Lecture 19 MEG Flashcards
(33 cards)
What does MEG measure?
Magnetic activity from neural currents, primarily cortical neurons
How is MEG different from EEG in terms of signal distortion?
1) Magnetic field permeates brain tissue, cerebrospinal fluid (CSF), skull, scalp, and air with little distortion
2) High permeability allows us to measure the magnetic field activity without contacting the scalp
What kind of environment does MEG require?
A magnetically shielded room and a super-high-fidelity device
What is a SQUID?
Superconducting Quantum Interference Device
* SQUIDs essentially act as amplifiers for the magnetic signal, with virtually no noise and high gain (Josephson effect in superconductivity)
How is the SQUID kept superconductive?
- By cooling the coil close to absolute zero (~0 kelvin) superconductivity occurs
- To achieve superconductivity, liquid helium (-269°C) is used to cool the SQUID sensors
What principle allows SQUIDs to detect magnetic signals?
Faraday’s law of induction and the Josephson effect
What is a magnetometer?
A single-loop coil that detects magnetic field strength but is sensitive to noise
What is a planar gradiometer?
A figure-8 coil that adds signals from the same source and subtracts noise
What is an axial gradiometer?
Two loops 50 mm apart that detect tangential sources and reduce distant noise
Why is a dewar used in MEG?
To house and thermally insulate the SQUID sensors
Why don’t MEG sensors need to contact the scalp?
Because magnetic flux penetrates tissue and air with little distortion
Why must participants be metal-free in MEG?
To avoid artifacts or safety issues due to magnetic field sensitivity
What are HPI coils used for?
To detect head movement (yaw, pitch, roll) during MEG
How are anatomical landmarks recorded in MEG?
Using a magnetic pen digitizer on fiducial points (e.g., nasion, preauricular)
What are OPMs in MEG?
Optically Pumped Magnetometers, newer sensors that don’t require cooling， can be mounted flexibly in a lightweight helmet to fit any head shape
What is the benefit of OPMs over SQUIDs?
Closer to scalp, flexible fit, higher sensitivity, no cryogenics needed
What generates the MEG signal?
Postsynaptic currents in apical dendrites of pyramidal neurons
What type of dipoles is MEG most sensitive to?
Tangential dipoles, such as those in cortical sulci
What type of dipoles is EEG most sensitive to?
Radial dipoles, such as those in cortical gyri
Why is MEG good for source localization?
Magnetic fields are less distorted by tissue and bone
What is the inverse problem in MEG?
A given MEG signal may correspond to multiple source configurations
How does signal depth affect MEG recordings?
Deeper sources produce weaker MEG signals
What affects sensor sensitivity in MEG?
Source orientation and distance from sensors
How many sensors can modern MEG systems have?
Up to 300, arranged in a helmet to cover the whole head
