Magnetoencephalography

Magnetoencephalography (MEG) is the magnetic counterpart of EEG.

History of MEG

The first recordings of magnetic brain activity were performed in 1968 by David Cohen at the Massachusetts Institute of Technology in Boston (Cohen D. Science 1968;161:784-786). Cohen was able to detect the magnetic equivalent of the electric α-rhythm using conventional (i.e., non-superconducting) coils. To record the exceptionally small magnetic fields produced inside the brain, the measurement was done in a magnetically shielded room. The data was averaged, time-locked to the simultaneously recorded EEG, to increase the signal-to-noise ratio.

The development of SQUIDs (Superconducting Quantum Interference Devices) in 1964/1965 allowed the recording of magnetic brain activity with high precision. SQUIDs are based on the Josephson Effect, which was discovered by the British physicist Brian David Josephson in 1962. The Josephson Effect refers to the phenomenon of current flow across two superconductors separated by a very thin insulating barrier, the Josephson junction. B. D. Josephson describes the discovery of tunnelling supercurrents in his Nobel Price lecture 1973.

The principle of MEG

MEG hardware

Modern MEG systems cover the entire head with coils that pick up the small magnetic field changes produced by the brain. The picture shows the sensor arrangement of the Elekta Neuromag 306 whole-head system. Earlier systems consisted of a single channel, eight channels or 37 channels (BTi Magnes).

Developing MEG systems is challenging, not only in engineering and scientific terms, but also economically. Several companies, such as Siemens and Phillips, started the development of MEG systems, but left the field after some time. For a summary of early MEG instruments the reader is referred to R. L. Fagaly, Neuromagnetic Instrumentation.

More recently, CTF and 4-D Neuroimaging (formerly BTi, Biomagnetic Technologies Inc.) had to close its doors due to financial difficulties. Thus, the swedish-finnish company Elekta Neuromag is the only remaining major manufacturer of MEG systems today. However, there is a smaller company that manufactures MEG systems, Advanced Technologies Biomagnetics of Italy.

MEG software

Free MEG software

BrainStorm is a Matlab toolbox for MEG and EEG visualization and analysis.

NUTMEG uses an adaptive eigenspace vector beamformer to reconstruct the time series of neural activity at every grid point within a brain volume of interest. A short paper describing NUTMEG appeared in Neurology and Clinical Neurophysiology 2004. pdf

Commercial MEG software

Curry integrates different modalities, e.g. MEG, EEG, fMRI and PET.

BESA (Brain Electrical Source Analysis). Software for source analysis and dipole localization in EEG and MEG research. For combined analysis of MEG / EEG and MRI / fMRI data, BESA has an easy link to BrainVoyager. Besa is a commercial programme and runs only under Windows.

SAM (Synthetic Aperture Magnetometry), provided by former CTF.

Own work

Sörös P, Dziewas R, Manemann E, Teismann IK, Lütkenhöner B. No indication of brain reorganization after unilateral ischemic lesions of the auditory cortex. Neurology 2006;67:1059-1061. PubMed Reprint

Sörös P, Michael N, Tollkötter M, Pfleiderer B. The neurochemical basis of human cortical auditory processing: Combining proton magnetic resonance spectroscopy and magnetoencephalography. BMC Biology 2006;4:25. PubMed Reprint

Sörös P, Cornelissen K, Laine M, Salmelin R. Naming actions and objects: cortical dynamics in healthy adults and in an anomic patient with a dissociation in action/object naming. Neuroimage. 2003;19(4):1787-801. PubMed Reprint

Sörös P, Teismann I, Manemann E, Michael N, Pfleiderer B, Ross B, Pantev C. Interindividual and interhemispheric differences of brain function: an MEG study of auditory short-term adaptation. In: Nowak H, Haueisen J, Gießler F, Huonker R, editors. Proceedings BIOMAG 2002, 13th International Conference on Biomagnetism. Berlin: VDE; 2002. p. 125-127. Reprint

Sörös P, Knecht S, Bantel C, Imai T, Wüsten R, et al. Functional reorganization of the human primary somatosensory cortex after acute pain demonstrated by magnetoencephalography. Neurosci Lett. 2001;298(3):195-8. PubMed Reprint

Sörös P, Knecht S, Manemann E, Teismann I, Imai T, Lütkenhöner B, Pantev C. Hemispheric asymmetries for auditory short-term habituation of tones? In: Nenonen J, Ilmoniemi RJ, Katila T, editors. Biomag2000, Proceedings 12th International Conference on Biomagnetism. Espoo: Helsinki University of Technology; 2001. p. 47-49. Reprint

Sörös P, Knecht S, Imai T, Gürtler S, Lütkenhöner B, et al. Cortical asymmetries of the human somatosensory hand representation in right- and left-handers. Neurosci Lett. 1999;271(2):89-92. PubMed Reprint

Further Reading

Cohen D. Magnetoencephalography: evidence of magnetic fields produced by α-rhythm currents. Science. 1968 Aug 23;161(843):784-6.

Cohen D. Boston and the history of biomagnetism. Neurology and Clinical Neurophysiology 2004:114 pdf

Josephson BD. The discovery of tunnelling supercurrents. Nobel Lecture 1973. pdf

Vrba J. Multichannel squid biomagnetic systems. 2000. pdf

Introduction to MEG

Biomagnetism by Jaakko Malmivuo and Robert Plonsey, online edition. This book is an excellent introduction to the physiology of biomagnetic phenomena, to the methods of biomagnetic recording and to the applications of those methods.

The basic mechanisms behind MEG. Short text provided by 4-D Neuroimaging (pdf)

A general overview about MEG and its applications. Presentation provided by 4-D Neuroimaging (pdf)

An MEG examination, step by step. Short text provided by 4-D Neuroimaging (pdf)

Links

The Biomag Blog

Magnetoencephalography at Wikipedia

SQUIDs at Wikipedia