Calculation of MRI-induced heating of an implanted medical lead wire with an electric field transfer function

School of Electrical and Computer Engineering, Purdue University, West Lafayette, Indiana 47907, USA.
Journal of Magnetic Resonance Imaging (Impact Factor: 3.21). 11/2007; 26(5):1278-85. DOI: 10.1002/jmri.21159
Source: PubMed


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.
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.
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.
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.

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    • "diathermy devices [8] [9]. Concerns about coupling in MRI have led to changes in counter-indicated exposure guidelines [10] [11]. A range of methods to mitigate temperature increases around leads during external coupling have been proposed [12], often aimed with counter-indication guidelines, of minimizing initial coupling. "
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    • "Sources of this electromagnetic energy include the pulsed radiofrequency energy from a head or body coil and the time varying magnetic fields used by the MR system for spatial localization of signals. The transfer of radiofrequency energy to heat and electrical energy is dependent on factors including: 1) the pulse sequence parameters, 2) the whole body averaged and local specific absorption rates (SAR) associated with a given sequence, 3) spatial relation and orientation of the anatomy to the transmit RF coil, and 4) lead factors (composition, length, geometry, configuration, and orientation) [51,54,55,63,94-97]. "
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    ABSTRACT: Variability of the tissue in which implant is embedded and the length of the bare part of the implants were studied with regard to MRI-related heating. The implants studied were thin and elongated with different effective resonant lengths. We compare two leads having different lengths of bare portions to investigate the heating effect. To support our conclusions, we compare the leads based on current distributions, SAR distributions, and their associated temperature rises in the tissue.
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