Imaging of current flow in the human head during transcranial electrical therapy

Abstract

Background It has been assumed that effects caused by tDCS or tACS neuromodulation are due to electric current flow within brain structures. However, to date, direct current density distributions in the brains of human subjects have not been measured. Instead computational models of tDCS or tACS have been used to predict electric current and field distributions for dosimetry and mechanism analysis purposes. Objective/Hypothesis We present the first in vivo images of electric current density distributions within the brain in four subjects undergoing transcranial electrical stimulation. Methods Magnetic resonance electrical impedance tomography (MREIT) techniques encode current flow in phase images. In four human subjects, we used MREIT to measure magnetic flux density distributions caused by tACS currents, and then calculated current density distributions from these data. Computational models of magnetic flux and current distribution, constructed using contemporaneously collected T1-weighted structural MRI images, were co-registered to compare predicted and experimental results. Results We found consistency between experimental and simulated magnetic flux and current density distributions using transtemporal (T7–T8) and anterior-posterior (Fpz-Oz) electrode montages, and also differences that may indicate a need to improve models to better interpret experimental results. While human subject data agreed with computational model predictions in overall scale, differences may result from factors such as effective electrode surface area and conductivities assumed in models. Conclusions We believe this method may be useful in improving reproducibility, assessing safety, and ultimately aiding understanding of mechanisms of action in electrical and magnetic neuromodulation modalities.

Convolutional neural network MRI segmentation for fast and robust optimization of transcranial electrical current stimulation of the human brain

“…Effects on neuronal excitability depend on the electric field (E-field) distribution [2]. However, and despite recent advances in in-vivo measuring techniques [3], the only approach to predict the E-field in the head with a high spatial resolution is via numerical methods, such as the finite element method (FEM) [4]. Such FEM requires geometric information about the tissues in the head, which is obtained from structural images, such as MRIs [5,6], and relies on knowledge about the electrical conductivity of the different tissues in the brain and the electrode montage (position, geometry and currents) [7].…”

“…This dataset includes 422 3D T1-w MRI scans from the ADNI dataset 2 with 166x256x256 voxels and size 1.2x0.9x0.9 mm 3 and also 108 3D T1-w MRI scans from the IXI dataset (see 3 for details of scanner parameters) with 256x256x150 voxels and size 0.9x0.9x1.2 mm 3 . Both datasets are normalized to an intensity range of 0 to 255 and converted to 256 3 dimensions and 1 3 mm 3 voxel size.…”

A flexible workflow for simulating transcranial electric stimulation in healthy and lesioned brains

“…Performing simulation studies based on such data can 52 become difficult due to challenges in the segmentation of low-contrast tissue such as skull using 53 standard segmentation approaches. Following the image segmentation, a surface-based meshing 54 approach is commonly used to create the head volume mesh. The advantage of this approach is a 55 maximum of control over the approximation of the boundaries of the sub-compartments of the head 56 model, which, on the other hand, must not intersect, restricting the topology of the included structures 57 and complicating the inclusion of irregular tissue such as lesioned tissue.…”

“…Promising approaches are electrical current density measurements obtained by the means of magnetic 590 resonance electrical impedance tomography (54) or in-vivo recordings of the electrical potential by 591 intracranial electrodes. TDCS simulations have been validated using intracranial recordings of epilepsy 592 patients before (55), (56).…”

Evaluation of magnetohydrodynamic effects in magnetic resonance electrical impedance tomography at ultra‐high magnetic fields

“…In MREIT, electric currents are injected into body through surface electrodes, in coordination with pulse sequences . Over the last decade, MREIT has advanced from phantom studies, to canine whole body imaging with disease models, to in vivo human imaging of lower leg and knee, to current flow and conductivity in the human brain . All these experiments have been aimed at clinical translation and were primarily performed at main magnetic field strengths of 3T.…”