Individual head models were performed for each participant by using a combination of freely available software tools and custom MATLAB scripts (MathWorks, Natick, MA, USA; www.mathworks.com), following a pipeline derived from (11). T1w MRI images of each participant were segmented into scalp, skull, air, cerebrospinal fluid (CSF), gray matter (GM), and WM masks using a MARS toolbox and an SPM 8 (www.fil.ion.ucl.ac.uk/spm/) segmentation toolbox (Figs. 2 and 3). Models of PITRODE PISTIM 3.14-cm2 Ag/AgCl electrodes (Neuroelectrics, Barcelona, Spain) were placed in the scalp positions corresponding to the international 10/10 electroencephalography (EEG) system. Finite-element meshes of the head and electrodes were created using Iso2Mesh, a MATLAB toolbox. The meshes were then imported to Comsol (Comsol Inc., Burlington, MA, USA; www.comsol.com), where the E-field calculations were performed. The manually segmented masks of the edema, necrotic core, and solid tumor were used to define the mesh nodes that belong to each of these tissues and used to change the electrical conductivity of the tissues to match that of the lesion. All healthy brain tissue nodes were assigned electrical conductivity values appropriate to DC low-frequency electrical currents: 0.330 S/m (scalp), 0.008 S/m (skull), 10−5 S/m (air), 1.790 S/m (CSF), 0.400 S/m (GM), and 0.150 S/m (WM). The conductivity of the necrotic core nodes was set to that of CSF. The conductivities of the solid tumor and edema nodes were defined on the basis of the average WM and GM conductivity values: half of this average conductivity in the solid tumor nodes (0.138 S/m) and two-thirds higher in the edema nodes (0.458 S/m; see Figs. 2 and 3 for examples), according to prior similar modeling work (38). In three participants, a reevaluation was performed after neurosurgery. Skull defects created by the surgical intervention were manually segmented on T1w images with the guide of the skull CT rendering. Head model regions representing skull defects were assigned a conductivity equal to that of CSF (Fig. 3).

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