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Magnetoencephalography. Matt Martin 09/17/2004 APPM 4380. Overview. Description Applications Issues. Magnetoencephalography - Definition. Non-invasive brain imaging technique Passive measurement of minute current dipoles and corresponding magnetic moments

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Magnetoencephalography l.jpg

Magnetoencephalography

Matt Martin

09/17/2004

APPM 4380


Overview l.jpg
Overview

  • Description

  • Applications

  • Issues


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Magnetoencephalography - Definition

  • Non-invasive brain imaging technique

  • 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)


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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


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Magnetoencephalography - Apparatus

  • Prototype noise reducing helmet by Los Alamos National Labs

  • 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


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Magnetoencephalography - Apparatus

  • Clinical System by VSM Medtech

  • Screen is used for patient stimulation for functional mapping


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Magnetoencephalography - Applications

  • Epilepsy diagnosis

  • Functional Imaging and Mapping


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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


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Magnetoencephalography – Epilepsy Diagnosis

Epileptic seizure scan data and postprocessing


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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.


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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


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Magnetoencephalography – Functional Imaging

  • VSM's DataEditor: An averaged somato-sensory evoked response from a tactile stimulation of the second right digit.

  • VSM's MRIViewer: Dipole fit results for this scan.


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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


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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/overview.htm

  • National Society for Epilepsy: Information on Epilepsy.http://www.epilepsynse.org.uk/pages/info/leaflets/explaini.cfm


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Additional Readings

  • Habib Ammari, et al: An Inverse Source Problem For Maxwell’s Equations In Magnetoencephalography.http://epubs.siam.org/sam-bin/getfile/SIAP/articles/37392.pdf

  • Takashi Suzuki: Parallel optimization applied to magnetoencephalography. http://www.sigmath.es.osaka-u.ac.jp/suzuki/preprint/pdf/04-4.pdf