Head CT scans of 24 patients (10 females and 14 males, age 35.4 ± 9.25 years) undergoing epilepsy surgery were provided by the “IRCCS Neuromed” (Pozzilli, Italy) and included in this study. Before the acquisition of CT scans, patients underwent craniotomy and ECoG electrodes were placed onto their brain surface. CT images were acquired by a General Electric LightSpeed Pro 16 Multi-Slice scanner. RX tube parameters were set to 120 kV, 600 mA for a total of 37 mAs. The CT gantry was not tilted. The pixel size ranged from 0.44 to 0.98 mm, and the slice thickness was 0.625 mm.

Flexible ECoG electrode arrays (Ad-Tech Medical Instrument Corporation, 400 West Oakview Parkway Oak Creek, WI 53,154 USA) were used. Each electrode consisted of a platinum–iridium disc with a diameter of 4 mm and a thickness of about 0.5 mm. Electrodes inter-distances were 10 mm (nominal center-to-center spacing). Electrode arrays were embedded in flexible sheets and arranged either in strips or grids (see Fig. 1). The strips contained 4 (Fig. 1a), 6, 8 or 12 electrodes, while the grids were composed of matrices of 8 × 8 (Fig. 1b) or 6 × 8 electrodes. They were conveniently placed where needed, onto the frontal, temporal or parietal cortex. Electrodes were connected to the recording device via CABRIO or TECH-ATTACH cables, ending with standard 1.5 mm safety socket DIN connectors. Grid electrodes enclosed a platinum marker to identify electrode numbered as 1 (see Fig. 1b). Table S1 (the current table and all the subsequent ones are available in supplementary materials) shows the number of implanted electrodes, strips, grids and other metal objects per patient.

Sketch of the ECoG electrodes arrays: a Strip of 4 electrodes; b Grid of 8 × 8 electrodes (www.adtechmedical.com, Catalog #: IS04R-SP10X-000, Catalog #: FG64C-SP10X-0C6)

As an example, Fig. 2 shows a 3D rendering (Fig. 2a) of the CT volume of Neuromed patient #15, along with the axial (Fig. 2b), sagittal (Fig. 2c) and coronal (Fig. 2d) cut planes. ECoG electrodes are indicated by red arrows.

CT volume of Neuromed patient #15: a 3D rendering, b–d axial, sagittal and coronal cut planes. The red arrows point to electrodes. In the 3D rendering both an 8 × 8 grid electrodes and some strip electrodes are clearly visible. Cranium cuts are clearly recognizable in the axial view

A smaller patient cohort from a database of CT volumes, made available by Mayo Clinic (200 First St. SW Rochester, MN 55,905, USA) on the IEEG public repository [18], was also considered. In particular, only the studies feature a complete head volume and a resolution comparable to electrodes size. The CT volumes including only ECoG electrodes were first considered in the analysis, that is, the ones with Mayo patient IDs #12, #16, #20, #22, #26, #28, #31 (two females and five males, age 17.4 ± 13.8 years).

Moreover, the CT volumes with penetrating depth electrodes (arrays of coaxial sleeve-shaped electrodes arranged on a thin tip) were also considered, i.e., the ones with Mayo patient IDs #5, #17, #27 (one female and two males, age 33.0 ± 6.56 years). In particular, the first two include only depth electrodes, while the third includes both depth and ECoG electrodes. Table S2 reports the number of implanted ECoG electrodes, strips, grids, depth electrodes, depth contacts and other metal objects per patient.

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