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Qiang, I am using gradient-echo, not spin-echo images, for my fieldmaps. In other words, I do not use symmetric and asymmetric spin echoes. I can tell you, however, that I have a phase map and magnitdue image for both of the gradient-echo scans. If I don't have two phase maps I cannot calculate the fieldmap. Fieldmap = difference in phase of protons between two time points. Hope this helps. -Carlos