Transmission Line Model of an Implanted Insulated Cable for Magnetic Resonance Imaging Radiofrequency Hazard Evaluation

Abstract

This paper demonstrates that one can model the power deposited at the electrode of an implanted insulated cable submitted to the radiofrequency field of a 1.5 T magnetic resonance imaging modality (64 MHz) with a transmission line model. This offers an alternative that is more related to physics to the usually used transfer function model. The equivalence between the models is shown through a finite difference model and a new analytical formula for the transfer function as a function of transmission line parameters. First, the possibility of modeling an insulated cable with a transmission line model was analyzed through full-wave numerical simulations. The assumption of a transmission line model underlying the transfer function model was shown to be right for a simple cable embedded in tissue imitating gel, and the transmission line parameters extracted from this analysis were consistent with analytical formulas derived from the laws of physics. The transmission line model predictions were first compared to experimental and simulated data of the cable transfer function and then to experimental and simulated data of the resonant behavior as a function of length of cables with different termination conditions. The measured and simulated transfer functions fit perfectly a transmission line model with an analytical expression of the propagating constant. The transmission line model extracted from the transfer function allows us to predict the resonant behavior of two cables with different termination conditions.