Magnetoencephalography (MEG) is the measurement of the magnetic fields produced by electrical activity in the brain, usually conducted externally, using extremely sensitive devices such as SQUIDs. Because the magnetic signals emitted by the brain are on the order of a few femtotesla (1 fT = 10 - 15T), shielding from external magnetic signals, including the Earth's magnetic field, is necessary. An appropriate magnetically shielded room can be constructed from Mu-metal, which is effective at reducing high-frequency noise, while noise cancellation algorithms reduce low-frequency common mode signals.
Modern whole-head systems have roughly 300 channels, and have a noise floor of around 5 to 7 fT above 1 Hz. The overall magnetic field of the brain is typically around 100 to 1000 fT, while signals from individual neurons are much weaker, generally well below the noise floor. The signals themselves derive from ionic currents flowing passively in the dendrites of neurons.
MEG is a relatively new technique that promises good spatial resolution and extremely high temporal resolution , thus complementing other brain activity measurement techniques such as Electroencephalography (EEG), Positron emission tomography (PET), and functional Magnetic Resonance Imaging (fMRI). The primary technical difficulty with MEG is that the problem of inferring charge motions in the brain from magnetic measurements outside the head (the "inverse problem") is ill posed, and is itself the subject of intensive research. The clinical uses of MEG are in detecting and localizing epileptiform spiking activity in patients with epilepsy, and in localizing eloquent cortex for surgical planning in patients with brain tumors.
See also
Electrophysiology, Functional neuroimaging, Magnetocardiography , Magnetometer, Magnetic source imaging , Mu-metal