During the course of the project, MRI compatibility of the wireless power and data transfer unit is validated:
MRI-Compatibility: According to international standards ISO/TS 10974:2018, to validate MRI-compatibility, the system’s heating should be observed both under power transfer to the system (electrical heating) and when the implanted part is put through MRI exposure (MRI-induced heating).
Heating simulations were conducted using the Electromagnetic Heating Module in COMSOL Multiphysics 5.2. Critical heating points, areas expected to experience the highest temperatures, on the receiver coil were determined through these finite element method simulations.
Electrical Heating Simulations: The coils were modelled to match their real-life geometries and were placed within a simulated tissue phantom medium with physical properties matching those of the experimental phantom. The entire phantom-coil complex was modelled in air, with a boundary condition of 37°C to simulate body temperature.
Magnetic field calculations were performed across the entire geometry, then the transmitter coil was excited with a time-varying voltage corresponding to the power amplifier output. The coils were positioned some distance apart, yielding the voltage coupling factor consistent with experimental settings.
Given that the physical parameters remained constant with temperature changes, both methods produced consistent results.
MRI-induced Heating Simulations: To simulate MRI exposure in COMSOL, a birdcage MRI coil was implemented to generate the RF field, with the RF Module used for field calculations.
A head model was positioned within the MRI system, incorporating the receiver coil geometry to simulate the actual MRI scenario. The head model’s physical parameters were chosen to match those of the tissue phantom used in experiments. A continuous RF excitation was applied without imaging sequences to observe the temperature increase at critical points and spatial power dissipation.
Analytical Solutions: The Joule heating analytical solutions were done to explore the heat generation due to resistive losses in the coil during power transfer, accounting for the geometry of the coil and the electrical properties of the materials.
The RF heating analytical solutions were done to represent the temperature rise induced by the interaction of the implantable coil with external radio frequency fields.
Heating Tests: All of the electrical and MRI-induced heating tests were done with the receiver coil submerged in a human-body mimicking phantom, while the previously detected critical points being measured continuously. Developed by a third party, the phantom has been tested at RF frequencies up to 128 MHz (the RF frequency of a 3T MRI). Key parameters for the tissue phantom, such as electrical conductivity, thermal conductivity, relative permittivity, and specific heat capacity, were selected according to ISO/TS 10974:2018 to accurately replicate human tissue properties during heating assessment.
Electrical Heating Tests: The power amplifier and rectifier circuit were adjusted according to their intended operational settings. The transmitted power level was swept across four levels, ranging from 720 to 1260 mW.
During wireless power transfer, temperatures at the identified critical points and a reference location were recorded continuously.
For these experiments, the rectifier was operated in voltage mode, the most power-intensive rectification setting. Coils were aligned along their axis to maximize induced current in the receiver coil, and the voltage coupling between coils was adjusted to approximately 0.15.
MRI-Induced Heating Tests: Heating measurements during MRI exposure followed a similar approach, with the same instrumentation used to ensure consistency.
