Measurement of induced currents in radio frequency magnetic fields based on near field antenna perturbations

Abstract

A measurement system is developed utilizing electromagnetic compatibility test equipment for the study of induced current in conductive materials subjected to radio frequency (RF) magnetic field strengths similar to the 1.5 T magnetic resonance imaging (MRI) B1 magnetic field at ∼65 MHz. The intent of developing such a system was to produce μT range RF magnetic fields in the laboratory to facilitate characterization of induction in conductive materials with modified surface electromagnetic properties to address unintended eddy current issues like Joule heating caused by implanted devices during MRI. A Helmholtz coil (HHC) is used as the RF magnetic field source, and the radiated field is monitored using a receiving loop antenna positioned coaxially outside the HHC. The measurement system operates in continuous wave and pulsed wave modes. Analytical models of the system were derived, which calculate the spatial distribution of RF magnetic flux and the induced current within a coaxially located sample in the transmission path between the HHC and receiving (R/C) loop from output voltage measurements at a single coaxial position. Induced currents were evaluated at multiple flux densities and at different frequencies, showing direct proportionality over the flux densities tested. Induced current results recorded in samples of different sizes and electrical conductivities (ranging from 0.1 to 5.8 × 107 (Ω m)−1 produced changes, matching trends predicted by conductive, closed-loop antenna theory. Induced currents were also used with simultaneous temperature rise measurements to characterize the effective surface conductivity for wire with non-uniform properties at 65 MHz.