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.

Frontal tDCS modulates orbitofrontal reality filtering

“…The main objective of the model was to predict whether significant current reached our region of interest for this study, i.e., the posterior medial OFC. A finite-element model was generated using previously described and validated protocols Truong et al, 2012Truong et al, , 2013Turkeltaub et al, 2012;Edwards et al, 2013;Antal et al, 2014). The model was based on a high-resolution magnetic resonance imaging of an adult male at 1-mm  1-mm  1-mm resolution, and segmented according to the following isotropic conductivity values: skin, fat, skull, cerebrospinal fluid, gray matter, white matter, or air (Soterix Medical, New York, NY, USA).…”

Technique and Considerations in the Use of 4x1 Ring High-definition Transcranial Direct Current Stimulation (HD-tDCS)

“…Several approaches have been investigated in order to "shape" current flow by varying inter-electrode distance 5 and increasing/decreasing pad size to decrease/increase modulation in cortical regions under the electrode 6 . Nevertheless, efforts to further target current flow while avoiding shunting of current between electrodes 7,8 remain of interest.…”

Limitations of ex vivo measurements for in vivo neuroscience

“…With the advent of powerful in vivo imaging methods (2), the study of cadavers has become less important in modern-day neuroscience research. However, cadavers are still regularly used to examine possible methodological artifacts in brain imaging studies because of the complete absence of neuronal activity with largely preserved anatomical structures (3,4). Numerous studies have relied on cadavers to establish the conductivity of differing tissue types, with notable discrepancies compared with in vivo measurements (5)(6)(7)(8).…”