Degaussing a Two-layer Mumetal Shield for Magnetoencephalography

Magnetoencephalography (MEG), which is sensitive to electrical currents in neurons in the brain, is an important diagnostic of diseases such as epilepsy. As the resulting magnetic fields are at the femtotesla level, optical magnetometers are very sensitive to fluctuations in the Earth's magnetic field and background EMF, which must be shielded in order to provide accurate signals. We have repurposed a 1 meter cubed two-layer mumetal box to perform MEG measurements. To achieve the highest shielding factor each layer must be properly degaussed. We will discuss the process used to degauss the shield as well as the optimal parameters, such as frequency, maximum current, profile, and isolation with transformers.