RF safety of wires in interventional MRI: using a safety index

With the rapid growth of interventional MRI, radiofrequency (RF) heating at the tips of guidewires, catheters, and other wire-shaped devices has become an important safety issue. Previous studies have identified some of the variables that affect the relative magnitude of this heating but none could predict the absolute amount of heating to formulate safety margins. This study presents the first theoretical model of wire tip heating that can accurately predict its absolute value. The method of moments was used to find the induced currents on insulated and bare wires that were completely embedded in the tissue. The induced currents caused an amplification of the local specific absorption rate (SAR) distribution near the wire. This SAR gain was combined with a semi-analytic solution to the bioheat transfer equation to generate a safety index. The safety index is a measure of the worst case in vivo temperature change that can occur with the wire in place. It can be used to set limits on the spatial peak SAR of pulse sequences that are used with the interventional wire. Under worst-case conditions with resonant wires in a poorly perfused tissue, only about 100 mW/kg/°C spatial peak SAR may be used at 1.5 T. But for ≤10 cm wires with insulation thickness ≥30% of the wire radius that are placed in well perfused tissues, normal operating conditions of 4 W/kg spatial peak SAR are possible at 1.5 T. We propose a simple way to ensure safety when using an interventional wire: set a limit on the SAR of allowable pulse sequences that is a factor of a safety index below the tolerable temperature increase.