MEG – Magnetoencephalography
Physical and Physiological Basis
Learning Objectives
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- Describe what signals magnetoencephalography (MEG) detects.
- Summarize the technology involved in MEG
Magnetoencephalography (MEG) is another technique for noninvasively measuring neural activity. The flow of electrical charge (the current) associated with neural activity produces very weak magnetic fields that can be detected by sensors placed near the participant’s scalp. The number of sensors used varies from a few to several hundred. Like electroencephalography (EEG), the source of the magnetic fields is dendritic current of pyramidal neurons that fire synchronously and in parallel. Axonal and synaptic currents and their magnetic fields cancel out.
MEG is the measurement of the magnetic field generated by the electrical activity of neurons. It is usually combined with a magnetic resonance imaging to get what is called magnetic source imaging. The technology that has helped record these minute magnetic fields is super-conducting quantum interference detector which is like a highly sensitive magnetic field meter. To maintain super conductors there needs to be an extremely cold environment, which is made by using liquid helium which is only 3° above absolute zero (−452°F or −270°C). This is all housed within a large machine that is above the subjects head, which can be seen in the Figures below.
Due to the fact that the magnetic fields of interest are so small, special rooms that are shielded from magnetic fields in the environment are needed in order to avoid contamination of the signal being measured. The actual sensors recording magnetic fields are magnetometers and/or gradiometers. There are 2 types of gradiometers — axial and planar. Magnetometers provide the best signal and are most sensitive to deep brain sources but are also more sensitive to competing magnetic noise. Gradiometers are better at noise reduction. Using these sensors, MEG fields pass through the head without any distortion and provides a high spatial and temporal resolution.
MEG records magnetic fields generated by electric currents in the brain. An electric current is always associated with a magnetic field perpendicular to its direction as per the right-hand rule. The magnetic permeability of biological tissues is almost the same as that of empty space and so the magnetic field is not distorted by scalp or skull. However, the magnetic fields diminish as 1/r3 with the distance of ‘r’.
The usual amplitude of magnetic fields created by the brain are extremely small, they do not exceed a few hundred femto tesla (10-15 T). Compared with this the Earth’s magnetic field is between 10-4 T and 10-5 T and an MRI is usually between 1.5 to 3 T.
MEG currently has two approved indications in the United States, one is for pre-operative Brain Mapping and the other is for use in epilepsy surgery.
Strengths and Limitations
Learning Objectives
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