MEG · Grey Matter

MEG (magnetoencephalography) records the tiny magnetic fields that the brain's electrical currents produce outside the head, and it is used to map where and when activity happens with millisecond timing, most often to locate the source of epileptic activity before surgery.

MEG. MEG (magnetoencephalography) records the tiny magnetic fields that the brain's electrical currents produce outside the head, and it is used to map where and when activity happens with millisecond timing, most often to locate the source of epileptic activity before surgery.

The same synchronous currents in pyramidal cells that give rise to the EEG also produce a magnetic field, of the order of tens to hundreds of femtotesla at the scalp (about a billionth of the Earth's field). Magnetic fields pass through the skull and scalp almost undistorted, which is MEG's main advantage over EEG, but they are so weak that the recording has to be made in a magnetically shielded room. MEG sees mainly currents that run parallel to the scalp (in the walls of the cortical folds) and is nearly blind to radial ones, the reverse of EEG's bias.

Classic MEG uses SQUIDs, superconducting sensors cooled by liquid helium in a fixed helmet a few centimetres from the head. The subject must stay still, and the gap to the scalp costs signal.

OPM-MEG uses optically pumped magnetometers: small sensors in which a laser reads how a vapour of alkali atoms responds to a magnetic field. They need no cryogenics, sit directly on the scalp in a wearable helmet, and move with the head; the first demonstration of a wearable system was published in 2018. Being closer to the brain, they pick up stronger signals than a SQUID helmet.

MEG shares EEG's fundamental limit. Many different arrangements of currents inside the head produce the same field outside it, so every map MEG makes rests on a model that picks one answer.

MEG trades the skull's blur for the room's noise.

The field passes through bone cleanly, so the hard part moves from the head to shielding out the magnetic noise of the world around it.

Questions: What does MEG see that EEG does not, and the other way round? Both record the same synchronous currents of pyramidal cells, but the skull treats them differently: it smears the electric potential that EEG measures and leaves the magnetic field almost untouched, so MEG maps sources more sharply. MEG is mostly blind to currents pointing straight out of the head (the crowns of the cortical folds) and sees best those running parallel to the scalp (the walls of the folds), while EEG picks up both orientations. EEG is cheap and portable; MEG needs a shielded room and, until optically pumped sensors arrived, a helium-cooled helmet. Recording both together gives two views of the same sources and constrains the inverse problem better than either alone. Why can a full helmet of MEG sensors still not say for certain where a signal came from? The answer is not unique. Helmholtz proved in 1853 that the currents inside a conductor cannot be recovered from the fields outside it, since some current patterns produce no external field and can be added to any solution unseen. A few hundred sensors facing many thousands of possible source locations leave infinitely many current maps that fit the data equally well. MEG source maps therefore come from adding assumptions (a few point sources, the smallest total current, sources confined to the cortical sheet of the subject's MRI), and different assumptions can place the same signal differently, especially for deep sources.