Model Validation and Inversion of Active Implantable Medical Devices
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The model validation and inversion for active implantable medical devices (AIMD) used for safety evaluations under magnetic resonance imaging (MRI) radio frequency (RF) coil emission were discussed. A mathematical derivation is presented to provide guidance on selecting meaningful pathways for the model validation. Suggested validation pathways from current ISO 10974 are used as examples. It is shown that these standard pathways are 1) inefficient since validations from several pathways are theoretically redundant and 2) incomplete or false since significantly different AIMD models can have identical validation outputs. Based on the developed guidance, two sets of pathways are proposed. It is demonstrated that for efficient and correct model validation, the tangential components of the incident fields along validation pathways should be orthogonal to each other or at least has low correlations between each other. These guidelines can be implemented for future AIMD model validations in ISO 10974. Based on the transmission line model, the AIMD model can be developed semi-analytically using a few direct measurements inside the ASTM phantom. Folded orthogonal pathways based on the Hadamard matrix are used in the model development to make the problem of the AIMD model development to be well-conditioned. Both induced voltage and heating models for the example AIMDs were developed to demonstrate the effectiveness of this method. Furthermore, the optimized validation pathways were designed according to a given AIMD model in a high electric field generator to make sure the validation pathways sufficient and have high signal noise ratios. Still, the circular validation pathways were proposed to be relevant to the clinical validation pathways.