MEG strength
I found some information on the strength of MEG in The Encyclopaedia of Medical Imaging Vol 1: Physics, Techniques and Procedures
by Gustav K. Von Schulthness. I'll need to get the reference when it shows up on the Library of Congress database. Here is the useful quote:
Neuromagnetic fields have amplitudes in the order of a few picotesla (
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MathType@MTEF@5@5@+=feaafeart1ev1aaatCvAUfeBSjuyZL2yd9gzLbvyNv2CaerbuLwBLnhiov2DGi1BTfMBaeXatLxBI9gBaerbd9wDYLwzYbItLDharqqtubsr4rNCHbGeaGqiVu0Je9sqqrpepC0xbbL8F4rqqrFfpeea0xe9Lq=Jc9vqaqpepm0xbba9pwe9Q8fs0=yqaqpepae9pg0FirpepeKkFr0xfr=xfr=xb9adbaqaaeGaciGaaiaabeqaamaabaabaaGcbaGaaGymaiaaicdadaahaaWcbeqaaiabgkHiTiaaigdacaaIYaaaaaaa@39F2@
tesla) and very sensitive instruments are needed to detect these extremely weak fields.
Ahh, here's a better one, from Jasper Daube's "Clinical Neurophysiology" [1].
Magnetoencephalography (MEG) is the recording of the small magnetic fields produced by the electric activity of neurons in the brain. These magnetic fields are generated by current flowing in neurons, with a small contribution from extracellular current flow in the volume conducting medium around the brain (generally less than the contribution of intracellular currents). These magnetic fields are extremely small, typically in the femptotesla or picotesla range (
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to
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T). They must be detected by a magnetic gradiometer connected to a special type of extremely sensitive amplifier called a superconducting quantum interference device (SQUID), which must be cooled by liquid helium. To eliminate noise signals caused by the much larger magnetic fields associated with electrical equipment, power lines, and the earth's magnetic field, a special magnetically shielded room is required. For all these reasons, MEG is a very expensive tool. Another disadvantage of MEG, compared with EEG, is that it cannot be used readily for the long-term recordings needed to capture and to localize an epileptic seizure, because the subject's head must be kept immobilized near the magnetic gradiometer array during the entire recording......Because magnetic fields created by a current source are always oriented along a tangent to a circle around the line of current flow, MEG is insensitive to radially oriented currents in cerebral cortex and is sensitive only to tangential currents, in contrast to EEG, which is sensitive to both (although more sensitive to radial than to tangential currents). Thus, in practice, MEG recordings are often combined with simultaneous conventional EEG recordings.
Hmm, the comment about extracellular flow versus current flowing in the neurons is interesting. I thought the typical claim is that EEG is mostly due to the extracellular current flow.
References
[1] J.R. Daube, Clinical neurophysiology, Oxford ; New York: Oxford University Press, 2002, p. 144.
Posted by torque at December 10, 2003 12:12 PM
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