Design of spectral‐spatial phase prewinding pulses and their use in small‐tip fast recovery steady‐state imaging

Abstract

Purpose Spectrally selective “prewinding” radiofrequency pulses can counteract B0 inhomogeneity in steady-state sequences, but can only prephase a limited range of off-resonance. We propose spectral-spatial small-tip angle prewinding pulses that increase the off-resonance bandwidth that can be successfully prephased by incorporating spatially tailored excitation patterns. Theory and Methods We present a feasibility study to compare spectral and spectral-spatial prewinding pulses. These pulses add a prephasing term to the target magnetization pattern that aims to recover an assigned off-resonance bandwidth at TE. For spectral-spatial pulses, the design bandwidth is centered at the off-resonance frequency for each spatial location in a field map. We use these pulses in the small-tip fast recovery (STFR) steady-state sequence, which is similar to balanced steady-state free precession (bSSFP). We investigate improvement of spectral-spatial pulses over spectral pulses using simulations and STFR scans of a gel phantom and human brain. Results In simulation, spectral-spatial pulses yielded lower normalized root mean squared excitation error than spectral pulses. For both experiments, the spectral-spatial pulse images are also qualitatively better (more uniform, less signal loss) than the spectral pulse images. Conclusion Spectral-spatial prewinding pulses can prephase over a larger range of off-resonance than their purely spectral counterparts.