Researchers at the University of Minnesota have announced a new noninvasive, painless method of diagnosing brain diseases. This is something we will be hearing a lot of in the future, because it represents new possibilities for less-stressful, pain free diagnosis for complex brain diseases such as Alzheimers, as well as a way to measure the effectiveness of different treatments for these diseases.
It also makes for a more objective assessment that current methods such as behavioral examinations and psychiatric interviews.
MEGs and SQUIDs
Magnetoencephalography (MEG) is an imaging technique for measuring magnetic fields produced by electrical activity in the brain. It works through extremely sensitive devices such as superconducting quantum interference devices (SQUIDs).
These measurements are commonly used in both research and clinical settings. There are many uses for the MEG, including assisting surgeons in localizing a pathology, assisting researchers in determining the function of various parts of the brain, neurofeedback, and others.
Using MEG to track tiny magnetic fields in the brain, University of Minnesota researchers recorded brain cells communicating with each other while research subjects stared at a point of light for 4560 seconds.
After applying various mathematic algorithms, the researchers were able to classify the 142 research subjects by diagnosis. Study participants fell into one of six categories, including people with Alzheimer’s disease, chronic alcoholism, schizophrenia, multiple sclerosis or Sjogren’s syndrome, as well as healthy controls.
This study was published in the Aug. 27, 2007 issue of the Journal of Neural Engineering.
“This elegantly simple test allows us to glimpse into the brain as it is working,” says lead researcher Professor Apostolos P. Georgopoulos, M.D., Ph.D. “We were able to classify, with 100 percent accuracy, the various disease groups represented in the group of research subjects.”
“This discovery gives scientists and physicians another tool to assess people’s disease progression,” he said. “In the future it could be applied when studying the effect of new treatments or drug therapies."
Activity and cognition in the brain involves networks of nerves continuously interacting; interactions that happen on a millisecond by millisecond timescale. The MEG is equipped with 248 sensors, each recording the interactions in the brain millisecond by millisecond, much more rapidly than current methods of evaluation like the functional magnetic resonance imaging (fMRI), which takes seconds to record. The MEG measurement records represent the workings of tens of thousands of brain cells.
More about the MEG
The MEG signals come from the 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. 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.
It is worth noting that action potentials do not usually produce an observable field, mainly because the currents associated with action potentials flow in opposite directions and the magnetic fields cancel out. However, action fields have been measured from peripheral nerves.