Hardware development for wireless MRI receive arrays

Magnetic resonance imaging (MRI) is an imaging modality effective for viewing soft tissue contrast that usually outperforms other modalities such as computed tomography (CT) or X-ray. However, while effective, MRI requires long scan times that can be onerous for patients and technicians. Considerable research has been done to decrease scan times, which would improve patient experience and scanning workflow. A major development that has improved scan times while maintaining good signal-to-noise ratio (SNR) is parallel imaging. This uses multiple receive coil elements and channels to collect spatial frequency data simultaneously before combining the data in the final image. The use of multi-coil receive arrays for parallel imaging can be cumbersome, due to bulky cables, baluns that heat up, and cable coupling. Hence, there has been a considerable push to develop wireless MRI receive coils that are easier for technicians to use and more comfortable for patients. The goal of this work is to develop low power system components and to study the challenges of wirelessly transmitting scanner state signals and phase correction information to an unconnected MRI receive coil, which would eliminate the cabling and associated issues with safety and convenience. My work begins with a focus on power reduction for MRI receive arrays. This is important since wireless coils are not connected to the scanner, so coils must be battery powered or harvest energy by wireless power transfer. One energy-intensive process is Q-spoiling, the process of decoupling receive array coil elements during MRI transmit. This is necessary to protect the receiver on-coil electronics and the patient. Q-spoiling conventionally uses PIN diodes for switching, drawing significant current. In my work, I applied Gallium Nitride (GaN) HEMT switches for Q-spoiling, which require negligible power and result in at least 100x in power savings. The use of GaN HEMT switches can provide sufficient decoupling at negligible power without affecting image quality. Finally, this work leads to wireless Q-spoil triggering and clock synchronization for wireless MRI receive arrays. Antennas used for wireless transmission must be constructed to operate within the MRI bore environment in the presence a patient. These antennas should not perturb the B1 transmit fields of the MRI scanner and must satisfy maximum path loss constraints. Using prototype shielded dipole antennas, I achieved wireless Q-spoiling with sub-microsecond delay, suitable for the majority of pulse sequences including extreme cases such as ultra-short TE. In addition, I implemented different wireless modulation schemes for use in clock synchronization between the on-coil receiver electronics and the scanner. MRI scans acquire phase sensitive data, and sampling of this data due to improper clock synchronization may appear as blurring or translational artifacts in the MRI image. These core technological components are key to enabling wireless coils for faster, safer, and more effective MRI scans that can benefit doctors, technicians, and patients.

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