Transmit Radio-Frequency Field Mapping In High Field Magnetic Resonance Imaging

Abstract

A fast approach to reconstruct transmit RF field (|B1+|) maps in high field parallel Magnetic Resonance Imaging is presented in this thesis. First the concepts of parallel imaging reconstruction and the Bloch-Siegert (BS) |B1+| mapping method are introduced. Then we present the idea to accelerate BS field mapping based on the fact that the |B1+|-to-phase encoding pulse for each transmit coil and off-resonance frequency applies a unique phase shift to the same underlying image. This enables joint reconstruction of all images in a BS acquisition from an augmented set of virtual receive coils. Simulations are shown to produce accurate |B1+| maps at high acceleration factors for both Cartesian and Non-Cartesian sampling. Phantom and in vivo experiments confirm the simulation results and endorse the proposed approach. It is also shown that reconstruction accuracy can be improved using disjoint sampling patterns between acquisitions. Noise performance analysis in simulation demonstrates that disjoint sampling produces less noise amplification when compared to same sampling across all the coils. The approach is general in that it can be applied using any auto-calibrated multi coil image reconstruction method.