Periodic Analysis of MR-Safe Transmission Lines

In this paper, a theoretical analysis is presented of transmission lines that are periodically interrupted by isolation transformers. Such lines have application as safe interconnects in MRI with internal coils, where the transformers act to block common-mode signals excited by the transmitter while passing differential-mode signals from a detector. The dispersion characteristics are derived, and it is shown that the line is a form of metamaterial supporting magneto-inductive (MI) waves. Propagation can take place in a series of bands, each close to one of the standing wave resonances of an isolated segment. However, it is shown that additional series capacitors are required to implement the protection strategy correctly and the line then operates on a low-frequency lumped-element MI band. Lines operating at 63.8 MHz frequency are constructed using coaxial cables and printed circuit board transformers, and the existence of multiple propagating bands is confirmed.