Magnetoencephalography Matt Martin 09/17/2004 APPM 4380

Overview Description Description Applications Applications Issues Issues

Magnetoencephalography - Definition Non-invasive brain imaging technique Non-invasive brain imaging technique Passive measurement of minute current dipoles and corresponding magnetic moments Passive measurement of minute current dipoles and corresponding magnetic moments Magnetic field generated by neurons on the order of tens of femtoTeslas Magnetic field generated by neurons on the order of tens of femtoTeslas High resolution in both space (2 - 3mm) and time (<1ms) High resolution in both space (2 - 3mm) and time (<1ms)

Magnetoencephalography - Apparatus Patient wears a helmet containing an array of 100+ sensitive magnetic field measurement devices Patient wears a helmet containing an array of 100+ sensitive magnetic field measurement devices Measurement devices are called SQUIDs – Superconducting Quantum Interference Devices Measurement devices are called SQUIDs – Superconducting Quantum Interference Devices Measurements must occur in costly magnetically shielded room Measurements must occur in costly magnetically shielded room

Magnetoencephalography - Apparatus Prototype noise reducing helmet by Los Alamos National Labs Prototype noise reducing helmet by Los Alamos National Labs Lead shell must be kept below 8 Kelvin for superconducting properties 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 Surface Meissner currents expel external magnetic fields, reducing noise by six orders of magnitude

Magnetoencephalography - Apparatus Clinical System by VSM Medtech Clinical System by VSM Medtech Screen is used for patient stimulation for functional mapping Screen is used for patient stimulation for functional mapping

Magnetoencephalography - Applications Epilepsy diagnosis Epilepsy diagnosis Functional Imaging and Mapping Functional Imaging and Mapping

Magnetoencephalography – Epilepsy Diagnosis Epilepsy is a condition wherein a patient suffers from repeated seizures that originate in the brain. Epilepsy is a condition wherein a patient suffers from repeated seizures that originate in the brain. Manifested by extraordinarily high localized brain activity. Manifested by extraordinarily high localized brain activity. MEG ideal to locate such epileptic centers for surgical removal 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 Previous method: Intracranial Electroencephalography Invasive surgery to lay EEG sensor network directly on the brain Invasive surgery to lay EEG sensor network directly on the brain Network connected to EEG monitor in hospital intensive care units. 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 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 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 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 VSM's DataEditor: An averaged somato-sensory evoked response from a tactile stimulation of the second right digit. 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. VSM's MRIViewer: 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 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 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 Mathematics of the reconstruction well beyond the scope of this presentation

Sources Belle Dumé: Brain Scans Made Easy. Belle Dumé: Brain Scans Made Easy. CTF MEG Systems: w.htm CTF MEG Systems: w.htm w.htm w.htm National Society for Epilepsy: Information on Epilepsy. plaini.cfm National Society for Epilepsy: Information on Epilepsy. plaini.cfm plaini.cfm plaini.cfm

Additional Readings Habib Ammari, et al: An Inverse Source Problem For Maxwell’s Equations In Magnetoencephalography. bin/getfile/SIAP/articles/37392.pdf bin/getfile/SIAP/articles/37392.pdf Takashi Suzuki: Parallel optimization applied to magnetoencephalography. u.ac.jp/suzuki/preprint/pdf/04-4.pdf u.ac.jp/suzuki/preprint/pdf/04-4.pdf

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