Magnetoencephalography

R.J. Ilmoniemi , R.J. Näätänen , in International Encyclopedia of the Social & Behavioral Sciences, 2001

seven Conclusion

Magnetoencephalography provides a completely noninvasive tool to probe into the existent-time operation of the human brain in experimental conditions that are suitable for studying sensory and cognitive brain functions as well as their disturbances.

In evaluating the futurity role of MEG in cerebral brain enquiry, it is to be borne in mind that 1000000 and EEG are the merely noninvasive methodologies that provide precise temporal data about the brain activation patterns in its interaction with the environs. Further, of these two methodologies, Meg locates sources considerably more than accurately. Of course, this advantage of Million over EEG only applies to the currents that are visible to One thousand thousand, i.east., to those that are not fully radial and not located very deep from the head surface. These limitations, still, should be seen equally a strength rather than a weakness of the method, for they could enable ane (a) to separately measure out a single brain process rather than an amalgamate of multiple temporally overlapping processes which cannot be unequivocally disentangled on the footing of location or orientation, and (b) then, by using this procedure-specific information, to disambiguate ERP information recorded from the aforementioned experimental situation. Therefore, the combined utilise of the 2 methodologies is strongly recommended in research aiming at an authentic description of the spatio-temporal activation patterns underlying the brain'due south cognitive operations. The use of both MEG and EEG, anticipated also in the current developments of the recording systems, will certainly non be fabricated useless or redundant by further developments of EEG bespeak-assay techniques which, considering of inherent limitations of EEG methodology, cannot challenge the superiority of the combined use. On the contrary, important developments can be made in the field of indicate analysis, particularly in view of this combined employ of the two methodologies, to make their combination peculiarly powerful and an attractive tool of cognitive brain inquiry, which cannot be replaced by whatsoever other methodology in the foreseeable hereafter.

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Epilepsy

Hermann Stefan , ... andrew C. Papanicolaou , in Handbook of Clinical Neurology, 2012

Abstract

In addition to conventional electroencephalographic (EEG) analysis, magnetoencephalography (MEG) provides the possibility of sophisticated electrophysiological analysis of epileptic network activities. In addition, combinations of Million/EEG with multimodal imaging provide promising approaches for delineation of functionally important and epileptic cortex. The following clinical applications of MEG in epileptology are discussed: localization of focal epileptic activeness sources to guide invasive electrophysiological procedures; localization of focal epileptic activeness sources to guide detailed planning of neurosurgical procedures (e.one thousand., with neuronavigation), with the goal of minimal tissue removal; elucidation of spatial relationships of epileptic spike generation and subtle anatomical lesions; postoperative follow-up and, in cases where the first neurosurgical handling has failed to render the patient seizure-free, facilitation of the decision apropos the possibility of a second operation; screening of patients who are possible candidates for epilepsy surgery.

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Pediatric Neurology Part I

Ritva Paetau , Ismail Due south. Mohamed , in Handbook of Clinical Neurology, 2013

One thousand thousand in specific pathologies

1000000 studies confirmed the intrinsic epileptogenicity of focal cortical dysplasias (FCD). MEG fasten sources were clustered within the MRI-defined cortical dysplasia (Bast et al., 2004). MEG clusters alone were more oft detected in Taylor's FCD, while clusters and circumstantial scattered Million spike sources were more than oftentimes seen in other types of FCD. Surgical outcomes did non differ provided the areas of the MRI lesion and the MEG cluster were resected (Widjaja et al., 2008). As MEG signals are generated by tangentially oriented neurons in cortical fissures, MEG can be less sensitive in detecting epileptiform discharges in brain malformations with loss of cortical fissures and tangential neurons such as polymicrogyria. Simultaneous EEG is necessary in those cases (Bast et al., 2005).

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Encephalon imaging

In Fundamentals of Cognitive Neuroscience, 2013

2.5 Magnetoencephalography: magnetic fields of the brain

Magnetoencephalography (MEG) measures the magnetic field produced past electrical activeness in the encephalon ( Figure v.13). Its spatial resolution is now budgeted a few millimeters, while its temporal resolution is in milliseconds. Because of the physics of magnetism, MEG is highly sensitive to dendritic menstruum at correct angles to the walls of the sulci (the cortical folds). MEG results must exist superimposed upon a structural prototype of the living brain. MEG uses a procedure called magnetic source imaging (MSI) to coregister the magnetic sources of brain activity onto anatomical pictures provided by MRI. One thousand thousand has the advantage of being entirely silent and noninvasive. As nosotros will come across, MRI is quite noisy, and, of form, depth electrodes crave surgery. Thus, Million is attractive for use with children and vulnerable people. One thousand thousand is easy for young children to tolerate as long equally they can stay relatively nevertheless.

Figure 5.13. Magnetoencephalography and its assay. The bailiwick is placed in the scanner that has a large fix of shielded sensors. The signals are derived from ionic currents flowing in the dendrites (lesser). Action potentials do not produce an observable field. Upper heart: Magnetic fields following painful (touch) stimulation, where (a) shows the recorded data, and (b) and (c) display residual magnetic fields obtained after filtering the somatosensory processing signals from the recorded data. The lesser two lines show the time course of the source strengths during the painful stimulation. Upper correct: Source locations of the Million information overlaid on MR images.

 Source: VSM MedTech.

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Traumatic Brain Injury, Part I

Sarah Schmitt , Marc A. Dichter , in Handbook of Clinical Neurology, 2015

Magnetoencephalography and traumatic brain injury

Magnetoencephalography (One thousand thousand) is a relatively new technique that is closely related to EEG. Much as EEG records the electric activity from the cognitive cortex, MEG records the fluctuations in the magnetic field around the head that result from current flow inside neurons. Recently, magnetoencephalography (MEG) has been used in the evaluation of patients with traumatic brain injury. Although all the studies to date have been pocket-sized, the information suggest that One thousand thousand may be useful in detecting abnormalities in individuals with a history of mild TBI. In ane group of 45 individuals with mild TBI, 1000000 was able to notice abnormal slow wave activity in 96% of individuals with injuries related to combat blasts and 77% of individuals whose injuries were non related to combat blasts ( Huang et al., 2009). MEG also detected abnormalities in 86% of individuals with postconcussive cognitive abnormalities; Million was significantly more sensitive than either SPECT or MRI, which detected abnormalities in 40% and eighteen% of patients, respectively (Lewine et al., 1999). MEG is currently bachelor at a relatively small number of medical centers worldwide, which limits its widespread utilize. Although these studies are promising, further studies are needed to better ascertain the role of MEG in the evaluation of patients with TBI.

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Clinical Neurophysiology: Footing and Technical Aspects

Richard C. Burgess , in Handbook of Clinical Neurology, 2019

Abstract

Magnetoencephalography (MEG) is the noninvasive measurement of the miniscule magnetic fields produced by electrical currents flowing in the encephalon—the same neuroelectric activity that produces the EEG. MEG is one of several diagnostic tests employed in the evaluation of patients with epilepsy, but without the need to expose the patient to any potentially harmful agents. MEG is especially of import in those being considered for epilepsy surgery, in whom accurate localization of the epileptic focus is paramount. While other modalities infer brain function indirectly by measuring changes in claret period, metabolism, oxygenation, etc., Million, as well as EEG, measures neuronal and synaptic function directly and, like EEG, MEG enjoys submillisecond temporal resolution. The measurement of magnetic fields provides data not only about the aamplitude of the current merely also its orientation.

MEG picks upward the magnetic field from neuromagnetometers surrounding the head in a helmet-shaped array of sensors. Clinical whole-caput systems currently take 200–300 magnetic sensors, thereby offer very loftier resolution. The magnetic signals are non distorted by anatomy, because magnetic susceptibility is the same for all tissues, including the skull. Hence, One thousand thousand allows for a more accurate measurement and localization of encephalon activities than does EEG. Because i of its primary strengths is the ability to precisely localize electromagnetic activeness within brain areas, MEG results are e'er coregistered to the patient's MRI. When combined in this style with structural imaging, it has been called magnetic source imaging (MSI), but MEG is properly understood as a clinical neurophysiologic diagnostic test.

Point processing and clinical interpretation in magnetoencephalography require sophisticated noise reduction and computerized mathematical modeling. Technological advances in these areas have brought Million to the point where it is at present function of routine clinical exercise. One thousand thousand has become an indispensable part of the armamentarium at epilepsy centers where MEG laboratories are located, especially when patients are MRI-negative or where results of other structural and functional tests are not entirely concordant.

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Whole-Brain Imaging

Matt Carter , Jennifer Shieh , in Guide to Enquiry Techniques in Neuroscience (Second Edition), 2015

Magnetoencephalography

MEG measures changes in magnetic fields on the surface of the scalp that are produced past changes in underlying patterns of neural electric action (Figure ane.16). Nigh 50000 neurons are required to produce a detectable signal with 1000000, a number that may seem large just is actually much smaller than what is required for an EEG indicate, which may require millions of neurons. 1000000 offers relatively poor spatial resolution but fantabulous temporal resolution compared with PET, SPECT, and fMRI. Therefore, One thousand thousand tin be thought of as a compromise technique: it offers excellent temporal resolution and much amend spatial resolution compared with EEG, simply non as expert spatial resolution as other imaging techniques. In item, MEG in combination with fMRI allows for fantabulous temporal and spatial resolution of neural activity. Unfortunately, MEG is a very expensive technique, requiring a room that tin can obstruct magnetic fields from outside sources; even something equally small as a coffee machine in a neighboring building tin can be detected if the room is not adequately insulated.

Effigy 1.16. A magnetoencephalography (MEG) setup.

(A) A subject wears a cap with magnetic field sensors and (B) is placed inside an imaging chamber. One thousand thousand equipment is expensive and requires a room with strong insulation.

Similar EEG, the temporal precision of MEG allows for the report of event-related changes in brain activity. The MEG equivalent of an event-related potential is referred to every bit an event-related field (ERF).

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The tools

Bernard J. Baars , Nicole M. Gage , in Cognition, Brain, and Consciousness (Second Edition), 2010

2.4 Magnetoencephalography (Million)

Magnetoencephalography (MEG) measures the magnetic field produced by electric action in the brain ( Effigy 4.20). Its spatial resolution at parts of the cortical surface is now approaching a few millimeters, while its temporal resolution is in milliseconds (Effigy 4.21).

Figure 4.xx. Meg is silent and not-invasive. MEG is piece of cake for young children to tolerate, as long as they can stay relatively all the same. The pictures at the bottom of the figure testify the vector fields of the One thousand thousand over the caput of the subject area.

 Source: 4D Neuroimaging, San Diego.

Figure 4.21. An example of a wavelet assay of Million data in response to speech and non-speech sounds. The non-voice communication sounds consisted of steady-state tones, tonal contours (TC) which contained dynamic changes; speech sounds consisted of steady-state vowels and consonant vowel (CV) syllables which independent dynamic changes. Using a wavelet analysis, with time on the ten-centrality and frequency on the y-axis, and color cogent the level of action, the authors investigated neural activity by stimulus contrasts and task (AUD, the subjects decided if ii sounds were exactly the same, CAT, the subjects decided if two sounds were in the same category). Results: high levels of activity in the alpha (8-13   Hz) ring, shown in red on the wavelet plots.

Because of the physics of magnetism, Million is highly sensitive to dendritic menstruation at correct angles to the walls of the sulci (the cortical folds), just much less sensitive to the bottom. 1000000 has excellent temporal resolution and somewhat improve spatial accuracy than EEG.

Like whatsoever other method that measures brain action, MEG results must be superimposed upon a structural image of the living brain. Million uses a process called magnetic source imaging (MSI) to co-annals the magnetic sources of brain activeness onto anatomical pictures provided by MRI. In this way, MSI combines the high spatial resolution of MRI with the loftier temporal resolution of Million. MSI techniques are used before brain surgery, to pinpoint brain regions with vital functions that must be protected during surgery.

MEG has the advantage of being entirely silent and non-invasive. As we will encounter, MRI is quite noisy, and of class depth electrodes require surgery. Thus, MEG is attractive for use with children and vulnerable people.

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Brain Imaging With Magnetoencephalography During Rest and During Speech and Language Processing

Srikantan S. Nagarajan , ... Keith A. Vossel , in Genomics, Circuits, and Pathways in Clinical Neuropsychiatry, 2016

Abstract

Magnetoencephalography (MEG) is a noninvasive method for sensing magnetic fields generated past the man brain. Brain imaging with MEG sensor data is enabled by the application of point processing and source reconstruction algorithms to Million information that generates highly dynamic maps of encephalon activation in a wide range of neural oscillatory frequencies with a high spatiotemporal precision unattainable by other imaging techniques. This imaging modality is ideally suited to monitor dynamic neuronal activity beyond widely separated brain regions and simultaneously characterizing their interactions. This technique also offers a reliable quantitative tool to measure contradistinct neuronal activity that is correlated to specific behavioral deficits observed in affliction atmospheric condition. In this chapter, nosotros offset review how brain activity and connectivity images tin can be reconstructed from Meg measurements and and so review the use of brain imaging with Meg in various neurodegenerative conditions.

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Developmental Disabilities and Metabolic Disorders

Mary Lee Gregory , ... Bruce Chiliad. Shapiro , in Neurobiology of Brain Disorders, 2015

Magnetoencephalography

Magnetoencephalography (Meg) is a method of mapping brain activeness by measuring magnetic fields produced by the electrical activity of neurons. Information technology is similar to EEG but measures magnetic instead of electrical activity resulting from postsynaptic evoked potentials. Magnetic fields are less distorted by the skull and scalp than electric signals, resulting in better spatial resolution. MEG but measures the activity of neurons in a specific orientation, so signals have less interference due to the position of the neuron compared with the recording device. Yet, Million is much more expensive than EEG and more than easily distorted past ambient signals in the surround. MEG requires not only specialized equipment but also shielded areas. 41

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