Asymmetric three-dimensional finite element analysis of a magnetically shielded room with access ports

As part of a new initiative in multimodal functional neuroimaging at Dartmouth, a novel design for an economical magnetically shielded room (MSR) has been custom fabricated and installed on site. A three-dimensional finite element analysis (FEA) model was developed in concert with the MSR for evaluation of critical aspects of the design and then determination of degaussing effectiveness. The FEA model generates a full three-dimensional map of the shielding factor as a function of frequency and is sensitive to the number, thickness, and spacing of wall shielding, as well as access ports such as the door, vents and feed through panel. This simulation uses quadratic vector Lagrange elements to solve for the total electric and magnetic potentials using a timeharmonic analysis. With this model, estimation of the spatial variation in the shielding factor for a modestly shielded room is demonstrated. The Dartmouth MSR has internal dimensions of 2.35m x 2.35m x 2.16m and has two high conductivity eddy current layers and one mu metal layer (two additional layers, one eddy current and one mu metal layer will be added in the near future). The simulation results indicate that the shielding factor at the center of the MSR does not vary significantly with incident angle of the applied magnetic field. However, spatial inhomogeneities in the shielding factor are evident near MSR access ports. The results of this simulation will be used in future work, where the degaussing of the room will be validated against modeling predictions. Our future modeling work will add a virtual patient and virtual instruments, electroencephalography (EEG), and near-infrared spectroscopy (NIRS), for evaluation of achievable multimodal contrast to noise.