Radio Frequency Induced Heating of a Medical Device with Vascular Flow Conditions
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Publisher:The Ohio State University
Series/Report no.:The Ohio State University. Department of Biomedical Engineering Undergraduate Research Theses; 2016
Magnetic Resonance Imaging (MRI) is a tool used to scan more than 40 million U.S. patients every year, but a major concern exists with the safety of patients with implanted medical devices. When implanted medical devices constructed of electrically conductive materials are subjected to an oscillating magnetic field, electric currents are produced according to Faraday’s Law of Induction. These electric currents cause potentially damaging Radio Frequency (RF) induced heating near the device. The current standard test method, ASTM F2182, for evaluating MR safety of medical devices defines the use of a phantom consisting of a gel with thermal and electrical properties that approximate tissue and does not incorporate convective blood flow. It was hypothesized that vascular flow would cause a significant reduction in RF induced heating during MRI. An ASTM phantom was modified to include a flow channel and a Zilver 635® Vascular Self-Expanding Stent within the channel. Experiments were performed in a Siemens 3-Tesla MRI system. Flow rate was varied from 0 to 2240 mL/min and transient temperatures were measured using fiber optic probes. It was found that constant flow significantly reduced the maximum temperature increase when the device was subjected to MRI-powered RF induced heating. The maximum temperature rise measured approximately 10°C without flow, while physiologic flow rates decreased temperature rises by up to 70%. These experimental results were used to validate a COMSOL MultiPhysics® simulation which was in agreement with the experimental data over the physiological range of flow. The agreement shows that the simulation can be utilized to accurately predict the influence of blood flow on RF induced heating of a vascular stent. The results of this study indicate that blood flow has a significant cooling effect and support the use of simulation tools to predict device heating under physiological conditions. These results will hopefully lead to more accurate evaluation of the MR safety of medical devices with the goal of ensuring that patients with unsafe devices are precluded from MRI scans, and those with safe devices have access to clinically indicated MRI scans.
Undergraduate Research Scholarship
Academic Major: Biomedical Engineering
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