Electromagnetic source imaging (ESI) is a model-based neuroimaging technique. It entails: 1) modeling the brain electrical activity, 2) modeling the head volume conduction process so as to link the modeled electrical activity to EEG/MEG, and 3) reconstructing the brain electrical activity from recorded EEG/MEG data. The first two modeling steps serve to solve the EEG/MEG forward problem; the third step is the inverse of the second step, thereby commonly known as the EEG/MEG inverse problem.
