Variable-flip-angle single-slab 3D GRASE imaging with phase-independent image reconstruction

Purpose: To develop a single-slab three-dimensional variable-flip-angle gradient- and spin-echo pulse sequence with phase-independent reconstruction that is highly energy-or encoding-efficient for high resolution isotropic imaging at high magnetic field. Methods: Amplitude modulation in the proposed pulse sequence was alleviated using a variable-flip-angle induced smooth signal evolution along the long echo train. To avoid phase modulation, instead of directly interleaving phase encoding signals with different off-resonance induced phase accumulation over multiple echoes, phase-independent image reconstruction was performed, wherein each echo image was separately reconstructed using convolution-interpolation with echo-interleaving self-calibration and then combined. Numerical and experimental studies were performed at 3.0 T for generation of clinical T₂-weighted contrast to investigate the effectiveness of the proposed method over existing methods. Results: Compared with conventional techniques, the proposed method produces smooth amplitude variation, no ghosting artifacts (no phase modulation), and competitive signal-to-noise ratio. An energy-efficient variable-flip-angle gradient- and spin-echo reduces specific absorption rate by 71% without significant loss of signal-to-noise ratio, while an encoding-efficient one decreases imaging time by 54% with a slight loss of signal-to-noise ratio. Conclusion: We successfully demonstrated that the proposed variable-flip-angle gradient- and spin-echo can be a highly promising energy- or encoding-efficient alternative for high resolution isotropic imaging.