← Back to Course Home

Week 09: Magnetoencephalography (MEG)

Magnetoencephalography measures the minute magnetic fields ($10-100\text{ fT}$) produced by synchronous intracellular neuronal currents. Unlike electrical potentials, magnetic fields pass undistorted through the skull. This week covers SQUIDs, magnetically shielded rooms, gradiometer coils, and source localization.

Interactive Demos & Activities

• Interactive SQUID Magnetometer vs. Gradiometer Simulator

  Signal Processing Simulator · Noise Cancellation

  Experience why gradiometer configurations (two opposing coils) are critical for measuring femtotesla neural signals: observe how uniform ambient magnetic noise is subtracted out while near-field cortical dipole signals are preserved.

• Vector Field & Curl Explorer

  Electromagnetic Principles · Biot-Savart Law

  Visualize vector fields and rotational curl ($\nabla \times \vec{B}$), reinforcing the right-hand rule and magnetic field lines generated by primary intracellular dendritic currents.

• MEG Modality Card Game

  Educational Card Game · SQUIDs & Source Localization

  Review superconducting quantum interference devices (SQUIDs), helium cryogenics, magnetically shielded rooms, and presurgical language mapping.

Topics Covered

• The Right-Hand Rule & Primary vs. Volume Currents: Why MEG is predominantly sensitive to tangential current dipoles in the sulci, whereas EEG detects both radial (gyral) and tangential sources.
• Cryogenic SQUIDs & Shielding: Liquid helium cooling (4 Kelvin), physical flux transformers, and multi-layer mu-metal magnetically shielded rooms (MSR).
• The Inverse Problem & Source Reconstruction: Helmholtz's ill-posed inverse problem, equivalent current dipoles (ECD), minimum norm estimation (MNE), and beamforming.