Goal of this tutorial

This page is to give an intuitive understanding of how dipolar sources in the brain project to the sensor array in MEG.

Source orientation and distance to sensors

The magnetic field wraps around dipolar sources.
In the first image on the right, a single dipolar source is shown (red arrow), along with the theoretical magnetic field it projects to the magnetometer array (recorded on a 4D/BTI MEG system, from the Human Connectome Project). The color scale is such that the most yellow/blue colors correspond to a signal of 10 fT.
The dipole is located in the dorsomedial prefrontal cortex and revolves horizontally without changing position.
Sensors are almost blind to radially oriented sources.
In the second figure, a dipole located at the same location as above revolves vertically. One can note the decrease of signal when the dipole is vertical (either pointing up or down).
Field strength decreases with the square of the distance to sensors.
In the third figure, a dipole is located deeper in a posterior temporal location. The amplitude color scale is the same as above.