A wireless power transfer system compatible with MRI scanners

Wireless technology has impacted just about every aspect of modern life. As the number of portable electronic devices expands for phones, robotics, internet of things (IoT), and wearable devices, battery-free operation and the elimination of charging adapter cables have become major technology drivers. This is equally true for health care monitoring devices and the needs for mobility in the hospital environment. Wireless power transfer (WPT) has made major inroads for wireless charging of battery-operated devices, and efforts are underway by companies like WiTricity for powering devices in the home. In magnetic resonance imaging (MRI), there are many motives for using wireless technology to create completely cable-free receive arrays. There is a continuous drive toward a greater number of receiver channels in an effort to improve the signal-to-noise ratio (SNR) or imaging speed. Increasing channel counts require a commensurate increase in the number of RF cables and connectors for the array. The added bulk of these cables can cause increased patient discomfort, as well as requiring longer setup times which reduce patient throughput. Bulky cable traps are necessary to prevent the induction of RF current on the cables during B1 transmit, and avoidance of RF burn hazards. To remove all of these cables, there needs to be a method to wirelessly provide power to the receive array. Non-magnetic batteries are one option, but it would be difficult to ensure that they maintain a full charge. This would lead to limits on scan time and additional time lost swapping batteries out between scans. Batteries also restrict the voltage and power available, and physically large batteries can also suffer from vibrations due to eddy currents from the large gradient fields. As a result, it would be ideal to use wireless power transfer to continuously provide power to the wireless coil array. This dissertation demonstrates a wireless power transfer system based upon resonant inductive coupling and explores the adaptations of WPT in terms of electromagnetic coupling, physical deployment and noise emissions for compatibility with MRI. To minimize added noise and decouple the wireless power system from MRI coils, restrictions are placed on the coil geometry of the wireless power system, which are shown to limit its efficiency. The final WPT system includes a class-E power amplifier, RF MEMs automated impedance matching, a primary coil array employing RF MEMs power steering, and a flexible secondary coil with class-E rectification. A major challenge was the identification and suppression of noise and harmonic interference, by gating, filtering, and rectifier topologies. Efficiency can be traded off to reduce noise through additional filtering and rectifier choice, and by replacing the switching supply to the power amplifier with batteries. An ultimate SNR performance within 6 dB of the ideal can be achieved while continuously transferring power. For the present system, the feasibility of wireless power transfer within an MRI scanner has been fully demonstrated, and the realization of completely wireless MRI receive arrays appears within reach.

Keywords: Electrical Engineering.
Advisor: Pauly John
Nishimura Dwight George 
Scott Greig Cameron