A functional neuroimaging technique for mapping brain activity by recording ultra-faint magnetic fields produced by intracellular neuronal currents.
Key takeaway: While EEG is sensitive to volume conduction and records widespread cortical layers, MEG is highly sensitive to the tangentially oriented intracellular currents flowing in the cortical sulci. Magnetic fields pass through the skull and scalp mostly undistorted, granting MEG superior spatial resolution compared to EEG, while perfectly matching its millisecond-level temporal resolution.
Physiological Basis
Source Orientation (Sulci vs. Gyri)The right-hand rule of electromagnetism.
Because pyramidal neurons sitting inside a sulcus run parallel to the skull's surface, their summed intracellular currents create magnetic fields that project out toward the sensors.
Neurons located at the crest of a gyrus are oriented radially (pointing straight out). Their magnetic fields remain parallel to the scalp and are notoriously difficult for standard MEG sensors to detect.
Transparency of the SkullWhy MEG wins in spatial resolution over EEG.
Unlike electrical currents (which are heavily smeared and attenuated by the highly resistive skull), magnetic fields interact minimally with organic tissues.
This means the magnetic field pattern measured at the scalp faithfully represents the underlying neural source, greatly simplifying the "inverse problem" (calculating where the signal came from).
Hardware Technologies
SQUIDs (Superconducting Quantum Interference Devices)The traditional gold-standard sensors.
SQUIDs are exquisitely sensitive magnetometers that must be bathed in liquid helium (-269°C / 4 Kelvin).
They require large, rigid, heavily insulated helmet "dewars" that force the subject's head to be fixed via padding. Variations in head size lead to sub-optimal sensor proximity.
OPMs (Optically Pumped Magnetometers)The next generation of wearable MEG.
OPMs use a laser to interrogate the spin states of a heated vapor (usually Rubidium) which is highly sensitive to external magnetic fields.
Because they don't require cryogenic cooling, OPM sensors can be placed directly on the scalp in a flexible, 3D-printed wearable cap, improving signal dramatically and allowing limited subject movement.
Magnetic ShieldingBlocking out the modern world.
Since the brain's magnetic fields are measured in femtoteslas (10-15 T)—billions of times weaker than the Earth's magnetic field—an MEG scanner must be kept inside a multi-layer Mu-metal Magnetically Shielded Room (MSR).
Applications
Pre-Surgical MappingAvoiding eloquent cortex and pinpointing seizures.
MEG is heavily utilized in pre-surgical planning to map out language, motor, and sensory areas before tumor resection.
It is highly effective at identifying the precise origin zone of intractable epileptic spikes in 3D space, aiding the surgeon prior to invasive ECoG.
Cognitive NeuroscienceTracking network dynamics.
Often combined with structural MRI to project cortical activity onto the folded surface of the individual's exact anatomy.
Interactive SQUID Gradiometer Simulator
Why do MEG scanners use Gradiometers? Watch how an ambient magnetic noise wave (like a passing car or power line) completely overwhelms a standard magnetometer, while an axial gradiometer uses two opposing coils to perfectly subtract the distant uniform noise, revealing the tiny brain signal.
― Actual Brain Signal (femtoTesla)― Magnetometer Output (1 Coil)― Gradiometer Output (2 Coils)