Radiofrequency pulses for improved simultaneous multislice magnetic resonance imaging

Abstract

Simultaneous multislice (SMS) imaging is a scan acceleration method where mul- tiple slices are simultaneously excited using a multiband pulse and the aliased slice images are separated in reconstruction using the receive coils’ sensitivity maps. At high main field strengths, SMS brain imaging suffers from artifacts caused by non- uniform and subject-dependent transmit RF fields and large magnetic susceptibility differences near air-tissue interfaces such as the frontal sinus and the middle ear. Another significant engineering challenge is the increase in peak power of multiband pulses with the number of excited slices. In this research work, we propose novel radiofrequency pulses and pulse sequences to address these SMS imaging problems. Low peak power multiband spokes excitation pulses are proposed to mitigate the image shading artifacts caused by inhomogeneous transmit RF field in multiple si- multaneously excited slices. Results from simulations and in vivo experiments at 7 T demonstrate that images excited using multiband spokes pulses have reduced center brightening artifact than conventional multiband pulses. We propose a novel pulse sequence called multispectral z-shim to reduce the through-plane signal loss artifact in structural and functional MR imaging. In vivo experiments show that the multispectral z-shim sequence recovers signal in regions of susceptibility difference in multiple brain regions while maintaining signal elsewhere. To reduce the peak power of conventional pulses, we present a method to design root-flipped multiband pulses. Simulations and experiments demonstrate that for a fixed peak amplitude, the root-flipped pulses excite the desired slices with a pulse duration lower than that of pulses proposed earlier. The work presented in this dissertation will improve high field SMS imaging research in areas such as functional MRI, susceptibility-weighted imaging and diffusion-weighted imaging.