Modeling and measurement of lead tip heating in implanted wires with loops

<i>Objective</i>. In MRI, conductive lead tip heating caused by radiofrequency (RF) power deposition is an important safety issue for patients with implanted devices. In this work, we investigate lead tip heating in different wire configurations that contain loop(s) through theoretical models and experimental measurements.<i>Approach.</i>We have previously proposed analytical transfer function models to predict relative heating of implanted, straight metallic leads. Here we extend the models' application to leads containing loops, that are widely used in the clinic. Maximum temperature rise caused by RF heating was measured at 1.5 T on twenty (20) insulated, capped wires with various loop and straight segment configurations. The experimental results were compared with predictions from the previously reported simple exponential and adapted transmission line models, as well as with a long-wavelength approximation.<i>Main results.</i>Both models effectively predicted the trends in lead tip temperature rise for all the wire configurations, with the adapted transmission line model showing superior accuracy. In a typical MRI configuration where the RF electric field is predominantly in the superior/inferior (S/I) direction, wires oriented in the same direction showed decreased heating as the number of loops increased. However, when wires were oriented right/left (R/L) where the corresponding component of the electric field is negligible, additional loops increased the overall heating.<i>Significance.</i>The simple exponential and the adapted transmission line models previously developed for, and tested on, straight wires require no additional terms or further modification to account for RF heating in a variety of loop configurations. These results extend the models' usefulness to manage implanted device lead tip heating and provide theoretical insight regarding the role of loops and electrical lengths in managing RF safety of implanted devices.