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Basic principles in neuroimaging > 6. MEG > Flashcards

6. MEG Flashcards

1
Q

What does SQUID stand for?

A

Superconducting Quantum Interference Device

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2
Q

Electrical current flow in a wire produces a magnetic field _____ to the current flow…

A

perpendicular

  • Think of the right-hand rule where thumb points in the direction of the current flow and the fingers wrapping around the wire determine the direction of the magnetic field
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3
Q

Which neurons does MEG focus on?

A

Pyramidal neurons (they are arranged in columns in the neocortex)

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4
Q

What are radial fields and what is one issue with detecting them?

A

they are magnetic signals produced by neuronal activity at the top of the gyri

  • they are less detectable by MEG
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5
Q

What are tangential fields?

A

they are magnetic signals produced by neuronal activity in the sulci

  • they are detectable by MEG
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6
Q

What are the potentials we primarily measure with MEG and how do they differ to the ones we measure with EEG?

A

Post-synaptic potentials (these can be excitatory or inhibitory) DON’T CONFUSE WITH ACTION POTENTIALS

  • When an impulse arrives at a synapse from an activated presynaptic neuron, a neurotransmitter is released causing channels in the membrane of the postsynaptic neuron to open creating a shift in the resting membrane polarization.

This polarization leads to activation or inhibition of voltage-gated channels responsible which cause action potentials

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7
Q

In MEG we measure the strength of the ______ (___)

A

magnetic field (in Tesla)

  • aka magnetic flux density
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8
Q

What is magnetic permeability (μ0)?

A

How much a material magnetizes if we apply a magnetic field to it

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9
Q

Ferromagnetic materials

A

Retain magnetism after being magnetised (really bad around fMRI scanners)

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10
Q

Paramagnetic materials

A

Amplify magnetic fields but do not stay magnetized (make magnetic fields bigger when passing through them)

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11
Q

Diamagnetic materials

A

Weaken applied fields (repulse them, will do anything to drive a magnetic field to 0)

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12
Q

How is MEG cooled?

A

liquid helium

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13
Q

What are superconductors?

A

Materials which lose all of their electrical resistance when cooled to a low enough temperature

  • So, if we put a current into a superconducting loop, it will continue infinitely (as long as we maintain the low temperature)
  • In reverse, applying a magnetic field to a superconducting loop will induce a current to cancel out the field

SQUIDS in MEG use such superconductors (~4K/ -269.15C)

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14
Q

Magnetometers…

A

measure the magnetic field (a coil with wrapped one way in a circle - 1 loop)

  • very prone to background noise
  • more sensitive to deeper parts of the brain
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15
Q

Gradiometers…

A

Measure the magnetic field (a coil wrapped into two loops going in opposite directions)

  • more of a noise resistant measurement
  • comes at a cost of being less sensitive to deeper parts of the brain
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16
Q

How does MEG work?

A

Magnetic signal present during post-synaptic potentials induces a current in the pickup coil.

This current is transferred to another coil located just under the squids where it is turned back into a magnetic field which can be measured by the SQUID

  • this mechanism is known as a flux transformer

A current is put into the SQUID and we measure how much current we have to inject to cancel a magnetic field

(From our POV what we’re measuring is the magnetic field next to the person’s head)

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17
Q

What is the approx MEG sampling rate? (At York)

A

~1000Hz (1000 times per second)

measurements down to 1ms

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18
Q

Physiological MEG artifacts

A
  • Cardiac
  • Breathing
  • Eye movements
  • Muscle movements
19
Q

Non-physiological MEG artifacts

A
  • Magnetic material in the room
  • Cars
  • Electronics
  • Other magnetic “noise”
  • Poorly installed equipment
20
Q

What is a butterfly plot? (MEG)

A

Shows magnetic flux/field vs time for all of the channels

21
Q

What is a sample period? (MEG)

A

amount of time between samples (1 / sample frequency (Hz))

More generally:
Duration = 1/frequency

22
Q

What is a nyquist frequency?

A

Maximum theoretical frequency we can see

in theory: sample rate / 2

  • in reality, we limit the signals to a maximum of 200Hz (this is know as our badwidth)
23
Q

What is frequency resolution?

A

depends on how much data we have, we can tell our signal granularity

Calculated as: 1/(data length in s)

e.g. 1/10 = 0.1Hz (we can tell differences that are 0.1Hz apart)

the more data the better the resolution

24
Q

“high” Gamma

A

70-150Hz:

  • cog processing
25
Q

very high freq oscillations

A

> 150Hz:

  • low level perceptual processing / epilepsy
26
Q

Distance between the “peaks” of the dipole pattern (in MEG) gives a clue as to…

A

How deep the dipole is

  • Dipole strength drops off as 1/r (cubed) with increasing depth

meaning that deeper dipoles will have a weaker magnetic signal

HOWEVER
- deep focused source will give the same/v similar signal as a more distributed shallow source

27
Q

What is an Equivalent Current dipole (ECD)

A

A summation of currents of many neurons with the same positive-negative direction can be mimicked as one strong dipole.

They are comprised of an:

  • orientation
  • location
  • strength (Am)
28
Q

What is the unit of an ECD (equivalent current dipole)?

A

Am (Ampere-metres; i.e. current)

Strength of ECD tells us how much current is flowing

29
Q

From an ECD (equivalent current dipole), we can predict…

A

what the MEG sensors would show

This is the FORWARD model:
- going from a description of an electrical source (orientation, location, strength) in the head and we predict what it would look like on the MEG sensors

30
Q

(MEG) evoked (think Event) responses are also known as…

A

phase-locked responses (or ERF - evoked response fields)

They are the same every time, you can get a few responses and average them starting from the trigger (Like ERPs in EEG)

31
Q

(MEG) induced responses are also known as…

A

Non-phase-locked responses

Unlike ERFs (Evoked response fields - equivalent of ERPs from EEG), these can be jittery and misaligned, thus, cannot be averaged

Analysis involves averaging the frequency domain representation:
- create freq(y) time(x) plots for each trial and average them out

32
Q

Phase-locked (____) responses are a ____ part of the overall MEG signal

A

(Evoked: think Event), small

Total response = phase locked (evoked) + non-phase-locked (induced)

33
Q

What does sensor space analysis mean? (MEG)

A

Looking at the data on the sensors

34
Q

What is source space analysis? (MEG)

A

Trying to work out what is happening at a particular location in the brain based on what we observed outside

  • Where is something happening
  • when is it happening
  • what exactly is happening

This is known as the “INVERSE PROBLEM”
- opposite of the forward model (I.e. inferring from a magnetic field what is happening in the neurons)

35
Q

Dipole modelling is…

A

the simplest form of inverse problem solution which involves fitting a single dipolar source

  • most suitable for simple responses hypothesised to come from a single neuronal source (single location of grouped neurons)
36
Q

Minimum Norm estimates is a form of … solution

A

inverse problem

  • Like dipole modelling, attempts to explain all of the data on the MEG sensors (at selected times) with a single solution
37
Q

What is Beamforming

A

MEG scanning technique

  • Takes each source (scans brain) location in turn and estimates activity at that point
  • for a given point in space a beamformer is a SPATIAL FILTER which attempts to focus on particular spatial location and attenuate (reduce) responses from other locations

!Simply a weighted sum of sensor activity we see

  • once we have the weights for each “location” in the brain, we can produce a “virtual electrode”
38
Q

What is a “virtual electrode” in MEG?

A

An estimate of the current (in Am) flow at a point in the brain

39
Q

What is good about MEG?

A
  • It is a measure directly coupled to brain activity (not via haemodynamics)
  • excellent temporal resolution
  • Doesn’t suffer from spatial “smearing” of EEG, signals are much more focal (magnetic signal doesn’t smear because of various tissues it travels through)
  • Can ask interesting questions about connectivity and networks beyond just “where is this happening”. How do different areas work together(induced and evoked responses)?
40
Q

Individual MEG sensors can see ___ but not ____ radial sources

A

tangential, radial

41
Q

Whether an MEG channel is a magnetometer or gradiometer setup is determined by:

A

The configuration of the pickup (sensor) coils

42
Q

Minimum norm is a _____ analysis method

A

source-space

43
Q

(EEG) Evoked activity is analysed by…

A

taking a direct average of the time-series MEG data.

Basic principles in neuroimaging (8 decks)

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