Independent component analysis reveals dynamic ictal BOLD responses in EEG-fMRI data from focal epilepsy patients.
BACKGROUND
Seizures occur rarely during EEG-fMRI acquisitions of epilepsy patients, but can potentially offer a better estimation of the epileptogenic zone than interictal activity. Independent component analysis (ICA) is a data-driven method that imposes minimal constraints on the hemodynamic response function (HRF). In particular, the investigation of HRFs with clear peaks, but varying latency, may be used to differentiate the ictal focus from propagated activity.


METHODS
ICA was applied on ictal EEG-fMRI data from 15 patients. Components related to seizures were identified by fitting an HRF to the component time courses at the time of the ictal EEG events. HRFs with a clear peak were used to derive maps of significant BOLD responses and their associated peak delay. The results were then compared with those obtained from a general linear model (GLM) method. Concordance with the presumed epileptogenic focus was also assessed.


RESULTS
The ICA maps were significantly correlated with the GLM maps for each patient (Spearman's test, p<0.05). The ictal BOLD responses identified by ICA always included the presumed epileptogenic zone, but were also more widespread, accounting for 20.3% of the brain volume on average. The method provided a classification of the components as a function of peak delay. BOLD response clusters associated with early HRF peaks were concordant with the suspected epileptogenic focus, while subsequent HRF peaks may correspond to ictal propagation.


CONCLUSIONS
ICA applied to EEG-fMRI can detect areas of significant BOLD response to ictal events without having to predefine an HRF. By estimating the HRF peak time in each identified region, the method could also potentially provide a dynamic analysis of ictal BOLD responses, distinguishing onset from propagated activity.
LeVan
Pierre
P
Montreal Neurological Institute, McGill University, 3801 University Street, Montreal, QC, Canada. pierre.levan@mail.mcgill.ca
Tyvaert
Louise
L
Moeller
Friederike
F
Gotman
Jean
J
D000328:Adult; D016477:Artifacts; D001284:Atrophy; D001921:Brain / Q000033:anatomy & histology*; D001923:Brain Chemistry; D001932:Brain Neoplasms / Q000473:pathology / Q000556:secondary; D002540:Cerebral Cortex / Q000002:abnormalities / Q000473:pathology; D002648:Child; D054221:Classical Lissencephalies and Subcortical Band Heterotopias / Q000473:pathology; D003627:Data Interpretation, Statistical; D004569:Electroencephalography / Q000706:statistics & numerical data*; D004828:Epilepsies, Partial / Q000097:blood / Q000473:pathology*; D005260:Female; D005625:Frontal Lobe / Q000473:pathology; D006801:Humans; D007091:Image Processing, Computer-Assisted / Q000706:statistics & numerical data; D016014:Linear Models; D008175:Lung Neoplasms / Q000473:pathology; D008279:Magnetic Resonance Imaging / Q000706:statistics & numerical data*; D008297:Male; D008875:Middle Aged; D010100:Oxygen / Q000097:blood*; D025341:Principal Component Analysis; D012640:Seizures / Q000473:pathology; D055815:Young Adult
D010100:Oxygen