Shielding of low-frequency magnetic interference in weak-field MRI by a single-layer cylindrical coil

Shielding of detection coil from low-frequency magnetic-field interference is one of the main problems in weak-field MRI methods that utilize cycling of main magnetic field (MRI in the Earth's magnetic field, for example). In such cases, the best solution is usually to shield the detection coil by shorting one of the resistive coils of the system, typically the magnetization coil. The optimization of this shielding method has been done by studying the total magnetic field on the axis of a shorted single-layer cylindrical coil placed in a homogeneous oscillating magnetic field parallel to the coil axis. The results are generalized for the case when series impedance is added to the shielding coil. The optimal added impedance that gives the largest shielding factor in the center of the coil is calculated. Because the theoretical treatment is confined to the coil axis, the results are in simple form ready to use in applications. The results of the calculation are verified experimentally and implemented in Earth's field MRI system. (C) 1997 Academic Press.