RF heating due to conductive wires during MRI depends on the phase distribution of the transmit field

In many studies concerning wire heating during MR imaging, a “resonant wire length” that maximizes RF heating is determined. This may lead to the nonintuitive conclusion that adding more wire, so as to avoid this resonant length, will actually improve heating safety. Through a theoretical analysis using the method of moments, we show that this behavior depends on the phase distribution of the RF transmit field. If the RF transmit field has linear phase, with slope equal to the real part of the wavenumber in the tissue, long wires always heat more than short wires. In order to characterize the intrinsic safety of a device without reference to a specific body coil design, this maximum-tip heating phase distribution must be considered. Finally, adjusting the phase distribution of the electric field generated by an RF transmit coil may lead to an “implantfriendly” coil design. Magn Reson Med 48:1096 –1098, 2002. © 2002 Wiley-Liss, Inc. In the rapidly developing field of interventional MRI, localized RF heating near the tips of long metal devices used during MR scanning has become a major safety concern (1‐3). Common to all studies of RF heating of wire-shaped devices is the presence of resonant device lengths at which RF heating is the most intense. This resonance had been empirically observed but not quantitatively modeled nor accurately predicted until our recent study of RF heating at the tips of totally inserted wires (4). The effects of varying wire properties (diameter, insulation thickness) and the properties of the tissue it was placed in (electrical conductivity, thermal conductivity, perfusion) were explored. This theoretical understanding and experimental validation established safety thresholds on the SAR (specific absorption rate) of pulse sequences used with totally inserted wires. The existence of a resonant length, at which a wire has the greatest heating potential, suggests that to improve safety wire lengths should be either longer or shorter than this resonant length. This leads to the nonintuitive notion that putting more metal in the body may actually be safer, in some cases, to avoid this resonance. We will show that the resonance phenomenon depends on the characteristics of the RF transmitter, particularly the phase distribution of the incident electric field. There exists an electric field distribution in which a wire will show no resonant length but will cause more and more heating as the wire gets longer. Present designs for volume transmit coils will likely have near uniform phase distributions since this maximizes the coil’s quality factor. However, older designs may have nonuniform phase distributions. Therefore, the RF heating characteristics of a conductive wire cannot be stated without reference to a specific transmit coil design. However, it is possible to formulate the transmit phase distribution that will generate the maximum tip-heating for a given wire so that its safety characteristics can be stated independently of the specific MR transmit coil used. In addition, the results of this study suggest a design criterion for “implant-friendly” RF transmit coils.