Effective Suppression of Residual Magnetic Interference in a Conductive Shielded Room for Ultra-Low Field Nuclear Magnetic Resonance

Abstract

Residual magnetic interference induced by applied magnetic field pulses inside a conductive shielded room (SR) has been a common issue in ultra-low-field (ULF) nuclear magnetic resonance (NMR). The rapid cutoff of the applied pre-polarizing field (B_p) induces eddy currents in the walls of the SR, which produces a decaying residual magnetic interference that may cause severe image distortions and signal loss. In this study, a pair of cancellation coils (CC) and control electronics were designed for the suppression of the residual magnetic interference in a SR. Simulations show that this method was effective in suppressing the residual magnetic field (B_r) after removal of the pre-polarizing magnetic field. Then, a small-scale SR was designed and the effectiveness of this cancellation scheme was experimentally verified. The test results showed a good agreement with the simulation, which indicated that the cancellation scheme was capable of reducing B_r field to a much lower level. The scheme proposed in this study provides a solution for suppressing the residual magnetic field in the ULF NMR system. After decoupling the eddy–current field, the effect of the suppression may be further improved by optimization of the cancellation coil in further work.