Modular scalable techniques for advanced magnetic resonance systems and radio-frequency instrumentation [electronic resource]

Magnetic resonance imaging (MRI) is unique among imaging modalities in providing exceptional versatility for examining a wide spectrum of anatomy and physiological functions. Modern MRI systems are moving toward higher magnetic field strengths for increased signal, and depend on rising receiver and transmitter channel counts to enhance speed, image quality, specificity, and safety. This has left researchers in a predicament. Commercial MRI scanners are becoming ever more capable and complex, yet by their nature inevitably lag the leading edge of science, failing to anticipate new research directions. Moreover, their closed proprietary hardware, software, and interfaces frustrate any expansion or adaptation to new techniques and experiments. Developed to address these challenges, the Medusa MRI console is a modular scalable open platform for advanced MRI research and development. Medusa enables MR methods and applications that would have been challenging or impossible using only commercial hardware, and does so with an efficient flexible architecture designed to grow to meet future needs. The Medusa MRI console delivers the complete set of functionality required for MRI including multi-channel radio-frequency (RF) excitation and reception, gradient waveform generation, coil and amplifier gating, and an open software platform for pulse sequencing and experimental development. Key features of Medusa include the use of direct-conversion digital RF components, distributed processing and memory for modularity and flexibility, and USB 2.0 High-Speed (480Mb/s) interfaces to the host PC. Medusa was first tested with the novel Stanford University Pre-Polarized MRI scanner, and is now used regularly for experiments on a diverse range of commercial and custom-built imaging systems. A significant application of Medusa has been the investigation and development of MRI Parallel Transmit techniques (PTx), which hold promise for improving MR safety and high-field image quality. Yet PTx methods demand high-fidelity delivery of RF pulses at kilowatt power levels across multiple channels into an unknown load. Vector Iterative Predistortion (VIP) was developed to address this challenge, using Medusa to encode the non-ideal response from RF amplifiers using sensors and iteratively pre-distort the input to achieve desired output. Even when used with cost-efficient low-fidelity RF power amps, VIP drives errors to below 0.1~dB and +/-1~degree phase, and is capable of correcting time-dynamic memory effects where traditional look-up table methods fail. Accurate RF sensing is essential in any RF control system, and on-coil and in-line vector RF sensing approaches were developed to enable real-time current, power, and impedance measurements. Notably, this monitoring permits not only the hardware characterization and control needed for VIP, but can also detect patient motion, cardiac and respiratory rhythms, and has the potential to assess and improve MR safety for patients with implanted devices.

Keywords: Electrical Engineering
Advisor: Nishimura Dwight George primary advisor. Thesis advisor
Pauly John (John M.) primary advisor. Thesis advisor
Scott Greig Cameron