Novel Methods of RF-induced Heating Assessment and Heating Reduction Techniques for Active Implantable Medical Devices in MRI




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Radiofrequency (RF)-induced heating of active implantable medical devices (AIMDs) is a significant concern during magnetic resonance imaging (MRI). According to the tier-three approach proposed in ISO 10974, the device model (i.e., the transfer function [TF]) of AIMD should be developed and validated in a tissue-simulating medium and further used for RF-induced heating prediction. In this dissertation, a new approach to determining the dielectric parameters of the tissue-stimulating medium in which the TF shall be developed is proposed. Applying the equivalent medium method, a more accurate TF model for AIMD can be derived. Two novel techniques are proposed for a more convenient and effective TF validation procedure. First, the validation procedure can be made more convenient and faster by using a group of optimized rotation-invariant pathways on a circular plate in a standard rectangular phantom. Second, the TFs of AIMD at 1.5 T and 3 T can be developed and validated by applying a series of highly orthogonal pathways in a novel electromagnetic field generator. This strategy provides a method that is more economic and effective than the one recommended by ISO 10974. The remainder of this dissertation focuses on RF-induced heating reduction on AIMDs. A novel technique is proposed and validated to mitigate heating at the lead tip of AIMDs. After introducing a decoying conductor and dissipating rings into the lead structure, the induced energy could be coupled out from the stimulation conductor and dissipated near the added rings. Both simulation and experiment studies are described. Additionally, an experiment-based study regarding the conditionality of abandoned leads in patients is presented. Three different configurations of the proximal ends of abandoned leads are considered and compared (metal-end, bare-end, and plastic-end). The findings of three AIMD studies reveal that the metal-end configuration is the optimized treatment of the proximal end of abandoned leads to lower heating in human bodies. Capping the proximal end with plastic or an insulator should be avoided because this can lead to significantly increased heating.



Magnetic resonance imaging (MRI), active implantable medical device (AIMD), transfer function, heating reduction.


Portions of this document appear in: Wang, Yu, et al. "On the development of equivalent medium for active implantable device radiofrequency safety assessment." Magnetic resonance in medicine 82.3 (2019): 1164-1176; and in: Wang, Yu, et al. "A Fast and Accurate Transfer Function Validation Strategy Using Optimized Rotation-Invariant Lead Trajectories." IEEE Transactions on Electromagnetic Compatibility (2020); and in: Wang, Yu, et al. "A Novel Device Model Validation Strategy for 1.5-and 3-T MRI Heating Safety Assessment." IEEE Transactions on Instrumentation and Measurement 69.9 (2020): 6381-6389.