Electromagnetic source imaging (ESI), is the reconstruction of the spatiotemporal activation of brain sources from magnetoencephalography (MEG) and electroencephalography (EEG) measurements. MEG is the measurement of tiny magnetic fields, created by neural activity in the brain, non-invasively outside the head. MEG measurements are achieved by superconducting quantum interference devices (SQUIDs), that have adequate sensitivity to detect tiny magnetic fields (on the order of femto-Teslas). EEG is the measurement of electric potentials, due to neural activity in the brain, on the scalp. ESI is enabled by sophisticated mathematical algorithms for reconstruction of brain activity from MEG and EEG measurements and visualized by overlaying such activations on structural magnetic resonance images. In contrast to other functional brain techniques that measure indirect hemodynamic and metabolic changes due to neuronal activity, such as functional magnetic resonance imaging (fMRI) and positron emission tomography (PET), MEG and EEG measurements offer the unique capability of measure direct neural activity in the millisecond time-scale with high temporal resolution. ESI is unique among function imaging techniques for its ability to provide spatiotemporal brain activation profiles that reflect not only where activity occurs in the brain but also when this activity occurs in relation to the presentation of the external stimulus and the activity in other brain regions.
ESI can be integrated with neuro-navigational systems to provide intraoperative guidance to the surgical team. ESI mapping has been shown to be important in preoperative planning and complements intraoperative mapping by delineating retained areas of function non-invasively and in advance, reducing the time needed for intraoperative mapping.