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Hi again, Dear Lulu, I searched information about the scanning method of the hardi data. One author said that whether should we blip phase encoding directions depending on the strategy chosen to correct for susceptiblity artifacts. - the first one relies on the acquisition of a calibration fieldmap to correct for these susceptiblity artifacts; in this case, the phase encoding direction should be set to P/A (eg A/P + 180°); the correction is done using an unwarping technique where the correction along the phase axis is driven by the acquired fieldmap (see Peter Jezzard landmark paper for further details) - the second one relies on a double acquisition with the phase encoding set along R/L direction but with a reversal of the blip sign; this is the technique used for instance in the HCP project; inverting the blip sign between the two acquisition double the distorsions between the two scans, and therefore increases the robustness of the non rigid registration that is absolutely not driven by any preliminary calibration like the first strategy. But the second strategy doubles the scan time. So I am thinking that the first strategy would be better for us. Is the EDDY/TOPUP would do susceptiblity artifacts correction of the first strategy? the (Jezzard) fieldmap technique you describe is used to directly measure a fieldmap, which can then be used to correct the EPI images. The corresponding thing to that is to use topup with one (or several) pairs of EPI images with reversed blips. To correct such a pair doesn't take any longer than it takes to collect a fieldmap. Sorry, "correct" should be "collect". J Either strategy should give you good correction for susceptibility, but the topup strategy is easier to combine with eddy. Jesper Thanks so much for your concern. Yours, Lulu