxygen level-dependent (BOLD) signal, which is sensitive to local magnetic fields. To test whether these currents can affect concurrent fMRI recordings we performed conventional gradient echo-planar imaging (EPI) during transcranial direct current (tDCS) and alternating current stimulation (tACS) on two post-mortem subjects. tDCS induced signals in both superficial and deep structures. The signal was specific to the electrode montage, with the strongest signal near cerebrospinal fluid (CSF) and scalp. The direction of change relative to non-stimulation reversed with tDCS stimulation polarity. For tACS there was no net effect of the MRI signal. High-resolution individualized modeling of current flow and induced static magnetic fields suggested a strong coincidence of the change EPI signal with regions of large current density and magnetic fields. These initial results indicate that (1) fMRI studies of tDCS must consider this potentially confounding interference from current flow and (2) conventional MRI imaging protocols can be potentially used to measure current flow during transcranial electrical stimulation. The optimization of current measurement and artifact correction techniques, including consideration of the underlying physics, remains to be addressed.

Keywords: Brain; Modeling; Post-mortem; fMRI; tACS; tDCS.

Figure 1

Transcranial Direct Current Stimulation (tDCS) in a post-mortem subject 1 produces significant polarity specific gradient-EPI magnitude signal. Top row: Reconstructed masks of main tissue from 86 year old female subject with tDCS electrode montage shown. Middle: EPI t-score during AC, M1 anodal, and M1 cathodal stimulation. Bottom: Sections of EPI z-score and current flow density maps predicted using FEM analysis of M1 anodal tDCS.

Figure 2

Transcranial Direct Current Stimulation (tDCS) in a post-mortem subject 2 produces montage specific polarity specific gradient-EPI magnitude signal. Likewise montage-specific current clustering is also predicted by the FEM model. Top row: Reconstructed masks of main tissue from a 78 year old female subject with one tDCS electrode montage shown. Middle and Bottom rows: Sections of EPI t-score and current flow density maps predicted using FEM analysis for both tested montages; M1 anodal (middle) and PT anodal (bottom).

Figure 3

Comparison of correlation, r, with predicted magnetic field in Z-axis and current density over the entire head including scalp. Top: Correlation r as reported by fMRI analysis software (BrainVoyagerQX). Middle: Predicted B_z field in units of Tesla (10⁻⁹T). Bottom: Predicted current density intensity in the whole head in units of A/m2. Stimulation intensity was 1mA as in the post-mortem experiment.

Figure 4

Comparison of EPI signal during tDCS (post-mortem, no BOLD signal) with physiological BOLD response during conventional finger tapping experiment. Left: Histogram of raw EPI signal across voxels and time (only mean and linear trend have been removed). As a reference, the median EPI signal in tissue for cathodal tDCS, anodal tDCS and finger tapping was 929, 930, and 765 respectively. Right: Histogram of mean EPI difference across voxels (difference between ON/OFF, and tapping/no-tapping respectively)