The MEG system and laboratory facilities
What is MEG?
Magnetoencephalography (MEG) is a brain imaging modality that maps brain activity by sampling tiny magnetic fields produced by electrical currents occurring naturally in the brain. MEG measures the brain activity in the scale of neuronal systems and networks. It is closely related to the older imaging modality electroencephalography (EEG), which measures brain activity by measuring electrical currents from the scalp.The strengths of MEG are its great temporal accuracy, good spatial accuracy, and non-invasiveness. On the temporal side, MEG is unrivaled since it can accurately measure brain activity within a range of a few milliseconds. Optimal and interference free measurements yield the possibility of good spatial accuracy with MEG. The problem with spatial accuracy is the ill-posed inverse problem. As the brain is composed of around 86 billion neurons and around 15 billion cortical neurons, the MEG with its 300 sensors can’t reliably decode brain activity as such. That said, the advances in computing and modern approaches to the problem allow the MEG to map brain activity with relatively good spatial accuracy. Many of the approaches utilize a priori information, aka. additional information through which the sensor data can be decoded to yield a single solution to the inverse problem. Finally, the MEG is excellent in its non-invasiveness. Compared to EEG, MEG can record just with the subject sitting in the measurement seat with their head in the MEG helmet. Often additional electrodes and coils are attached to the subject’s head, but this is not explicitly necessary. Compared to say magnetic resonance imaging (MRI), in MEG the subject is seated comfortably and there is little to no noise from the equipment.
Some of the weaknesses of MEG are its cost, somewhat limited clinical uses, and sensitivity to interference. A modern full head MEG system costs upwards of 1-2 million USD, and the ones that rely on SQUID-sensors have maintenance costs and might require helium refills. MEG scanners are quite sensitive to external magnetic fields as a side product of their operating mechanism. To combat the effects of external magnetic fields the MEG scanners are housed in a shielded rooms and preferably separated from the foundation of the rest of the building. There are also hardware and software shielding mechanisms that can be used to attenuate the magnetic interference. One of the limiting factors of wide scale adoption of MEG scanners is their relatively limited clinical use. MEG scanners have been used in epilepsy diagnostics and both tumor and epilepsy surgery as a presurgical method for identifying and mapping brain areas of interest.
Many of the MEGs weaknesses have been reduced with state-of-the-art technology in both hardware and software advancements in the last 2 decades. Also, there are numerous emerging neurophysiological uses for MEG, and other functional neuroimaging methods, being investigated and validated around the world.
Image of MEG.