Simulation and Measurements of Transient Fields From Conductive Plates of Shielded Room for SQUID-Based Ultralow Field Magnetic Resonance Imaging

In order to enhance the signal-to-noise ratio (SNR) of ultralow field magnetic resonance imaging (MRI) measurements, a prepolarization (Bp) coil is usually introduced to generate a magnetic field with magnitude in the order of tens of mT or even stronger to prepolarize the sample before MRI sequences. The macromagnetization M of the sample is now directly determined by the Bp strength, thus the SNR is effectively improved. However, the rapid turn-off of Bp current induces eddy currents in the conductive shielded room and the surrounding conductors, which in turn causes unwanted transient magnetic fields in the imaging area. In this paper, according to the relative position of the Bp coil and the shielded room, two physical models of the conductive shielded room are described to simulate the transient fields at the sample position: the Bp coil plane parallel to the ceiling and floor of the shielded room, and the coil plane perpendicular to the walls. The simulation is in good agreement with the experimental results. This paper will help us design the structure of the conductive shielded room as well as develop the eddy current cancellation technique.