What is MEG?

Magnetoencephalography (MEG) is an imaging technique which has millisecond time resolution and an excellent ability to localize brain function precisely. It is entirely non-invasive, with no applied magnetic fields, radiation or injections of any kind. Preparation and measurement times are short. Unlike MRI, it is completely silent and well tolerated, even by children.

Currently, the main clinical applications are for epilepsy and pre-surgical functional mapping; however there are many other uses, both in neuroscience research and clinical practice. MEG examinations have FDA approval and are reimbursable in the US and other countries.

Clinically, MEG leads to better surgical outcomes and brings an institution’s technology to the level of competing neuroscience programs. A strong MEG program leads to centre distinction and helps attract top level physicians. Scientifically, MEG always leads to numerous research and collaborative programs. It attracts world class research scientist.

The number of MEG labs in operation has been increasing since the introduction of the technology in the early 1990s. The global total is currently close to 200.

How does it Work?

MEG operates by detecting at several hundred locations the extremely tiny magnetic fields generated by currents within the neurons of the brain. The pattern of these fields is used to precisely determine the parts of the brain that are functionally active. These locations can then be accurately superimposed on an MRI or CT scan to provide information about both the anatomy and function of the brain.

This is entirely different from information provided by CT or MRI alone, which provide only structural information. MEG shows active areas, whether they be important for normal brain function or a marker of pathology.

MEG is much more spatially accurate than EEG, because the skull and the tissue surrounding the brain affect the magnetic fields much less than they do the electric. Therefore, MEG offers greater localization accuracy than traditional EEG tests. EEG can only achieve the accuracy of MEG if the electrodes are placed directly on the cortex, a highly invasive and sometimes dangerous procedure.

Despite this, MEG is well placed as a multi-modality imaging tool. It combines synergistically with EEG, since each technology detects an orthogonal component to the other. For example, MEG detects epileptic spikes in about 75% of patients, whereas EEG detects them in about 60%. When MEG and EEG are combined, almost all spikes are detectable. MEG combines with MRI or CT scans to give a functional-anatomical image known as a Magnetic Source Image (MSI). fMRI and PET each provide vascular information, which add confirmations to the direct neurophysiological measurement of MEG.

Historically, MEG systems required a cryogenic dewar containing liquid helium for the cooling of “Superconducting Quantum Interference Device” (SQUID) sensors. With the York Instruments design, no liquid Helium is required and more sensitive HyQuid™ sensors are used instead.

The unique features of MEG include:

  • Direct measure of brain function. This is unlike fMRI, PET and SPECT, which are secondary measures of metabolism.
  • High temporal resolution. Events with millisecond duration/frequency can be resolved, unlike fMRI, PET and SPECT, which have much longer time scales.
  • High spatial resolution. Sources can be localized with millimetre accuracy. This includes those who have had past brain surgery, where EEG is severely distorted.
  • Completely non-invasive, safe and silent.

What has it been Used For?

MEG has a long history in neuroscience research. Initially, MEG was found to be ideal for localizing various sensory and motor related responses. Such measurements with MEG were often many years ahead of any other modality. MEG was used to separate responses generated in the primary and secondary somatosensory cortices by response timing and orientation. This proved to be important for understanding disorders of these networks. Determination of the organization of the auditory and visual cortices soon followed.

In the 1990s breakthroughs were made with MEG in the measurement of brain rhythms and cognitive processing. Much work was done studying activation patterns and cognition in, for example, language processing and brain plasticity. The study of oscillatory brain activity became a focus that MEG was uniquely suited for.

More recently, MEG has been embraced in the study of cognition, connectivity and neural development. Research emphasis has shifted to higher cognitive function, including language and social interaction. Normal brain development and its disorders are increasingly being understood. Connectivity between different areas of the brain is being studied with coherence estimation. ‘Resting-state’ networks are being found to be of great importance.

How does it Benefit Patients?

For patients with intractable epilepsy, surgery is often the best solution to end seizures. MEG is used to localize interictal (between seizure) activity, which is usually the source of the seizures themselves. MEG is particularly useful when MRI is negative (does not show a lesion) or routine EEG is inconsistent. MEG then guides the surgeon to a successful resection. Some findings may suggest a more complex situation, with a need for more investigations, or even the impossibility of surgery.

Another routine application of MEG is pre-surgical functional mapping (PSFM). This is useful for patients who have a tumours, other lesions, vascular malformations, epilepsy and/or brain injury. MEG is used to map the exact location of the healthy areas near the pathology, so that surgery does not result in postoperative weakness or loss of function. These functional areas, known as eloquent cortex, can include those used for audition, vision, motor control, language, etc.

Beyond the routine clinical indications for MEG, there are many other emerging applications, including mild traumatic brain injury, post-traumatic stress disorder, Alzheimer’s disease, autism, stroke recovery, dyslexia, stuttering and others.

Due to its fidelity and high temporal resolution, MEG has the ability to discern human brain networks with unprecedented accuracy. Increasingly, neuroscientists believe that many clinical disorders are caused by brain network interruptions. For example, evidence has shown that disruptions in the brain’s network can lead to both Alzheimer’s and autism. This positions MEG as the brain imaging modality of choice for studying and diagnosing these disorders.

The NIH funded Human Connectome Project is mapping the human brain as accurately as possible to gain information about connectivity, function and parcellation of the brain. It is largely focused on communication networks. The principle investigators have stated that because MEG measures neuronal activity in “real time” the connections activated either at rest or during task can be measured, giving a picture of the dynamic interactions among brain networks. Furthermore, because it has much greater temporal resolution than fMRI, MEG-based analysis provides correlates of connectivity, time-frequency content and temporal interactions. MEG provides the information that can reveal information flow between nodes of key cognitive networks.