An auto‐calibrated, angularly continuous, two‐dimensional GRAPPA kernel for propeller trajectories

The k‐space readout of propeller‐type sequences may be accelerated by the use of parallel imaging (PI). For PROPELLER, the main benefits are reduced blurring due to T2 decay and specific absorption ratio (SAR) reduction, whereas, for EPI‐based propeller acquisitions, such as Turbo‐PROP and short‐axis readout propeller EPI (SAP‐EPI), the faster k‐space traversal alleviates geometric distortions. In this work, the feasibility of calculating a two‐dimensional (2D) GRAPPA kernel on only the undersampled propeller blades themselves is explored, using the matching orthogonal undersampled blade. It is shown that the GRAPPA kernel varies slowly across blades; therefore, an angularly continuous 2D GRAPPA kernel is proposed, in which the angular variation of the weights is parameterized. This new angularly continuous kernel formulation greatly increases the numerical stability of the GRAPPA weight estimation, allowing for generation of fully sampled diagnostic quality images using only the undersampled propeller data. Magn Reson Med 60:1457–1465, 2008. © 2008 Wiley‐Liss, Inc.