Purpose:To develop and demonstrate a method to calculate the temperature rise that is induced by the radio frequency (RF) field in MRI at the electrode of an implanted medical lead. Materials and Methods:The electric field near the electrode is calculated by integrating the product of the tangential electric field and a transfer function along the length of the lead. The transfer function is numerically calculated with the method of moments. Transfer functions were calculated at 64 MHz for different lengths of model implants in the form of bare wires and insulated wires with 1 cm of wire exposed at one or both ends.Results: Heating at the electrode depends on the magnitude and the phase distribution of the transfer function and the incident electric field along the length of the lead. For a uniform electric field, the electrode heating is maximized for a lead length of approximately one-half a wavelength when the lead is terminated open. The heating can be greater for a worst-case phase distribution of the incident field. Conclusion:The transfer function is proposed as an efficient method to calculate MRI-induced heating at an electrode of a medical lead. Measured temperature rises of a model implant in a phantom were in good agreement with the rises predicted by the transfer function. The transfer function could be numerically or experimentally determined. IMPLANTED MEDICAL DEVICES such as deep brain stimulators and cardiac pacemakers interact with the magnetic fields in magnetic resonance imaging (MRI) (1-5). One of the interactions is tissue heating. The heating arises from the scattered electric field due to interaction of the MRI radio frequency (RF) magnetic field (B 1 ) and a medical implant. The greatest, and potentially dangerous, temperature rises occur at the ends of elongated implants, such as at the electrode of an implanted lead (6,7).In vitro testing in phantoms has been undertaken to characterize the RF-induced temperature rise (8). Recently, there has been an increased interest in mathematical modeling to assess the in vivo temperature rise. Numerical modeling based on the finite difference time domain (FDTD) method has been used to calculate the RF-induced electric field in human (9,10) and phantom models (11). It is difficult to numerically solve human and lead models simultaneously due to the complex structure of medical lead systems. It may be advantageous to model the lead and the human separately and then combine the results to determine the heating. The focus in this work is on modeling the lead. We describe the transfer function of a lead that relates the incident electric field to the scattered electric field in the vicinity of the electrode. From knowledge of the transfer function, the heating at the electrode and the resonant behavior of leads in phantom and human models can be predicted. Figure 1 shows the concept of the transfer function of a lead wire. An incident electric field with a tangential component E tan couples with the lead. The overall length of the lead is L, is the dis...

There are three principal magnetic fields in magnetic resonance imaging (MRI) that may interact with medical implants. The static field will induce force and torque on ferromagnetic objects. The pulsed gradients are of audio frequency and the implant may concentrate the induced currents, with a potential for nerve stimulation or electrical inference. The currents induced in the body by the radio frequency (RF) field may also be concentrated by an implant, resulting in potentially dangerous heating of surrounding tissues. This paper presents basic information about MRI interactions with implants with an emphasis on RF-induced heating of leads used for deep brain stimulation (DBS). The temperature rise at the electrodes was measured in vitro as a function of the overall length of a DBS lead at an RF frequency of 64 MHz. The maximal temperature rise occurred for an overall length of 41 cm. The method of moments was used to calculate the current induced in the lead. From the induced currents, the RF power deposition near the electrodes was calculated and the heat equation was used to model the temperature rise. The calculated temperature rises as a function of lead length were in good agreement with the measured values.

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