Active-Passive Shielding for MRI Acoustic Noise Reduction: Network Analysis

We have modeled the effect of passive copper shielding applied to the outside of an actively shielded, axisymmetric z-gradient coil assembly, with the aim of substantially reducing induced eddy currents in the cryostat inner bore that create acoustic noise. For the purpose of calculation, the cylindrical cryostat inner bore and the passive copper shielding are coaxial and are imagined to be sliced into thin ring sections. Each ring section has a finite thickness and is further divided into several concentric layers. The thin cylindrical sections become elements in an electrical network that includes the actively shielded gradient coil. We calculate eddy currents both for single frequency excitations and for time-dependent excitations by modeling a series of trapezoidal pulses. Our results take into account time dependence, diffusion of eddy currents among cylindrical sections and skin depth effects. Two configurations are analyzed. The first is a thin copper layer wrapped around the outer diameter of the gradient assembly, and the second extends the copper over the ends of the gradient assembly. A 2-mm-thick copper layer around the gradient assembly reduces power deposited in the cryostat inner bore by 13.5 dB for a 1 kHz gradient excitation. Extending the passive shield, for the same 2 mm thickness, to cover the ends of the gradient reduces cryostat inner bore power deposition, by 26.7 dB for the same frequency