Measurement of neuromagnetic brain function in pre-school children with custom sized MEG

Magnetoencephalography is a technique that detects magnetic fields associated with cortical activity [1]. The electrophysiological activity of thebrain generates electric fields - that can be recorded using electroencephalography (EEG)- and their concomitant magnetic fields - detected byMEG. MEG signals are detected by specialized sensors known as superconducting quantum interference devices (SQUIDs). Superconductingsensors require cooling with liquid helium at -270 °C. They are ontained inside a vacumm-insulated helmet called a dewar, which is filled withliquid. SQUIDS are placed in fixed positions inside the helmet dewar in the helium coolant, and a subject's head is placed inside the helmet dewarfor MEG measurements. The helmet dewar must be sized to satisfy opposing constraints. Clearly, it must be large enough to fit most or all of theheads in the population that will be studied. However, the helmet must also be small enough to keep most of the SQUID sensors within range of thetiny cerebral fields that they are to measure. Conventional whole-head MEG systems are designed to accommodate more than 90% of adult heads.However adult systems are not well suited for measuring brain function in pre-school chidren whose heads have a radius several cm smaller thanadults. The KIT-Macquarie Brain Research Laboratory at Macquarie University uses a MEG system custom sized to fit the heads of pre-schoolchildren. This child system has 64 first-order axial gradiometers with a 50 mm baseline[2] and is contained inside a magnetically-shielded room(MSR) together with a conventional adult-sized MEG system [3,4]. There are three main advantages of the customized helmet dewar for studyingchildren. First, the smaller radius of the sensor configuration brings the SQUID sensors into range of the neuromagnetic signals of children's heads. Second, the smaller helmet allows full insertion of a child's head into the dewar. Full insertion is prevented in adult dewar helmets because of thesmaller crown to shoulder distance in children. These two factors are fundamental in recording brain activity using MEG because neuromagneticsignals attenuate rapidly with distance. Third, the customized child helmet aids in the symmetric positioning of the head and limits the freedom ofmovement of the child's head within the dewar. When used with a protocol that aligns the requirements of data collection with the motivational andbehavioral capacities of children, these features significantly facilitate setup, positioning, and measurement of MEG signals.