Magnetoencephalography (MEG) is the measurement of the magnetic activity of the brain, usually conducted externally, using extremely sensitive devices such as SQUIDs. MEG needs complex and expensive measurement settings. Because the magnetic signals emitted by the brain are on the order of a few femtoteslas (1 fT = T), shielding from external magnetic signals, including the Earth's magnetic field, is necessary. An appropriate magnetically shielded room is very expensive, and constitutes the bulk of the expense of an MEG system.
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 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 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.