Calculation of MRI-induced heating of an implanted medical lead wire with an electric field transfer function.
ABSTRACT 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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ABSTRACT: The magnetic-field-driven heat generation in neuromodulation systems consisting of implanted and skin-surface-mounted components gives rise to the potential of discomfort, cell damage, and possible necrosis. The skin-surface-mounted component, commonly termed the antenna, serves the function of the primary of a transformer, and the implant is the secondary. Heating occurs in both of these components during the recharging of a battery situated in the implant. Previously reported experimental data for the heat generation characteristics of three commercially available neurostimulation systems has been enhanced by further experiments carried out as part of this investigation. A numerical simulation of the temperature distribution in tissue beds adjacent to the implant and the antenna has been performed here. The aggregated data have been used as input information to a bio-heat-transfer model which yields both the spatial and temporal variations of the temperature field. It was found that during long-duration recharging periods, the temperature of the tissue rises in response to the heat generation. This information enables the identification of the magnitude and location in the tissue of the hot-spot temperature. The temporal temperature variation at the hot spot was employed in conjunction with a tissue-damage integral to identify the possibility of cell damage and/or necrosis. It was found that two of the three investigated neuromodulation systems did not give rise to temperature levels that may cause tissue damage. However, the third of the systems caused temperatures of sufficient elevation so that for recharging periods on the order of 2 h, necrosis was found to be likely in situations where heat transfer is suppressed at the surface of the skin.International Journal of Heat and Mass Transfer. 01/2009;
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ABSTRACT: The in-vitro and in-vivo temperature rises during MRI for a sample passive implant in the form of a metal rod of 8 mm diameter and 118 mm length in the right humerus were calculated with FDTD and the heat equation. The temperature rise in a phantom test was calculated for a background medium with electric properties specified in ASTM F2182-09 and for a medium with electrical properties of the cancellous bone that surrounds the implant. Scaled to local background SAR, in-vitro rises are greater for the bone. For a patient in the MRI coil with landmark in the torso and a whole body SAR of 2 W/kg, the calculated temperature rise after six minutes of RF power deposition was 1.3°C for 64 MHz and 2.4°C for 128 MHz. The numerical methods presented here could be extended to determine the temperature rises that would occur in the phantom and in the patient for other implants.General Assembly and Scientific Symposium, 2011 XXXth URSI; 09/2011
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ABSTRACT: There is a growing interest in the use of deep brain stimulation (DBS) for the treatment of medically refractory movement disorders and other neurological and psychiatric conditions. The extent of temperature increases around DBS electrodes during normal operation (joule heating and increased metabolic activity) or coupling with an external source (e.g. magnetic resonance imaging) remains poorly understood and methods to mitigate temperature increases are being actively investigated. We developed a heat transfer finite element method (FEM) simulation of DBS incorporating the realistic architecture of Medtronic 3389 leads. The temperature changes were analyzed considering different electrode configurations, stimulation protocols and tissue properties. The heat-transfer model results were then validated using micro-thermocouple measurements during DBS lead stimulation in a saline bath. FEM results indicate that lead design (materials and geometry) may have a central role in controlling temperature rise by conducting heat. We show how modifying lead design can effectively control temperature increases. The robustness of this heat-sink approach over complimentary heat-mitigation technologies follows from several features: (1) it is insensitive to the mechanisms of heating (e.g. nature of magnetic coupling); (2) it does not interfere with device efficacy; and (3) can be practically implemented in a broad range of implanted devices without modifying the normal device operations or the implant procedure.Journal of Neural Engineering 07/2012; 9(4):046009. · 3.28 Impact Factor