Download Magnetoencephalography

Magnetoencephalography
(MEG)
Overview
• Description
• Applications
• Issues
Magnetography
• Term used to describe a recording of the
magnetic field that accompanies a bioelectric
event
• Movement of electric charge gives rise to a
magnetic field
• Depolarization & repolarization 
translocation of ions  movement of charge
 give rise to magnetic field  detected with
suitable eqipment
• Magnetogram & electrogram: common origin
 reflect the same event but provides a
different type of information
• Magnetic fields pass freely through living
tissue  little attenuation of magnetic field
• No need for contact with the body to record
magnetogram  electrodeless recording
• Magnetic field by active tissues  measured
by voltage induced in search coil
• Detector  represent first derivative of
bioelectric event
• Field low intensity  necessary to use
magnetic shielding and/or signal averaging to
obtain adequate SNR
SQUID : Superconducting Quantum
Interference Device
MEG
• MEG : non-invasive tool to study epilepsy and
brain function.
• MEG measures small electrical currents arising
inside the neurons of the brain.
• These currents produce small magnetic fields.
• MEG generates a accurate representation of
the magnetic fields produced by the neurons.
• To some degree, MEG is similar to EEG.
• An important difference is that the skull and
the tissue surrounding the brain affect the
magnetic fields measured by MEG much less
than they affect the electrical impulses
measured by EEG.
• The advantage of MEG over EEG is therefore
greater accuracy owing to the minimal
distortion of the signal.
• This allows for more usable and reliable
localization of brain function.
• The combination of the images of and MRI
extremely helpful
– for identifying areas of the brain that may be
generating a potential for seizures
– for localizing the electrical activity in normal brain
function.
MEG
• Passive measurement of minute current
dipoles and corresponding magnetic moments
• Magnetic field generated by neurons on the
order of tens of femtoTeslas
• High resolution in both space (2 - 3mm) and
time (<1ms)
Why is an MEG performed?
• In the evaluation of epilepsy, MEG is used to
localize the source of epileptiform brain
activity.
• Usually performed with simultaneous EEG.
• MEG may be helpful in the following
situations:
– Seizure localization
– Lesion
– Tumours
• It can improve the detection of potential
sources of seizures by revealing the exact
location of the abnormalities, which may then
allow physicians to find the cause of the
seizures.
• It can help when MRI scans show a lesion but
the EEG findings are not entirely consistent
with the MRI information.
• In patients who have brain tumors or other
lesions, the MEG may be able to map the
exact location of the normally functioning
areas near the lesion prior to surgery.
• In patients who have had past brain surgery,
the electrical field measured by EEG may be
distorted by the changes in the scalp and brain
anatomy. If further surgery is needed, MEG
may be able to provide necessary information
without invasive EEG studies.
Magnetoencephalography - Apparatus
• Patient wears a helmet containing an array of
100+ sensitive magnetic field measurement
devices
• Measurement devices are called SQUIDs –
Superconducting Quantum Interference
Devices
• Measurements must occur in costly
magnetically shielded room
Magnetoencephalography - Apparatus
• Lead shell must be kept
below 8 Kelvin for
superconducting properties
• Surface Meissner currents
expel external magnetic
fields, reducing noise by six
orders of magnitude
Magnetoencephalography - Apparatus
• Screen is used for
patient stimulation for
functional mapping
Magnetoencephalography Applications
• Epilepsy diagnosis
• Functional Imaging and Mapping
Magnetoencephalography – Epilepsy
Diagnosis
• Epilepsy is a condition wherein a patient
suffers from repeated seizures that originate
in the brain.
• Manifested by extraordinarily high localized
brain activity.
• MEG ideal to locate such epileptic centers for
surgical removal
Magnetoencephalography – Epilepsy
Diagnosis
Epileptic seizure scan data and postprocessing
Magnetoencephalography – Epilepsy
Diagnosis
• Previous method: Intracranial
Electroencephalography
• Invasive surgery to lay EEG sensor network
directly on the brain
• Network connected to EEG monitor in hospital
intensive care units.
Magnetoencephalography –
Functional Imaging
• Functional Imaging utilizes the high temporal
resolution to generate real time brain scans
• Doctors can use these scans to determine how
the brain reacts to various stimuli
• Multiple scans allow a brain map to be built,
providing a base to begin research linking
neural activity to specific classes of stimuli
Magnetoencephalography –
Functional Imaging
• An averaged somatosensory evoked
response from a tactile
stimulation of the
second right digit.
• Dipole fit results for this
scan.
Issues
• Noise – the background magnetic field of the
earth is roughly 60 microTesla, approximately
9 orders of magnitude greater than that
generated by the neurons of the brain
• Reconstruction of imagery is inherently ill
posed, as it is an inverse problem of Maxwell’s
Equations
• Mathematics of the reconstruction well
beyond the scope of this presentation
Sources
• Belle Dumé: Brain Scans Made Easy.
http://physicsweb.org/articles/news/8/5/5
• CTF MEG Systems:
http://www.ctf.com/products/meg/ctf/software.htm
,
http://www.ctf.com/products/meg/meg_apps/overv
iew.htm
• National Society for Epilepsy: Information on
Epilepsy.
http://www.epilepsynse.org.uk/pages/info/leaflets/e
xplaini.cfm
Additional Readings
• Habib Ammari, et al: An Inverse Source
Problem For Maxwell’s Equations In
Magnetoencephalography.
http://epubs.siam.org/sambin/getfile/SIAP/articles/37392.pdf
• Takashi Suzuki: Parallel optimization applied to
magnetoencephalography.
http://www.sigmath.es.osakau.ac.jp/suzuki/preprint/pdf/04-4.pdf
Questions