The Glossary provides short definitions of vocabulary specific to the MEG and general neuroimaging concepts. Many entries are taken from MNE-Python.

anatomical landmark๐Ÿ”—
anatomical landmarks๐Ÿ”—
fiducial point๐Ÿ”—
fiducial points๐Ÿ”—

Fiducials are objects placed in the field of view of an imaging system to act as known spatial references that are easy to localize. In neuroimaging, fiducials are often placed on anatomical landmarks such as the nasion (NAS) or left/right preauricular points (LPA and RPA).

These known reference locations are used to define a coordinate system for localizing sensors (hence NAS, LPA and RPA are often called โ€œcardinal pointsโ€ because they define the cardinal directions of the head coordinate system). The cardinal points are also useful when co-registering measurements in different coordinate systems.

Due to the common neuroimaging practice of placing fiducial objects on anatomical landmarks, the terms โ€œfiducialโ€, โ€œanatomical landmarkโ€, and โ€œcardinal pointโ€ are often (erroneously) used interchangeably.

boundary element model๐Ÿ”—
boundary element method๐Ÿ”—

BEM is the acronym for boundary element method or boundary element model. Both are related to the definion of the conductor model in the forward model computation. The boundary element model consists of surfaces such as the inner skull, outer skull, and outer skin (scalp) that define compartments of tissues of the head.

Continuous Head Position Indicator๐Ÿ”—
Head Position Indicator๐Ÿ”—

Head position indicators (HPI, sometimes cHPI for continuous head position indicators) are small coils attached to a subjectโ€™s head during MEG acquisition. Each coil emits a sinusoidal signal of a different frequency, which is picked up by the MEG sensors and can be used to infer the head position.

Internal Helium Recyler๐Ÿ”—

The internal helium recycler collects helium gas from the MEG dewar passively during the day and liquefy it for reuse at night. Measurements must not be conducted during liquefaction.

superconducting quantum interference device๐Ÿ”—

A superconducting quantum interference device (SQUID) is a type of magnetometer that uses superconducting materials to sense magnetic fluctuations. Standard low-temperature SQUID sensors typically found in MEG systems operate at temperatures within a few degrees of absolute zero (e.g., below \(4 K\)).

signal space projector๐Ÿ”—
signal space projectors๐Ÿ”—

A projector, also referred to as Signal Space Projection (SSP), defines a linear operation applied spatially to MEG data. A matrix multiplication of an SSP projector with the data will reduce the rank of the data by projecting it to a lower-dimensional subspace.