Magnetoencephalography (MEG) Flashcards
Why does MEG have a higher spatial resolution than EEG?
Because magnetic fields generated by the brain are not distorted by the scalp or the skull like electric fields are.
What is the main challenge of MEG?
The neuromagnetic signal is small (around 100 fT in strength) so it is hard to detect.
State the MEG forward problem
Given a known current distribution in the brain, can we compute the magnetic field distribution outside the head?
What are the two key approximations made to solve the MEG forward problem
The spherically symmetric conductor approximation: assume that the head/brain is a single sphere with homogeneous conductivity.
The current dipole approximation: when viewed from a distance, the field from a large number of synchronised post-synaptic currents looks like the field of a single current dipole.
What is an alternative to using the spherically symmetric conductor approximation?
Using more complex models from a patient’s MRI scan.
When is the current dipole approximation reasonable?
When the region of the brain activated is small.
Give the equation for the total current that can contribute to an MEG signal
J(r’) = total current
Jp(r’) = primary current
Jv(r’) = volume current
State the Geselowitz formula
B(r) = magnetic field
J^P(r’) = primary current
r’ = location inside the brain
R = r - r’
V = electric scalar potential
n(r’) = vector normal to the surface element
∇A x ∇B = ?
∇ x (A∇B)
State Stokes’ Theorem
How can the Geselowitz formula be derived?
- Define total current.
- Define volume current in terms of conductivity and ∇V (E = - ∇V).
- Use the Biot-Savart law to define the B-field outside of G.
- Substitute the primary and volume current in and separate the terms.
- Redefine the volume current using ∇A x ∇B = ∇ x (A∇B) and Stokes’ theorem.
State the Geselowitz formula after applying the two forward problem approximations
B(r) = magnetic field
Q = dipole magnitude and direction
r_Q = position of the current dipole
e_r = vector normal to the plane of the magnetic field detector
r = position
What does the Geselowitz formula represent after approximations have been applied?
An analytical forward model solution for a single current dipole in a spherically symmetric conductor.
What parts of the brain is MEG most sensitive to? Why?
Shallow sources in the cortex because the field decays with the square of distance.
MEG is only sensitive to dipoles oriented ___________.
Tangentially
State the MEG inverse problem
Can we calculate a current distribution inside the brain given a known magnetic field distribution outside the head?
Why is the MEG inverse problem ill-posed?
There are infinite solutions so an estimate is required using optimisation techniques.
What is a lead field?
An N-dimensional vector containing the magnetic field that would be measured at each end of N sensors in response to a single current-dipole source of unit amplitude at a pre-determined location in the brain.
What does the lead field represent?
The solution to the forward problem for all sensor locations for all possible source locations and orientations.
Describe the matrix system for MEG data
m = sensors
L = lead field at the i-th sensor form a unit of the j-th dipole
q = current dipoles
How does the lead field model help to solve the MEG inverse problem?
It allows the field to be calculated at each channel where each channel scans a section of the brain.
In reality, there are ~______ locations in a ~1mm voxelisation of the brain and only ~___ channels.
20,000
300
Give the matrix equation for MEG data
m = sensors
L = lead field
q = current dipoles
Describe how dipole fitting for MEG data works
- The number of active dipoles at r_Q is assumed to be 1.
- Knowing this, Q and r_Q are iteratively varied to minimise the sum squared difference between the model (m_model) and the data (m).
- Minimising the error reveals the optimum location, orientation, and strength of the dipole in the brain.
How is interference reduced in MEG systems?
MEG systems are housed in a magnetically shielded room (MSR).
Why does interference have to be reduced for MEG systems?
Because brain fields are small so their signal can be obscured by larger fields such as mains electricity, vehicles, and other biomagnetic systems like the heart.
Give the equation for the shielding factor
SF = shielding factor
B = magnetic field
Why is it necessary to calculate the shielding factor?
To quantify the performance of an MSR
Give the equation for the shielding factor in units of dB
SF_dB = shielding factor in dB
SF = shielding factor
What type of material are MSRs made of? Why?
A material with high magnetic permeability (e.g. Mu metal) because these materials provide an easy path for flux, diverting it from the scanner.
What does SQUID stand for?
Superconducting QUantum Interference Device
What are SQUIDs?
Magnetic field detection devices that are sensitive to <5 fT fields. They need liquid helium at 4K to operate.
How is the sensitivity of a SQUID boosted?
The SQUID is coupled to a pickup coil which further rejects noise.
Describe the design of a magnetometer SQUID
A changing field induces a current in the loop. The current is proportional to the field, B.
State one cost and one benefit of magnetometers
Cost: Intrinsically sensitive to noise
Benefit: Sensitive to deep sources
Describe the design of an axial gradiometer
Adding a second loop produces a gradiometer. It measures the difference in magnetic field between the two loops separated axially. It is sensitive to shallow sources at the edge of the coil.
Describe the design of a planar gradiometer
Adding a second loop produces a gradiometer. It measures the difference in magnetic field between the two loops separated in a plane. It is sensitive to sources directly beneath the pair.
Give the equation for the gradient of a gradiometer
G = B1 - B2
Describe the shape of a graph displaying the strength of a magnetic field with distance from the source to show how position impacts gradiometer output
For nearby sources, G is ______. Placing coils close to the head highlights brain activity.
Large
For distal sources, G is _____, even if the amplitude is ______.
Small
Large
Give 3 reasons why MEG is hard to carry out
- MSR is expensive
- Helium is expensive
- Cryogenic equipment is fiddly to work with
How many MEG systems are there worldwide?
~200
Briefly describe OPM-MEG
An upcoming version of MEG sensors that targets the main problems of MEG as it does not require cryogenics.
