Modern MEG systems now provide hundreds of channels that can provide whole-head coverage (See ). This makes it possible to map activity throughout the cerebral cortex and is critical for presurgical mapping of language areas, and for detecting propagating or widespread epileptic activity. Due to interference from extraneous magnetic fields, all MEG measurements must be performed in a magnetically shielded room, which typically consists of two to four layers of aluminum and multiple layers of ferromagnetic shielding.
Typical modern MEG device positioned in the upright position facing a back projection screen.
Unlike other hemodynamic techniques (fMRI and PET), both MEG and electroencephalography (EEG) directly measures electric brain activity. The neural generators of the MEG and EEG are identical. There are critical differences that make them both useful and complementary. MEG preferentially detects activity in superficial, non-radial areas of cortex, such as the fissural cortex of the cerebral hemispheres. This is particularly advantageous if the area of the area of interest is also radial, such as the primary somatosensory cortex, which lies in the walls of the sulci.
Much of the neural activity measured by MEG originates as the post-synaptic activity in the pyramidal cells of the cerebral cortex. MEG measures the vector sum of post-synaptic potentials, as contrasted with BOLD fMRI and some form of PET imaging that reflects neural activity indirectly through changes in blood flow. MEG localizes neural activity more accurately than EEG because magnetic fields are less perturbed than electrical potentials by overlying brain structures: scalp, skull, cerebrospinal fluid, meninges, and vascular structures. Recently, statistical combination of structural MRI, functional MRI, with MEG have taken a great stride forward by yielding the maximum benefit from each technique into a single image [3
The calculation of the magnetic field is more straightforward than that of the electric field because of the symmetries and conductivity distribution of the human head. Since the EEG also is influenced by the extracellular volume current, which are difficult to model accurately. All currents, both intracellular and extracellular, generate magnetic fields, but, due to the near spherical shape of the head, one can calculate the resultant magnetic fields due to primary currents without taking into account the conductivity layers of the head.