Abstract: A wearable and customizable multi-shell MEG helmet comprising an inner shell and outer shell, wherein the inner shell interior surface is customized to conform to the patient's head shape so that the helmet assembly moves in unison with the patient's head movement and sensor locations are controlled and remain fixed relative to the brain. This invention improves data quality and user comfort since head movements may be permitted and their effects on data integrity is minimized. The outer shell is generic and may fit over any customized inner shell. The outer shell holds a group of sensors, which may be, but not limited to, optically pumped magnetometers. This generic outer shell may mate with the inner shell, allowing sensors to be easily pushed into the inner shell to be in closer proximity to the patient's head.