Interictal electric source imaging with open-source software and routine low-density electroencephalography: A systematic comparison of four inverse methods in drug-resistant epilepsy.

OBJECTIVE: This study was undertaken to assess the diagnostic accuracy of interictal electrical source imaging (ESI) performed using open-source software and routine low-density electroencephalography (LD-EEG), comparing four inverse methods at two spike time points. METHODS: We retrospectively included patients with drug-resistant epilepsy (DRE) who underwent epilepsy surgery, had at least 1 year of postoperative follow-up, and underwent routine LD-EEG. Interictal ESI was performed in Brainstorm using four inverse methods-standardized low-resolution electromagnetic tomography (sLORETA), linearly constrained minimum variance (LCMV) beamformer, minimum norm (MN), and Equivalent Current Dipole (ECD)-at two time points: the half-rising phase and the peak amplitude of averaged spikes. Concordance with the postsurgical cavity and Engel class I outcome were used as the reference standard for evaluating diagnostic performance. Intermethod concordance was assessed at the diagnostic, lobar, and sublobar levels. RESULTS: Seventy-nine patients were included (Engel I, 72.2%). ESI at the half-rising phase was significantly more accurate than at peak amplitude for sLORETA (p = .012) and ECD (p < .001). sLORETA at the half-rising phase achieved the highest diagnostic performance (accuracy 81.0%, sensitivity 82.5%, specificity 77.3%), significantly outperforming MN (p = .003). Diagnostic accuracy of sLORETA at the half-rising phase was maintained across subgroups, including patients with extratemporal epilepsy (84.6%) and nonlesional magnetic resonance imaging (80.8%) and patients requiring stereo-EEG (80.0%). Diagnostic agreement among the four methods was substantial, with sLORETA, LCMV beamformer, and ECD forming a high-concordance cluster (Cohen κ = .842-.857). Agreement was substantial at the lobar level (κ = .755) and moderate at the sublobar level (κ = .465). SIGNIFICANCE: Interictal ESI performed with open-source software and routine LD-EEG achieves high diagnostic accuracy for localizing the presumed epileptogenic zone in DRE, with robust results across traditionally challenging populations. These findings support the broader clinical adoption of ESI as a valuable tool in presurgical evaluation.