Magnetoencephalography

Magnetoencephalography

Magnetoencephalography (MEG) is a method used to map brain activity by capturing magnetic fields generated by natural electrical currents within the brain, utilizing arrays of SQUIDs (superconducting quantum interference devices). MEG finds application in fundamental research on perceptual and cognitive brain functions, pinpointing regions affected by pathology prior to surgical intervention, identifying the roles of different brain regions, and providing neurofeedback.

The brain's magnetic field arises from synchronized neuronal currents, producing weak magnetic fields measuring 10 femtotesla (fT) for cortical activity and 10^3 fT for the human alpha rhythm. Compared to the typical urban magnetic noise of approximately 10^8 fT or 0.1 µT, the brain's magnetic field is significantly smaller. Biomagnetism faces the challenge of detecting signals amidst this weak signal-to-noise ratio.

Brain-generated electric currents also produce EEG signals. Both MEG and EEG signals stem from the net effect of ionic currents flowing within neurons' dendrites during synaptic transmission. In line with Maxwell's equations, any electric current generates a magnetic field orthogonally oriented to it, which is then measured. These net currents can be envisioned as electric dipoles—currents possessing position, orientation, and magnitude but lacking spatial extent. As per the right-hand rule, a current dipole generates a magnetic field circulating around its vector axis.

Approximately 50,000 active neurons are required to generate a detectable signal. Pyramidal cells, situated perpendicular to the cortical surface, predominantly contribute to measurable magnetic fields. These neurons, oriented tangentially to the scalp surface, emit measurable portions of their magnetic fields outside the head, typically within the sulci. Although researchers are exploring signal processing techniques to detect deep brain signals, no clinically useful method currently exists.

Action potentials generally do not produce observable fields because their associated currents flow in opposing directions, resulting in canceled-out magnetic fields. However, action fields have been measured from peripheral nerves.

Due to the brain's emitted magnetic signals being in the femtotesla range, shielding from external magnetic fields, including the Earth's magnetic field, is imperative. Adequate shielding can be achieved through aluminum and mu-metal construction for reducing high-frequency and low-frequency noise, respectively.

The primary challenge of MEG lies in determining the location of electric activity within the brain based on the induced magnetic fields outside the head. This problem, known as an inverse problem, lacks a unique solution, making the identification of the "best" solution a subject of ongoing research. Localization algorithms utilizing given source and head models aim to identify plausible locations for underlying focal field generators.

MEG and EEG signals originate from the same neurophysiological processes but exhibit notable differences. MEG offers better spatial resolution than EEG due to magnetic fields being less distorted by the skull and scalp. While EEG detects activity in both the sulci and cortical gyri, MEG selectively measures activity in the sulci. MEG is particularly sensitive to superficial cortical activity, making it useful for studying neocortical epilepsy. Additionally, MEG is reference-free, unlike scalp EEG, which relies on an active reference, complicating data interpretation.