Parallel Imaging with RASER using Multiband Frequency-modulated Excitation Pulses

Introduction: The recently described RASER sequence is a conceptual novelty in MR imaging [1]. Based on frequency-swept excitation, and timeencoding, RASER is a rapid, pure T2-weighted scheme without signal voids, ghosting and distortion artifacts present in conventional echo-planar imaging (EPI). These properties render it attractive for functional imaging (fMRI). However, in the previous implementation of single-shot RASER, these benefits came only at the expense of coverage and spatial and temporal resolutions. Parallel imaging (PI) can alleviate such limitations. With RASER, this must utilize unconventional approaches since the sequence does not employ conventional phase-encoding. For this, multi-band frequency-swept excitation pulses were employed, and it is demonstrated that such a PI method can be used in RASER to extend its ability for spatial coverage with reduced time demands and achieve echo times optimal for T2-weighted blood oxygenation level dependent (BOLD) contrast in fMRI.