Intolerable Heating by Resonating RF’ Waves around Guidewires

Introduction In the rapidly developing field of interventional MR, there has been growing concern about RF-induced heating in metallic devices[3]. In the work reported here, we concentrate on the RF heating of a widely used intravascular guidewire, the Terumo guidewire (Tokyo, Japan), having a nitinol (Nickel-Titanium) core. Since nitinol is not ferromagnetic, the Terumo meets an important condition for MR safety. Nevertheless, nitinol is a good electrical conductor. According to Maxwell’s theory, one may discriminate between three mechanisms by which heating can be evoked in conductors by RF radiation: I) eddy currents, 2) induction loops, e.g. when using ECG leads [2,3], and 3) resonating RF waves along conducting wires. The first two effects have been studied for many devices [l], and have in common that no storage of electrical energy within the device takes place. The thermal energy is produced directly and instantaneously by the incident RF radiation. In this work however, we concentrate on the third effect. Resonating waves around the wire is the only mechanism by which excessive heating of a straight guidewire can be explained. The purpose of our study is to show that excessive RF heating can be evoked near the guidewire tip in an experimental set-up that resembles a MR guided endovascular intervention. The capricious nature of resonance phenomena (as explained in the theory section below) pose specific problems to the formulation of safety guidelines. At the end, the experimental results are discussed in the light of MR safety.