A Transmission Line Model for Active Implantable Medical Devices under MRI
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The use of magnetic resonance imaging (MRI) is widely restricted for patients with active implantable medical devices (AIMDs) due to safety concerns. In the United States alone, there are millions of people having AIMDs and a significant number of them need follow-up MRI scanning during their lifetimes. Some of the major MRI safety concerns come from the interaction between the electromagnetic fields generated by the MRI radio-frequency (RF) coil and AIMDs. Such interaction can result in RF-induced heating in human tissues, which can cause tissue damage, and RF-induced voltages on devices, which can cause device damage or malfunction. When analyzing the MRI RF safety problems, full-wave electromagnetic simulation approaches are often computationally prohibitive due to the complexity of the structure of the AIMDs, and in vivo measurement approaches are often not feasible. Therefore, an electromagnetic model is necessary to simplify the problem. In this dissertation, a transmission line model is proposed to describe the electromagnetic properties of AIMDs. The model is derived using simplified leads and generalized to practical AIMD leads. Numerical and experimental validations were performed for the model. A method to use a simplified equivalent solid lead to replace the complex AIMD lead is also proposed. The method significantly reduces the computational cost of simulating AIMD leads, and it enables the full-wave simulation of an AIMD together with a human model and an MRI RF coil. Based on the transmission line model, various studies were performed on the assessment and improvement of the MRI RF safety for patients with AIMDs. The analytical relation between the MRI RF safety and impedances in the transmission line model is derived and validated. The relation shows that adding properly chosen lumped elements or filters at the two ends of an AIMD can significantly improve the MRI RF safety. The influences of various physical parameters on the transmission line parameters are analyzed. An approach to search for the best tissue simulating medium is proposed, and an approach to determine the MRI RF-induced voltage transfer function scaling factor is proposed.