Magnetoencephalography in the diagnosis of concussion

Magnetoencephalography (MEG) is a biomedical technique which measures the magnetic fields emitted by the brain, generated by neuronal activity. Commercial whole-head MEG units have been available for about 15 years, but currently there are only about 20 such units operating in the USA. Here, we review the basic concepts of MEG and list some of the usual clinical indications: noninvasive localization of epileptic spikes and presurgical mapping of eloquent cortex. We then discuss using MEG to diagnose mild traumatic brain injury (mTBI; concussions). Injured brain tissues in TBI patients generate abnormal low-frequency magnetic activity (delta-waves: 1-4 Hz) that can be measured and localized by MEG. These abnormal delta-waves originate from neurons that experience deafferentation from axonal injury to the associated white matter fiber tracts, also manifested on diffusion tensor imaging as reduced fractional anisotropy. Magnetoencephalographic evaluation of abnormal delta-waves (1-4 Hz) is probably the most sensitive objective test to diagnose concussions. An automated MEG low-frequency (slow wave) source imaging method, frequency-domain vector-based spatiotemporal analysis using a L1-minimum norm (VESTAL), achieved a positive finding rate of 87% for diagnosing concussions (blast-induced plus nonblast), 100% for moderate TBI, and no false-positive diagnoses in normal controls. There were also significant correlations between the number of cortical regions generating abnormal slow waves and the total postconcussive symptom scores in TBI patients.