PROPELLER DUO: Applied to Diffusion-Weighted Imaging

Introduction: Fast Spin Echo (FSE) could suffer from the non-CPMG condition that results in destructive echo interference, unstable echo train and signal cancellation. Diffusion-weighted (DW) PROPELLER [1], an FSE-based sequence, mitigates the non-CPMG issue by XY phase cycling [1] combined with split-blade acquisition [2]. However, XY phase cycling requires refocus flip angle be close to 180° to stabilize echo train, resulting in long scanning time on 3T due to SAR limitation. DW SPLICE PROPELLER [3] is another approach without the high flip angle constraint. SPLICE [4] uses imbalanced readout gradient and extended acquisition window to split all the echoes in pathways into two selfcoherent echo groups, E1 and E2. Destructive interference between E1 and E2 is naturally avoided because they are acquired and reconstructed separately. The tradeoff of SPLICE PROPELLER is the increased echo space and scanning time. In this work, a novel PROPELLER sequence is presented. Instead of using imbalanced readout gradient, “phaser” gradients are used to separate E1 and E2 into two perpendicular blades in a single k-space. Echo interference is avoided by the phase correction during PROPELLER reconstruction. The sequence is named as PROPELLER DUO, based on the facts that the “two” echo groups (E1 and E2) are separated and form “two” blades in a single shot.