Magnetoencephalography (MEG) is a sophisticated, non-invasive neuroimaging technique that provides a direct window into the live electrical activity of the human brain. This method measures the tiny magnetic fields generated by neuronal currents, allowing clinicians and researchers to map brain function with remarkable precision. MEG captures and maps neural activity as it occurs, offering a functional view of the brain rather than just an anatomical one. It is used for evaluating the brain’s ongoing processes and localizing the source of abnormal activity.
The Physics: How MEG Detects Brain Activity
Brain activity is fundamentally an electrical process, where ionic currents flow through neurons, primarily the pyramidal cells in the cerebral cortex. When a large population of these neurons fires synchronously, the collective flow of electrical current produces an extremely weak magnetic field that extends outside the head. These neuromagnetic fields are the signals that Magnetoencephalography is designed to detect.
The magnetic fields generated by the brain are incredibly faint, often a billionth of the strength of the Earth’s magnetic field, and are measured in femtotesla (fT). Detecting such minute signals requires highly specialized, ultra-sensitive sensors known as Superconducting Quantum Interference Devices, or SQUIDs. SQUIDs function as magnetometers, converting the weak magnetic flux into a measurable voltage. A significant advantage of measuring the magnetic field is that it is not distorted by the skull, scalp, and other biological tissues. Unlike electrical signals, which become smeared as they travel through these insulating layers, magnetic fields pass through unimpeded. This property allows MEG to pinpoint the source of neural activity with greater spatial accuracy.
The Patient Experience and Specialized Equipment
The MEG procedure is non-invasive and painless, involving no radiation or injections. Patients typically sit or lie down comfortably while their head is positioned within a helmet-shaped array that houses the SQUID sensors. To avoid interference from external magnetic sources, the entire MEG system is housed within a magnetically shielded room (MSR).
The SQUID sensors rely on superconductivity to achieve their extreme sensitivity, requiring them to be kept at extremely low temperatures. This is achieved using cryogenics, with the sensors immersed in liquid helium. The sensors are contained within a specialized, vacuum-insulated vessel known as a dewar. Patients are asked to remove any metal objects, such as jewelry or clothing with metal fasteners, that could disrupt the magnetic field measurements. During the scan, the patient’s head must remain as still as possible. The duration of a clinical MEG scan is usually between 40 to 60 minutes.
Clinical and Research Applications
MEG is a valuable tool in both clinical medicine and neuroscience research, primarily due to its ability to capture the timing of brain events with high fidelity.
Clinical Applications
MEG is primarily used in clinical settings for pre-surgical evaluation.
- Pre-surgical evaluation of patients with medically refractory epilepsy. MEG records spontaneous brain activity to precisely localize the irritative zone where seizures originate, guiding neurosurgeons.
- Functional brain mapping before tumor removal or other neurosurgical procedures. This involves mapping the eloquent cortex—areas responsible for functions like movement, sensation, and language—to ensure these regions are preserved during surgery.
Research Applications
In research, MEG is widely used to investigate the temporal dynamics of cognitive processing and sensory perception. Scientists can track the millisecond-by-millisecond communication between different brain regions, studying how the brain processes information related to vision, hearing, and memory. This capability advances the understanding of neurological and psychiatric conditions, including autism, Parkinson’s disease, and schizophrenia.
Distinguishing MEG from Other Neuroimaging Tools
Magnetoencephalography is often considered alongside Electroencephalography (EEG) and functional Magnetic Resonance Imaging (fMRI). The primary factor distinguishing these modalities is the trade-off between temporal resolution (precision in time) and spatial resolution (precision in location).
MEG and EEG both directly measure the electromagnetic products of neuronal activity, granting them excellent temporal resolution and capturing brain events on a millisecond scale. In contrast, fMRI measures the hemodynamic response, an indirect proxy of neural activity based on blood oxygenation changes, resulting in a temporal resolution measured in seconds. However, fMRI offers superior spatial resolution, providing images with millimeter-scale anatomical detail. While EEG also boasts high temporal resolution, its electrical signals are significantly distorted by the skull, which limits its spatial accuracy. MEG offers the millisecond-level timing accuracy of EEG, but with significantly better spatial localization because the magnetic fields are unaffected by the scalp and skull.