Self‐calibration method for radial GRAPPA/k‐t GRAPPA

Generalized autocalibrating partially parallel acquisitions (GRAPPA), an important parallel imaging technique, can be easily applied to radial k‐space data by segmenting the k‐space. The previously reported radial GRAPPA method requires extra calibration data to determine the relative shift operators. In this work it is shown that pseudo‐full k‐space data can be generated from the partially acquired radial data by filtering in image space followed by inverse gridding. The relative shift operators can then be approximated from the pseudo‐full k‐space data. The self‐calibration method using pseudo‐full k‐space data can be applied in both k and k‐t space. This technique avoids the prescans and hence improves the applicability of radial GRAPPA to image static tissue, and makes k‐t GRAPPA applicable to radial trajectory. Experiments show that radial GRAPPA calibrated with pseudo‐full calibration data generates results similar to radial GRAPPA calibrated with the true full k‐space data for that image. If motion occurs during acquisition, self‐calibrated radial GRAPPA protects structural information better than externally calibrated GRAPPA. However, radial GRAPPA calibrated with pseudo‐full calibration data suffers from residual streaking artifacts when the reduction factor is high. Radial k‐t GRAPPA calibrated with pseudo‐full calibration data generates reduced errors compared to the sliding‐window method and temporal GRAPPA (TGRAPPA). Magn Reson Med 57:1075–1085, 2007. © 2007 Wiley‐Liss, Inc.