MEG – Magnetoencephalography

from wikipedia: http://en.wikipedia.org/wiki/Magnetoencephalography

Magnetoencephalography (MEG) is an imaging technique used to measure the magnetic fields produced by electrical activity in the brain via extremely sensitive devices such as superconducting quantum interference devices (SQUIDs).

History of the MEG

 The MEG was first measured by University of Illinois physicist David Cohen in 1968.

At first, only a single SQUID detector was used, to successively measure the magnetic field at a number of points around the subject’s head. This was cumbersome, and in the 1980s, MEG manufacturers began to increase the number of sensors in the Dewar to cover a larger area of the head, using a correspondingly larger Dewar. Present-day MEG Dewars are helmet-shaped and contain as many as 300 sensors, covering most of the head. In this way, MEGs of a subject or patient can now be accumulated rapidly and efficiently.

The basis of the MEG signal

 

Synchronized neuronal currents induce very weak magnetic fields that can be measured on MEG. However, the magnetic field of the brain is considerably smaller at 10 fT (femtotesla) for cortical activity and 10³ fT for the human alpha rhythm than the ambient magnetic noise in an urban environment, which is on the order of 10⁸ fT. Two essential problems of biomagnetism arise: weakness of the signal and strength of the competing environmental noise. The development of extremely sensitive measurement devices, SQUIDs, facilitates analysis of the brain’s magnetic field and confronts the aforementioned problems.

 

The MEG (and EEG) signals derive from the net effect of ionic currents flowing in the dendrites of neurons during synaptic transmission. In accordance with Maxwell’s equations, any electrical current will produce an orthogonally oriented magnetic field. It is this field which is measured with MEG. The net currents can be thought of as current dipoles which are currents defined to have an associated position, orientation, and magnitude, but no spatial extent. According to the right-hand rule, a current dipole gives rise to a magnetic field that flows around the axis of its vector component.

In order to generate a signal that is detectable, approximately 50,000 active neurons are needed.^[4] Since current dipoles must have similar orientations to generate magnetic fields that reinforce each other, it is often the layer of pyramidal cells in the cortex, which are generally perpendicular to its surface, that give rise to measurable magnetic fields. Furthermore, it is often bundles of these neurons located in the sulci of the cortex with orientations parallel to the surface of the head that project measurable portions of their magnetic fields outside of the head. Researchers are experimenting with various signal processing methods to try to find methods that will allow deep brain i.e., non-cortical, signal to be detected, but as of yet there is no clinically useful method available.

Source localization

The inverse problem

In order to determine the location of the activity within the brain, advanced signal processing techniques are used which use the magnetic fields measured outside the head to estimate the location of that activity’s source. This is referred to as the inverse problem. (The forward problem is a situation where we know where the source(s) is (are) and we are estimating the field at a given distance from the source(s).) The primary technical difficulty is that the inverse problem does not have a unique solution, i.e., there are infinite possible “correct” answers, and the problem of finding the best solution is itself the subject of intensive research. Adequate solutions can be derived using models involving prior knowledge of brain activity.