Magnetoencephalography

Your brain uses tiny bits of power to work.

This power makes very small magnetic fields. These fields are too weak to feel. We use a special helmet to find them.

The helmet has many sensors. These sensors act like tiny magnets. They help us map your brain. We must use a special room to block out noise. This keeps the signals clear. This helps doctors see how your brain works. It can even find parts that are sick.

Science helps us learn more every day.

Your brain is always working. Tiny electrical currents flow through your brain cells. These currents make very small magnetic fields.

Scientists use a tool called Magnetoencephalography, or MEG, to find them. MEG maps how your brain works by measuring these fields.

To find these weak signals, we use special sensors. Most machines use SQUIDs. These are sensors that use super-cold parts to work. Today, many machines use a helmet with 300 sensors. This helmet covers most of the head.

Some new machines use SERF magnetometers. These are smaller and do not need big cooling systems. They might even fit in a bike helmet one day!

Measuring the brain is hard. The magnetic fields from the brain are very small. Even the Earth has a magnetic field. This can act like loud noise. To fix this, we use a Magnetically Shielded Room, or MSR.

These rooms use layers of metal to block out noise. This helps the sensors hear the brain clearly. Doctors use MEG to find parts of the brain that are sick. It also helps them study how we think and see.

Magnetoencephalography, or MEG, is a special way to map how the brain works. It works by recording tiny magnetic fields. These fields are made by electrical currents that happen naturally in your brain. Scientists use very sensitive tools called magnetometers to find these signals.

This tool is very important for many reasons. Doctors use it to find parts of the brain that might be sick. It helps them plan surgeries to remove problems safely. Researchers also use MEG to study how we think and see things. It is a powerful way to look at the living brain in action.

How does this process actually work? Inside the brain, tiny electrical currents flow through parts of brain cells called dendrites. When these currents move, they create a magnetic field. This follows a rule of science where any electrical current makes a magnetic field. To get a signal strong enough to measure, you need about 50,000 active neurons working together. These neurons must be lined up in a certain way to help the signal grow.

Usually, these signals come from bundles of neurons located in the folds of the brain. The machine measures these fields to show where the activity is happening.

People have been working on this for a long time. In 1968, a physicist named David Cohen first measured these signals. He worked at the University of Illinois. Back then, he used a copper induction coil to find the signals. It was a very hard job because the measurements were noisy and not very clear. Later, Cohen moved to MIT and built a much better shielded room. He used one of the first SQUID detectors made by James E. Zimmerman at Ford Motor Company.

This new way made the signals much clearer than before.

Today, MEG machines use many different parts and numbers. Most machines use arrays of sensors called SQUIDs. These sensors are often placed in a helmet that looks like a vacuum flask. This helmet can hold about 300 sensors to cover most of the head.

The brain's magnetic signals are incredibly weak. They measure only about 10 femtotesla for some brain activity. This is much smaller than the magnetic noise in a city. To block this noise, scientists use a Magnetically Shielded Room, or MSR.

These rooms use layers of aluminum and special metal to keep the signals clear.

We can connect what we learn from MEG to other things we know. For example, you might have seen an MRI scan of a brain. Scientists can combine MEG data with MRI images to make a magnetic source image.

This helps them see the brain's activity sitting right on top of its shape. New technology is also making these tools easier to use. Scientists are testing SERF magnetometers, which are smaller and do not need huge cooling systems. They even made a prototype using a 3D-printed helmet. In the future, these might even be as easy to use as a bike helmet!

Magnetoencephalography, or MEG, is a functional neuroimaging technique used to map brain activity. It works by recording the extremely weak magnetic fields produced by natural electrical currents in the brain. This process is vital for both medical and scientific work. Clinicians use MEG to localize regions affected by pathology before surgical removal. Researchers use it to study cognitive processes and determine the functions of specific brain parts. It can also be used for neurofeedback, which helps people learn to regulate their own brain activity.

The mechanism of MEG relies on fundamental laws of physics. Inside the brain, ionic currents flow through the dendrites of neurons during synaptic transmission. According to Maxwell's equations, any electrical current will produce a magnetic field. These currents can be viewed as current dipoles, which are mathematical models representing a specific position, orientation, and magnitude. To create a signal strong enough for a detector to find, approximately 50,000 active neurons must work together. These neurons often belong to layers of pyramidal cells that sit perpendicular to the cortical surface. When these bundles of neurons are oriented tangentially to the scalp, their magnetic fields project outside the head, typically from the sulci, or the folds of the brain.

Measuring these signals is a massive technical challenge because the brain's magnetic field is incredibly faint. Cortical activity measures at about 10 femtotesla (fT), while the human alpha rhythm measures at 103 fT. In comparison, the ambient magnetic noise in an urban environment is around 10^8 fT, or 0.1 μT. Because the signal is so much smaller than the background noise, scientists must use highly sensitive magnetometers. Currently, the most common sensors are arrays of SQUIDs, which stands for superconducting quantum interference devices. Researchers are also investigating SERF (spin exchange relaxation-free) magnetometers for future use. These SERF sensors are smaller and do not require the bulky cooling systems that SQUIDs need.

The history of MEG began with physicist David Cohen at the University of Illinois in 1968. Initially, Cohen used a copper induction coil as a detector. However, these early measurements were noisy and difficult to use. To improve results, Cohen built a better magnetically shielded room at MIT. He then used one of the first SQUID detectors, which had been developed by James E. Zimmerman at Ford Motor Company.

This advancement made the signals nearly as clear as those from an EEG. In the 1980s, technology moved from single sensors to arrays of multiple sensors to cover larger areas of the head.

To protect these tiny signals, MEG systems require a Magnetically Shielded Room (MSR). An MSR is constructed using nested layers of aluminum and a high-permeability ferromagnetic material, such as molybdenum permalloy. The aluminum helps reduce high-frequency noise, while the ferromagnetic layer reduces low-frequency noise.

These rooms often use active shielding systems to provide three-dimensional noise cancellation. These systems use low-noise fluxgate magnetometers and noise cancellation algorithms to lower the noise floor. Modern systems can achieve a noise floor of around 2–3 fT/Hz^0.5 above 1 Hz.

A major part of MEG science is solving the "inverse problem." This is the challenge of determining exactly where the electrical activity is located inside the brain based only on the magnetic fields measured outside the head. This is difficult because the inverse problem does not have a unique solution; there are infinite possible answers. Scientists use various mathematical models to find the best estimate. Some use over-determined models, which assume the activity comes from a few specific points called equivalent dipoles. Others use under-determined models, which look for distributed source solutions across many areas. Techniques like beamforming or expectation-maximization help researchers narrow down these possibilities.

One powerful application is creating Magnetic Source Images (MSI). Scientists combine MEG data with Magnetic Resonance Imaging (MRI) to show brain activity on a structural map.

This is done through a process called coregistration, where specific points on the head are marked in both scans to create a common coordinate system. While these images often show colorful areas of activity, researchers note that these colors represent a probability cloud rather than a physiological certainty. Despite this uncertainty, MSI is a highly useful tool when it corroborates other medical data.

Future developments aim to make MEG more portable and accessible. While present-day arrays are often housed in large, helmet-shaped vacuum flasks containing about 300 sensors, new technology is changing this. In 2012, it was shown that MEG could work with chip-scale atomic magnetometers (CSAM). By 2017, researchers built a prototype using SERF magnetometers in 3D-printed helmets. These portable systems could eventually be as simple to use as a standard bike helmet, making brain mapping much easier to perform in different settings.