The Pros and Cons of

EEG and MEG Source Imaging 12-6-2011

John S. Ebersole, M.D.

The University of Chicago

Illinois MEG Center

Alexian Brothers Medical Center

American Epilepsy Society | Annual Meeting

Disclosure

Compumedics USA Speaker

American Epilepsy Society | Annual Meeting

Learning Objectives

• Identify the biophysical properties of EEG and MEG that contribute to their relative abilities

to record epileptiform brain activity.

• Determine how these strengths and weaknesses affect the overall clinical

usefulness of EEG and MEG source imaging.

American Epilepsy Society | Annual Meeting

Background 1

Localization of the epileptogenic focus is

the critical and rate-limiting step in an

evaluation for epilepsy surgery.

A variety of non-invasive localization

techniques are currently available –

MRI, PET, SPECT, fMRI, MEG, EEG

Background 2

Only EEG and MEG are:

Direct measures of epileptic pathophysiology

Performed in real time with msec resolution

Provide temporal sequencing of activity, e.g.

propagation

Biophysics of

EEG and MEG Fields

Sources of Magnetic

Fields

Basic Principles of MEG

Orientation of

Neurons

Skin

Skull

CSF

Cortex Active neurons

EEG Sensitivity

EEG requires >10 sq cm

EEG visualizes gyral and unopposed fissural sources, small sulcal sources cancel

EEG dipoles are deep to source cortex

For large sources, EEG favors center of activity

EEG is sensitive to all source orientations, but radial more so than tangential

A B

D

C

C A B

D

C

MEG Sensitivity

MEG requires 4-6 sq cm

MEG visualizes large unopposed sulci, fissures,

and tangential planes

MEG dipoles more accurately reflect source

depth

For large sources, MEG can favor an edge

Sensitive to a tangential source orientation

MEG and EEG Fields

EEG field orientation is the same as the

generator pyramidal cells

MEG field orientation is orthogonal to that of

the generator pryramidal cells

MEG field maxima are closer together than

associated EEG field maxima

MEG/EEG Complements

Volume conductor effects - + MEG

Spatial sampling - + MEG

Temporal sampling - + EEG

Source area - + MEG

Radial sensitivity - + EEG

Tangential sensitivity - + MEG

Deep source sensitivity - + EEG

Source Reconstruction

3D reconstruction of cortical sources of EEG or MEG requires a biophysical model

Multiple models:

Simple, point source – dipole

(unrealistic, easy to use and interpret)

Source Reconstruction

3D reconstruction of cortical sources of EEG

or MEG requires a biophysical model

Multiple models:

Complex, extended source – current density

(pseudo-realistic, needs “thresholding”)

MEG vs. EEG

Most patients have both EEG spikes and

MEG spikes

Their dipole source models commonly differ

by:

Orientation – radial vs tangential dominant

Location – mm vs cm

Timing – either can lead or lag

MEG/EEG Interpretation Scenarios

MEG EEG Synchrony Interpret: EEG adds

Tan Tan Sync nothing

Tan - - nothing

Tan Rad Sync radial comp

- Rad - location and orient *

Tan Rad MEG leads rad comp of propag

Tan Rad EEG leads origin and rad comp *

MEG/EEG Interpretation Scenarios

MEG EEG Synchrony Interpret: EEG adds

Tan Tan Sync nothing

Tan - - nothing

Tan Rad Sync radial comp

- Rad - location and orient *

Tan Rad MEG leads rad comp of propag

Tan Rad EEG leads origin and rad comp *

MEG/EEG Interpretation Scenarios

MEG EEG Synchrony Interpret: EEG adds

Tan Tan Sync nothing

Tan - - nothing

Tan Rad Sync radial comp

- Rad - location and orient *

Tan Rad MEG leads rad comp of propag

Tan Rad EEG leads origin and rad comp *

MEG/EEG Interpretation Scenarios

MEG EEG Synchrony Interpret: EEG adds

Tan Tan Sync nothing

Tan - - nothing

Tan Rad Sync radial comp

- Rad - location and orient *

Tan Rad MEG leads rad comp of propag

Tan Rad EEG leads origin and rad comp *

Practice Parameters

When the MEG and EEG fields are

synchronous, source characterization must

be a synthesis of MEG’s more accurate

dipole location and EEG’s more complete

dipole orientation.

When spike/seizure fields are radial there will

likely be no model-worthy MEG field.

EEG source modeling should be performed

MEG/EEG Interpretation Scenarios

MEG EEG Synchrony Interpret: EEG adds

Tan Tan Sync nothing

Tan - - nothing

Tan Rad Sync radial comp

- Rad - location and orient *

Tan Rad MEG leads rad comp of propag

Tan Rad EEG leads origin and rad comp *

MEG/EEG Interpretation Scenarios

MEG EEG Synchrony Interpret: EEG adds

Tan Tan Sync nothing

Tan - - nothing

Tan Rad Sync radial comp

- Rad - location and orient *

Tan Rad MEG leads rad comp of propag

Tan Rad EEG leads origin and rad comp *

Practice Parameter

When MEG and EEG fields are asynchronous,

dipole models of the leading field define

source origin, while those of the lagging field

help to characterize propagation.

MEG/EEG Interpretation Scenarios

In 2 of 6 scenarios, MEG provides superior or

the only localization of the spike source

In 2 of 6 scenarios, EEG provides superior or

the only localization of spike origin

In 2 of 6 scenarios, MEG and EEG each add

to the completeness/accuracy of the spike

interpretation

Bottom Line Comparison

MEG sees a window of brain activity (tangential fields) with more sensitivity and clarity than EEG

Localization of that activity with source models is

more accurate than with EEG EEG sees a more complete picture of brain activity

(tangential and radial), but less clearly than MEG Localization with EEG source models is less

precise, but orientation information is more complete than MEG

Conclusion

MEG and EEG strengths are complementary

and compensate for the weaknesses

of the other technology!