Development of compact and high efficiency MR-compatible switching power amplifiers for multi-coil shim and gradient arrays

PROJECT SUMMARY/ABSTRACT The objective of this project is to develop highly compact and efficient switching power amplifiers capable of in- bore operation as an enabling technology for magnetic resonance imaging (MRI) techniques based on multi-coil (MC) shim and gradient arrays. MRI is a non-invasive imaging modality that provides superb soft-tissue contrast for in vivo clinical diagnostics and physiological studies, but image and spectral quality – particularly in high field systems – is limited by B0 inhomogeneity arising from susceptibility-induced distortions. MC techniques not only outperform conventional spherical harmonic shimming available in clinical MRI systems but further provide a general mechanism for local magnetic field shaping and can simultaneously perform spatial encoding functions. This can eliminate dedicated gradient coils and drivers in applications such as low-field MR or localized area imaging, with the attendant cost reduction beneficial for improving accessibility to clinical-grade MRI systems. MC arrays also provide many exciting new capabilities for MRI, such as accelerated image acquisition and artifact mitigation. However, while existing MC power amplifiers generally provide sufficient performance for B0 shimming, they lack the capability to support many of these more powerful methods of spatially/spectrally selective excitation and image encoding that demand order-of-magnitude increases in coil current and bandwidth. The building block for an MC array comprises two key elements – the coil and the power amplifier driving current through it. Most current drivers for MR shim array coils use linear amplifiers which are low noise but inefficient, necessitating bulky cooling solutions outside the scan room, incurring significant cost and losses in cabling. Recent studies on switched-mode amplifiers for MR report efficiency gains, but even at low currents remain thermally limited without active cooling. Additional radiofrequency interference (RFI) due to switched-mode operation was also not fully resolved. This project will thus focus on developing the power amplifier element for a scalable MC building block through two approaches. The first is to use multi-MHz switching frequencies to reduce passive component volume for higher power density, as well as to increase separation between switching harmonics and MRI receiver bands. The second is to employ soft-switching techniques to reduce power dissipation, as well as minimize RFI generation at unintended frequencies due to excitation of circuit parasitics. We define two specific aims: (1) Design and build a soft-switching amplifier meeting high output current, efficiency, resolution, and bandwidth specifications. (2) Validate performance in a 3T MRI system by characterizing disturbance rejection, impact on noise floor, and any imaging artifacts. Successful completion of this project will provide the MR community with a key hardware component for scalable and high-performance MC arrays.