1. Image Selection T1 and T1 phantom images based on colin27 are used Segmentation Segmentation was performed using BrainSuite Finite Element Mesh Generation Volume meshes suitable for Finite Element calculations were generated using mimics Electric Field Calculation The electric field calculations were performed using Comsol The Electric Field on the Cortical Surface of the Human Brain during tCS A. Mekonnen 1, M. Lu 1, R. Salvador 1, G. Ruffini 2, P. C. Miranda 1 1 Institute of Biophysics and Biomedical Engineering, Faculty of Science, University of Lisbon, Lisbon, Portugal; 2 Starlab Barcelona, Barcelona, Spain. Introduction Transcranial current stimulation (tCS) is a noninvasive brain stimulation technique that has been shown to modulate the cortical functions in humans ([1]). The level of modulation by tCS depends on the induced electric field in the brain. The spatial distribution of the electric field in the brain, in particular the electric field magnitude and direction in the cortical sheet are important factors in understanding stimulus intensity and localization. We implemented a realistic Finite Element (FE) head model based on MR images to investigate the effect of tissue heterogeneity and electrode size on the electric field distribution. Two types of electrodes are investigated : (1) 50 mm square electrode, area 25 cm 2, height 6 mm; (2) 11 mm diam. circular electrode, area 1.0 cm 2, height 2 mm. Methods The realistic FE head was built in four steps: Results 1. Effects of tissue inhomogeneity : 2. Effects of Electrode size : 3. Superimposition on anatomy: Acknowledgments: This work was supported by the Foundation for Science and Technology (FCT), Portugal and project HIVE. The project HIVE acknowledges the financial support of the Future and Emerging Technologies (FET) programme within the Seventh Framework Programme for Research of the European Commission, under FET-Open grant number: 222079. Fig. 1. Finite element mesh of the head model: (a): scalp and electrodes; (b): interface between GM and CSF. In the model, the x axis points in the left to right direction, the y axis in the posterior-anterior direction and the z axis in the inferior-superior direction. Fig. 2. Spatial distribution of the magnitude of the electric field in a sagittal slice located under the large anode. The image in (a) includes all the tissues in the model as well as the anode. In (b) the electric field is shown only in the GM and WM. In these calculations, CSF = 1.79 S/m, GM = 0.33 S/m, GM = 0.15 S/m. In (c) CSF = GM = 0.33 S/m and in (d) CSF = WM = GM = 0.33 S/m. The color bars represent the norm of the electric field, in V/m. Fig: 3. Electric field distribution on the GM-CSF interface : the top row shows the normal component of the electric field whereas the bottom row shows its tangential component. The first column shows the electric field due to the 25cm 2 electrode whereas the data in the second row pertains to the 1 cm 2 circular electrode. Note the use of different scales in the plots. a b Discussion Fig. 4. The distribution of the magnitude of the electric field in the gray and white matter on a sagittal slice (x=56, near the hand knob) overlaid on the anatomical image (Colin27) in MRIcro, (a) for the square electrode, (b) for the circular electrode. The analysis of the electric field distribution within the gray matter and the white matter showed that the maxima of the electric field magnitude occur at localized hot spots at the bottom of the sulci, further away from the electrodes. In addition, the electric field in the cortex under the stimulation electrode has a strong tangential component. This is a significant departure from earlier findings that use spherical head models in which the maxima always appear close to the electrodes and where the electric field is predominantly normal to the brain surface. The difference is attributed to the high conductivity of the CSF and also to the fact that the cortex is highly convoluted. The 1.0 cm 2 circular electrode produced a more focal electric field distribution than the 25 cm 2 square electrode, but in both cases the high electric field region is not limited to the vicinity of the electrode. A method was devised to superimpose the electric field or the current density distributions on anatomical images, using MRIcro. As the user navigates through the brain the corresponding field data is displayed as an overlay. Conclusion The distinctive feature of this model is an accurate representation of the cortical sheet and of the CSF that fills its fissures. Using this model it is possible to investigate the complexity of the combined effects of tissue heterogeneity and the convoluted shape of the cortex on the electric field distribution in the brain. References [1] M A Nitsche, W Paulus, Excitability changes induced in the human motor cortex by weak transcranial direct current stimulation, J Physiol, 527(3): 633-9, 2000. A square 25 cm 2 cathode was placed over the right eyebrow and the anode, either a square 25 cm 2 or a circular 1.0 cm 2 electrode, was placed over M1. The injected current was always set to 1 mA. These results show that tissue heterogeneity has a considerable impact on the electric field distribution. When the conductivities of the intracranial tissues are set to the same value, the distribution becomes more like the one observed in spherical models. ab a |E n | c |E t | d b |E n | a b cd