Lecture 3: MEG Flashcards
MEG stands for
magnetoencephalography
Magneto in MEG means
magnetic
encephalo in MEG stands for
relating to the brain
graphy in MEG stands for
measurement
MEG uses sensors to measure small changes
in magnetic fields
The brain produces magnetic fields via (2)
- The brain produces electrical current
- Whenever we have electrical current flow (in pyramidal cells down dendrites), a magnetic field is also generated at right angles (perepndicular ; around the neuron)
Magnetic fields are also known as
‘B’ field
MEG is a non-inasive method as
changes in magnetic fields are detectable outside the head if the field is big enough
Like EEG, MEG signal gets weaker as it moves from the source
and gets further away
What are the 2 things that will affect the size of the magnetic field? - (2)
- Size of electrical current (more elec current , bigger magnetic field)
- Radius/Distance (further away from magnetic field, less able to detect it)
MEG also measures the activity of populations of neurons firing together (as still need many aligned cells to make a
dipole)
MEG is not measuring
action potentials
MEg also measures the activity of populations of neurons firing together as still need
many aligned cells to make a dipole
MEG still needs many
pyramidal cells lined up near the cortical surface to create a large enough dipole, to make a measurable magnetic field
The same activity looks different when measured with MEG vs EEG as looking at pyramdial and chuck of them being active and looking at brain activity but… - (2)
magnetic field is casued by electrical current
But has different orientaiton (90 degrees apart/perpendicular)
MEG measures the magnetic flux density which is measured in units called
Teslas (T)
MRI typically produces a 3T
magnetic field
Since the magnetic fields in the brain are very small, MEG usually measured in
femto Teslas (fT) - 10 to the power minus 15
Diagram shows different types of Teslas
There are 2 types of MEG systems that measure the small magnetic fields with two different kinds of sensors,
which is - (2)
- Traditional cryogenic MEG helmet with SQUIDS
- Wearable optically pumped magnetometers
Traditional MEGs uses SQUIDS which stands for
Superconducting QUantum Interference Devices
Superconductors are materials which display very
unusual properties when cooled down below their critical temperature
Superconductors lose all electrical resistance (thus allowing electricity to flow through them) when they get
very cold
A current in a superconducting loop will keep
flowing forever