The combination of EEG source imaging and EEG-correlated functional MRI to map epileptic networks

Functional electrophysiologic techniques such as electroencephalography (EEG) and magnetoencephalography (MEG) give insights into the dynamics of the networks involved in the generation of interictal and ictal epileptic activity and their interaction with physiologic brain activity. With recent advances in methodology and clinical validation, EEG source imaging (ESI) may now be used to map epileptic activity as well as evoked responses to external stimuli. By its ability to show hemodynamic changes time locked to epileptic activity in the whole brain, EEG-correlated functional magnetic resonance imaging (fMRI) (EEG-fMRI) is the natural counterpart of ESI, circumventing some of its limitations, the former adding data from the depths of the brain, and the latter temporal resolution. To better understand the potential and limitations of both techniques, this review starts with a description of the neurophysiologic mechanisms that give rise to the measured signals, followed by validation studies based on comparison with intracranial EEG and surgical outcome. We then discuss analysis strategies to combine both techniques by reviewing studies in epilepsy, current methodologic development, and future directions of this fast-developing field.