MRI safety: RF-induced heating near straight wires

The aim of this study was to develop an effective mathematical model for MR induced heating of metallic implants and to validate it with measurements. Bare and insulated wires were used to simulate biomedical implants. Current is induced in the wire by the electric field E₀ induced by the RF magnetic field. The Galerkin technique was used to determine the induced surface current on the bare or insulated wire. An integral equation was solved based on a reaction integral by using a piecewise sinusoidal approximation to determine the current as a function of distance z along the wire. The electric field in the medium with conductivity σ = 0.27 S/m and relative dielectric constant ε_r = 77 at the 64 MHz was determined from the current distribution along the wire. In order to test the computation by the moment method, RF induced heating was measured in a gelled saline phantom in an MRI body coil. The RF magnetic field was applied CW at 64 MHz. Temperature vs. time was measured with fiber optic probes. For the bare wire, the maximal temperature rise occurs at a length of 20 cm. For the insulated sires, the temperature rise increases with increasing length. The calculations are effective in predicting the heating as function of wire lengths. The study showed that there is no resonance length for insulated wires and the relative heating for capped and uncapped insulating wires depends on the insulation thickness.