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Vers une application sûre de l'IRM en présence d'implants actifs

Abstract : MRI is generally considered to be an exceptionally safe imaging method. However, in the presence of electrically conducting implants health risks exist, particularly in terms of RF heating of the tissues in contact with the implant. Some implants are cleared by the manufacturers or regulatory agencies for MR imaging of patients, but only under strictly limited conditions which often degrade image quality and exclude many configurations. The goal of this thesis project was to optimize and validate the methods for the assessment of MR safety in the presence of active implants. Increasing the predictability of the risk of RF heating in individual subjects should allow MRI to find wider applications in patients implanted with active devices.This project is based on three distinct approaches:- Measurements and MR method developments performed on test objects. Existing B1-mapping techniques were optimized for the specific needs of high dynamic range encountered in the presence of induced RF currents in conductors, leading to the “Actual Multiple Flip-Angle Imaging” technique. Further work has been performed on the optimization of rapid “Proton Resonance Frequency Shift” MR thermography.- The development of numerical simulations of the electromagnetic interactions between the RF resonator and implants as well as their thermal impact. A numerical RF resonator model was built and validated it using both theoretical and experimental studies. The optimization of the resonator has led to the development of an original method to rapidly and precisely adjust the individual capacitor values to obtain a given targeted current distribution. Separately, the measurement of RF currents induced in conductive wires, via B1 mapping, was developed. This method to measure RF currents in a specific configuration opens the possibility to evaluate RF safety in individual subjects using a low-SAR prescan prior to other acquisitions, for use in hypothetical future protocols on patients.- The construction of a simplified numerical model of deep brain stimulation electrodes, using transmission line theory. This model renders RF simulations tractable, while exhibiting the same electrical behavior as the real implant, allowing evaluation of RF heating in simulations covering the size of a whole-body MR resonator.The set of tools developed improve upon the currently available methods for the evaluation of RF safety in the presence of conductive implants.
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Melina Bouldi. Vers une application sûre de l'IRM en présence d'implants actifs. Electromagnétisme. Université de Grenoble, 2014. Français. ⟨NNT : 2014GRENY056⟩. ⟨tel-01363635⟩

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