RF Pulse Designs for Signal Recovery in T2*-Weighted Functional Magnetic Resonance Imaging.

Abstract

In blood-oxygenation-level-dependent (BOLD) functional magnetic resonance imaging (fMRI) using T2* contrast, images suffer from loss of signals at brain regions close to the air-filled cavities in the human head. The artifact arises from magnetic field distortion caused by the magnetic susceptibility difference between air and brain tissues, and hampers functional studies of important brain regions such as the orbito-frontal cortex. In this research project, I investigate two methods of designing radio-frequency (RF) pulses that can recover the signal loss. In addition to slice selective excitation, both pulse designs ``precompensate'' the through-plane dephasing that occurs between excitation and data acquisition. One method, which utilizes ``three-dimensional tailored RF pulses'', achieves these goals via three-dimensional spatially selective excitation. The other method uses spectral-spatial selective excitation, and relies on the assumption that through-plane dephasing is correlated with resonance frequency offset. All these sophisticated pulses are numerically designed using the iterative conjugate gradient method. To facilitate those design methods, I also propose new techniques applicable to general pulse designs, such as frameworks for pulse computation acceleration and joint design of excitation k-space trajectory and RF pulse. With phantom and human experiments, I demonstrate that the methods are efficacious in signal recovery, but not without costs and hurdles to overcome.