Lecture 6 - MEG Flashcards
Who used the first SQUID?
Cohen (1972)
What does SQUID stand for?
Superconducting Quantum Interference Device
MEG signal is ____ to EEG
perpendicular
What contributes to the MEG signal?
- PSPs, APs, or secondary volume currents?
post synaptic potentials
Which is faster, APs or PSPs?
APs
Where do radial fields originate? Do they contribute to the MEG signal?
From the top of gyri
Not really
Where do tangential fields originate? Do they contribute?
Folds in sulci, therefore magnetic field is perpendicular to sensors
Detectable by MEG
What unit of measurement is detected in MEG
B
Magnetic Flux Density / Induction (Tesla)
Tesla= [Wb/m2])
When trying to detect small magnetic fields, what two conditions are requires?
Sensitive detectors (SQUIDs)
Low noise measurement environment (MSR)
What are SQUIDS and how do they work?
Superconducting Quantum Interference Device (SQUIDs)
Very sensitive detector of magnetic flux
Cooled using liquid helium
Most modern MEG systems use DC-SQUIDS
Three configurations for signal (pickup) coils?
Magnetometer
Planer gradiometer
Axial Gradiometer
Magnetometers can see ____ in the brain
Gradiometers are regarded as ____
deep
shortsighted
Pros and cons of magnometers
can see deeper into the brain
However, they pickup more noise
How are SQUIDs and pickup coils connected?
Using a flux transformer
What are larger, SQUIDS or pickup coils?
pickup coils
SQUIDS are only 1mm3 ish
Examples of physiological artifacts?
Cardiac
Breathing
Eye movements
Muscle movements
Examples of non-physiological artifacts?
Magnetic material in the room
Cars
Electrics
Methods of artifact removal?
ICA
Epoch/Data rejection
EOG/ECG can provide information
MEG ___-___ data can be selected and averaged in a similar way to EEG data
sensor-space
Can choose which data points in sensor-space to include based on ____ information
trigger
What is the maximum frequency we can sample?
Sample rate/2
MEG systems often sample at 678.17Hz, so /2 = 339Hz
This is the Nyquist frequency
in reality, signals are limited to a maximum of 200Hz
Frequency resolution is determined by what?
The amount (s) of data we have Calculated as 1/(data length in seconds)
Time-domain representation of a waveform can be turned into a frequency-domain representation by what tranformation?
Fourier transformation
Time and frequency-domain representation can be transformed into what combined view?
time-frequency representation
What is oscillatory activity thought to arise from?
Assembles of neurones acting together
What do the red and blue parts of an equivalent current dipole?
Red shows fields coming “out” of the brain, blue shows fields going back “into” the brain
The source of activity in equivelent current dipoles is most likely found at what point?
the 0-point between dipole peaks
The distance between “peaks” in a dipole pattern gives a clue as how ___ the dipole is
deep
Dipole strength drops off as a ___/___ with increasing depth
1/r^3
A focal deep source and distributed superficial source will produce ___ magnetic fields
similar
ECD is composed of…
A position in the head
An orientation
A strength
What is the unit of an ECD?
Am (ampere-meters i.e. current)
From an ECD, we can predict what the MEG sensors would show. This is called the ____ model.
forward
An example of phase-locked response is…
Evoked responses to stimuli
ERP in MEG is known as…
ERF - evoked response fields
Analysis of phase-locked responses involves…
averaging the time representation
An example of a non-phased-locked response is…
Induced responses
Analysis of non-phase-locked responses involves…
Averaging the frequency domain representation
Changes in power are not time locked and may be jittered
Primary sensory responses tend to have more interesting ___-___ components, where as higher level functions and inter-area communications tend to involve ______-___ activity.
phase-locked
non phase-locked activity
What is an inverse problem?
When we attempt to make inferences about a system from a set of observations
In regards to the inverse problem in MEG, the things we know are…?
MEG sensor measurments and their positions
Structural information about the brain
Knowledge of how electrical currents relate to magnetic fields (via approximations of Maxwell’s equations)
Von Helmholtz (1853) showed there is ___ unique solution to explaining a set of magnetic fields outside a conducting sphere
NO
The problem is “ill-posed”
To “solve” the inverse problem, we need to apply ___
constraints
There are two main groups of constraint based inverse-problem solutions…
Fitting/Localization methods (Dipole Fitting)
Scanning or Imaging Methods (Minimum Norm Estimates; Beamforming)
To solve the inverse problem, we can model the ___ problem
Forward problem
The forward model describes…
Given a set of currents in the brain (position, orientation, strength), what would we expect to see on the sensors?
Most models use an ECD model for this.
When forward modelling, we need to model the brain. What are some brain models used?
Single Sphere
Multiple spheres
Some models can tesselate:
Boundary Element Models (BEM)
Finite Element Models (FEM)
After modelling the brain and ECDs in the forward model, we get a ____. This shows what the ___ ___ would ‘see’ for a given ____ at each ___ and ___ in the brain.
Leadfield MEG Sensors dipole location orientation
Simplest form of inverse solution is to fit a ___ ___ ___
single dipolar source
Single dipole source (inverse solution) minimises the ____ ___ between ____ pattern and ____ pattern on the sensors. This is known as ____-____ minimisation.
Squared errors
predicted
observed
least-squares minimisation
Although single dipole fitting is the simplest form of the technique, what are two alternative and more complex methods?
Multiple dipole fitting
Spatio-temporal dipole fitting
What does the algorithm RAP-MUSIC do?
Estimates how many dipoles should be fitted
Do you need to decide the number of dipoles to fit a-priori ?
yes
Minimum norm estimate?
Similar to dipole fitting, however, finds the most likely distributed source rather than a single point.
Beamforming
Does not try and minimise the error with respect to the MEG data for all source location at one
Takes each source location in turn and estimates activity at that point
Closely linked to radar
In relation to beamforming, what does the spatial filter do?
Attempts to focus on a particular spatial location. The filter tries to attenuate responses from other locations.
In beamforming, once we have the weights for each location in the brain, what can we produce?
A virtual electrode
What is a virtual electrode time series? What are the units?
An estimate of current moment at each “voxel” in the brain
VE units are the same as ECDs: Am
Methods to combat multiple comparisons?
Bonferroni Correction
Non-parametric
Random Field Theory
Advantages of MEG
Doesn’t rely on indirect measure of brain activity (e.g. haemodynamics)
Good temporal resolution
Doesn’t suffer from same spatial smearing of EEG
Good for network connectivity
Disadvantages of MEG
Complex analysis
Poorer resolution than fMRI
Expensive
Working in source space involves solving the ____ problem
inverse
What neural activity is thought to be the primary origin of the responses recorded in MEG?
Postsynaptic electrical activity from Pyramidal Cells
Whether an MEG channel is a magnetometer or gradiometer setup is determined by:
The configuration of the pickup (sensor) coils
What is the full name for a SQUID
superconducting quantum interference device
Individual MEG sensors can see ___ but not ___ sources
tangential
radial
What is the Nyquist freq?
sample rate / 2
Minimum norm is a ____ analysis method
Soure-space
Evoked activity is analysed by _____ of the time-series MEG data
taking a direct average
What did Josephson do?
Describes Josephson Junctions
Who recorded alpha rhythms using an induction coil?
Cohen (1968)
Electrical current flow (I) in a wire
produces a magnetic field (B)
_____ to the current flow
perpendicular
Why do secondary volume currents not really contribute to MEG signal?
Tend to cancel out
Ferromagnetic?
Retain magnetism after being exposed
Paramagnetic?
Amplify magnetic fields but do not stay magnetized
Diamagnetic?
Weaken applied fields
What is magnetic permeability?
How much does a material magnetize
when exposed to an external magnetic field?
Superconductors are perfect ___
diamagnets
MEG signal acquisition overview?
- Magnetic fields pass through the pickup coil
• Either magnetometer or gradiometer configuration - A current is induced in the pickup coil, and via the flux
transformer, it is coupled to the coil next to the SQUID - The measurement of this current tells us about the
magnetic field
Reference channels are automatically used to help ___ the data and we normally ___ ___ for data analysis purposes
‘de-noise’
ignore them
What is a butterfly plot?
Shows magnetic flux/field VS time for all the channels
‘high’ gamma?
70-150Hz - cognitive processing
Very high frequency oscillations?
150Hz+
Low level perceptual processing/epilepsy
Need ABOUT ____ neurons to detect signal (estimation ranges)
50,000
How much space do 50,000 neurons take up? ABOUT
50,000 neurons would need a patch about 0.63mm2 in area
The spatial resolution of MEG is _ ____ ____
a complex question
Evoked activity is analysed by taking a ____ of the ____ MEG data.
direct average of the time-series data
