Methods for MRI RF Pulse Design and Image Reconstruction.

Abstract

This thesis describes methods to improve magnetic resonance imaging (MRI) reconstruction and system calibration, namely, B1 field mapping which is to measure the spatial distribution of the magnetic field produced by radiofrequency (RF) coils. We also developed methods of RF pulse design and steady-state imaging sequence design for applications such as fat suppression and magnetization transfer contrast imaging. There are five projects: (a) We developed a framework of iterative image reconstruction with separate magnitude and phase regularization where compressed sensing is used for the magnitude and special phase regularizers that are compatible with phase wrapping are designed for different applications. The proposed method significantly improves the phase image reconstruction while accelerates the data acquisition. (b) A modified Bloch-Siegert B1 mapping was developed to efficiently acquire both magnitude and phase of the B1 maps of multi-channel RF transmission systems. A regularized method was developed to jointly estimate the B1 magnitude and phase to reduce low signal-to-noise ratio regions. Furthermore, we developed a method for coil combination optimization for this multi-channel B1 mapping sequence based on Cramer-Rao lower bound analysis, to improve the raw data quality for B1 estimation. (c) We developed a four dimensional spectral-spatial fat saturation pulse that uniformly suppresses fat without exciting water in the presence of main magnetic field and B1 field inhomogeneity. At 3T, we showed that the proposed pulse can work more robustly than the standard spectrally selective fat saturation pulse with half the pulse length. (d) We applied the proposed fat saturation pulse to spoiled gradient echo sequence and small-tip fast recovery imaging sequence, with a modified RF spoiling scheme. We tested these proposed sequences on clinical applications like cartilage imaging and MR angiography and demonstrated their ability to simultaneously produce fat suppression and magnetization transfer contrast. We show that the proposed sequences can reduce the minimal repetition time and potentially lower the overall RF power deposition. (e) We designed a small tip fast recovery imaging sequence combined with a post-processing method to separate water from fat and remove banding artifacts simultaneously.