Variable-flip-angle 3D spiral-in-out turbo spin-echo imaging using concomitant gradient compensation and echo reordering at 0.55T

Purpose: To develop single-slab 3D spiral turbo spin echo (spiral SPACE) for 1-mm3 isotropic whole-brain T2-weighted imaging on a high-performance 0.55T scanner, with high scan efficiency from interleaved spiral-in-out trajectories, variable-flip-angle refocusing radiofrequency (RF) pulses, echo reordering, and concomitant-field compensation. Methods: A stack-of-spirals (in-out waveforms) turbo-spin-echo acquisition was implemented with T2-weighed contrast. Gradient infidelity was corrected using the gradient impulse response function (GIRF), and concomitant-field compensation was used to correct for phase errors among echoes and during the readout windows. To maintain a long echo train (600ms) within each shot, variable-flip-angle refocusing RF pulses were generated using extended-phase-graph analysis. An echo-reordering scheme provided a smooth signal variation along the echo direction in k-space. Images from spiral SPACE with and without concomitant-field compensation were compared with those from Cartesian SPACE in phantoms and 6 healthy volunteers. Results: Phantom results demonstrated the improved performance of concomitant-field correction via sequence-based modifications and of GIRF-based trajectory estimation. Volunteer data showed that with concomitant-field correction and echo reordering, system imperfection associated image artifacts and blurring were substantially mitigated in spiral SPACE. Compared with Cartesian SPACE, spiral SPACE had an overall 15%-25% signal-to-noise ratio (SNR) improvement in both white matter and gray matter. Conclusion: A 3D spiral-in-out SPACE acquisition with variable-flip-angles, concomitant-field compensation, and echo-reordering was demonstrated at 0.55T, showing promising gains in SNR, compared with Cartesian SPACE.