36. Influence of data processing pipelines on EEG-fMRI results in patients with epilepsy

Abstract

Simultaneous EEG-fMRI is increasingly used for the noninvasive pre-surgical evaluation of epileptic patients to localize the epileptogenic zone. In this retrospective study of EEG-fMRI data in patients with pharmacoresistant epilepsy, we compared a wide range of data processing strategies using validation with resection masks after successful epilepsy surgery. The aims of this study were to find how various data processing strategies influence EEG-fMRI results, and to identify the best approach for data processing. Thirteen subjects (9 F, 4 M) with pharmacoresistant epilepsy and good outcome after epilepsy surgery were included in the study. Simultaneous EEG-fMRI data (1.5 T scanner) was acquired before the surgery (300 scans per session, TR = 3 s). The position of interictal discharges (IED) was marked and used as onsets for event-related regressors in statistical model. SPM8 software was used for data processing. In total, 240 statistical analyses were calculated for each subject comprising all possible combinations of the used variants of preprocessing and GLM settings. The resection mask was created individually for each patient using clinical MR images acquired 3 months after the surgery. Several parameters (e.g. sensitivity, cosine criterion) were calculated for each dataset and processing pipeline to evaluate the concordance between spike-informed EEG-fMRI results and the resection mask. Multivariate statistical analysis was performed in SPSS software. We found that the preprocessing type (mainly basic pipeline vs. correction for cardiac artifact) does not affect the results. The study revealed two main findings. The first is the optimal processing pipeline – only canonical HRF as a basis function, IED stimulation time series shifted 2 s earlier than positions from EEG description, and massive filtering of artifact (24 movement regressors, signals from white matter and CSF, and global signal). The second finding is related to a more general understanding of the influence of various processing options on results. The superiority of canonical HRF over more flexible basis functions is probably due to our concordance measure, which is based on a single epileptic focus represented with a resection mask, and some type of similarity between the activation map and the mask. The finding of earlier BOLD responses is in concordance with the predominantly prespiking character of the BOLD response presented in previous studies.