Magnetoencephalography (MEG) uses sensitive superconducting detector coils to non-invasively record the magnetic signals of brain cell communications with millisecond time resolution. The magnetometer does not produce any magnetic fields or electrical currents, it only passively listens to the magnetic fields produced spontaneously in a person's brain. This recording occurs in a manner similar to how EKG and EEG listen to electrical currents produced by a person's heart and brain.
Brain regions communicate with each other in time spans less than 10 milliseconds. Complex cognitive decisions, involving multiple communications between several brain regions, can occur in 200 to 300 milliseconds. The rapid response time of MEG allows this technology to track communications between groups of brains cells occurring in one hundredth to one third of a second. Although a three-dimensional scan of the millisecond by millisecond activity can be obtained with MEG, the MEG signal does not inherently contain anatomical information.
The MEG signals of interest are extremely small, several orders of magnitude smaller than other signals in a typical environment that can obscure the signal. Thus, specialized shielding is required to eliminate the magnetic interference found in a typical urban clinical environment.
Recent Advances in MEG Technology
The technological development of the whole head magnetometer (275 sensors) has occurred in the past few years. A single spontaneous or evoked response study can now be done in about an hour, rather than the 6-7 hours required when instruments had only 30 - 70 sensors and the instrument had to be repositioned over the head multiple times to complete a study. This advance has moved the length of the study into the time duration a child or adolescent can tolerate. Advances in the software for 'deciphering' the MEG signals have progressed in the last few years, greatly improving the quality of information we can obtain about brain processes. It is the combination of these two advances that make MEG viable for the study of children and adolescents.
The MEG Systems at CCHMC
The MEG Center at Cincinnati Children's Hospital Medical Center (CCHMC) comprises approximately 676 square feet of dedicated research laboratory space located in the Concourse A Rome 180 on the 7th floor in the department of Neurology. The lab includes a whole head MEG system which is housed in a magnetic shield room, for enhanced passive noise reduction. The MEG center also contains an array of stimulus and response devices such as Optical Button Boxes. The center has all necessary equipment such as audio-video monitor system, air-temperature alarm system for testing small children and clinical patients. The MEG system is a CTF MEG system (VSM MedTech Ltd, Coquitlam, BC, Canada) with 275 MEG channels and 128 electroencephalogram (EEG) electrodes. The MEG system allows real-time signal processing for fast analysis capability with sampling rates of up to 12 kHz. The fully integrated MEG and EEG subsystems can minimize phase distortions; the synthetic third-order gradient noise cancellation technology can minimize environmental disturbances.
Comparison to EEG
Both MEG and EEG are able to record changes in brain signals in the millisecond range. Magnetic fields pass through the skull unimpeded. Electrical currents must pass through the poorly conductive skull before they can be recorded at scalp electrodes. The sources of brain activity can be more easily determined with MEG than EEG, because the conductivity of the skull does not need to be known for MEG localization. This difference can be important for persons with medication-resistant seizures who require brain surgery to stop their seizures. The non-invasive EEG findings often have to be verified first by a surgical procedure to record seizures directly from the brain surface over the course of a week or more. The greater accuracy of non-invasive MEG localization may allow some patients to avoid that first surgery, and proceed directly to brain surgery to stop their seizures.
Comparison to MRI
MRI and MEG both use superconducting technology, but are completely different. MRI uses an external (machine-made) magnetic field that is applied to the head to look at brain anatomy and blood flow patterns. MEG records brain physiology by measuring the spontaneous magnetic fields generated by the brain cells themselves as they communicate with each other. The strength of MRI is in anatomical localization. The strength of MEG is in the time resolution of brain cells sending and receiving signals. MEG studies will be integrated with anatomical MRI studies to improve the anatomic localization of MEG recordings. MEG will provide the millisecond time resolution that is so important to understanding brain processes. The millisecond time resolution is not available with an MRI study.
The combination of the excellent tracking speed of MEG combined with the excellent anatomical representation of MRI may provide us with very good information how the brain functions normally and how to help children with neurologic disorders.
Clinical Applications of MEG
Epilepsy is a debilitating disease that is especially prevalent among children. In many cases, it can be surgically cured by removing the brain tissue identified as causing epileptic seizures. The major challenge, however, lies in pinpointing the exact origin of the epileptic activity and removing only that part of the brain that is causing symptoms. The current 'gold standard' for localization, electrocorticogram (ECoG), involves a highly invasive surgical procedure whereby a sensor grid is placed directly on the patient's brain and connected to a portable EEG that the patient must wear in the Intensive Care Unit. Not only can this procedure be extremely traumatic for the patient, it poses its own potential health risks. The temporal and spatial resolution of MEG is comparable to ECoG, raising the possibility that it can be used for localization of epileptic foci as a non-invasive procedure. MEG has also already proven to be a highly useful technique for investigating epileptic activity, and a significant source of information for guiding surgical decisions. Additionally, MEG can detect smaller zones of epileptic activity through more precise information (millimeter spatial accuracy).
About 30 MEG systems are currently being used for epilepsy localization at a number of high-profile clinical sites worldwide. As a non-invasive technology, MEG offers many powerful benefits: (1) Improved patient diagnosis can be achieved non-surgically; (2) Results are achieved at a lower cost and, most importantly, with less trauma and lower risk to the patient.
In cases of intractable partial seizure disorders, surgery may be the only treatment course to provide a significant reduction in seizures. For intracranial procedures, it is critical that areas of the brain responsible for key functions such as sight, hearing, language and motor skills are not damaged. MEG allows surgeons to accurately identify these regions of eloquent cortex. MEG data can be transferred to neuronavigational equipment to ensure that these areas emerge from surgical procedures untouched.
MEG is currently being investigated for use in the diagnosis, assessment and treatment of:
- Angelman Syndrome
- Alsheimer's Disease
- Attention Deficit Hyperactivity Disorder
- Brain Tumors
- Headaches (Migraines)
- Parkinson's Disease
- Multiple Sclerosis
- Traumatic Brain Injury (TBI)
- Cochlear Implants
- Pharmacological Agents
- Stereotactic Radiotherapy