计算SENSE重建图像中的g-factor,这是并行磁共振成像SENSE算法的关键一步-G-factor is the metric to quantify the amplificaiton of noise power in reconstructing SENSE accelerated image. The detail was presented in Pruessmann s 1999 Magn. Reson. Med. paper. In theory, g-factor is the pixel-by-pixel ratio of the image variance between the SENSE reconstructed image and un-accelerated image. And g-factor depends on: (1) the k-space data trajectory (2) coil sensitivit profiles (3) noise covariance matrix of the array coil