An Ultra-low Vibration Helium Reliquefier for Dual-Helmet Magnetoencephalography (MEG)

Abstract

An ultra-low vibration helium reliquefier has been developed that can be integrated into an innovative magnetoencephalography (MEG) system with dual helmets for adult and pediatric use. The compact reliquefier establishes a closed-cycle helium loop for the MEG dewar allowing for uninterrupted, maintenance-free operation. The reliquefier, employing a Cryomech 4 K pulse tube cryocooler (model PT420), provides a liquefaction rate of 21 L/day from room temperature helium gas. Several vibration reduction measures have been applied to the system. The reliquefier incorporates a ∼1.5 m long, horizontal, flexible liquid return line allowing it to be mounted outside of the magnetically shielded room (MSR). This arrangement greatly reduces vibrational, acoustic, and magnetic interference. In the prototype tests, this setup reached noise levels of ∼18 fT/ÖHz without the use of noise cancellation software. The spontaneous brain activity signal showed nearly identical signal quality with the reliquefier turned on and off.