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Hi, Matt Thank you for the information. I am not trying to combine them like diffusion MRI. But is it true that we will probably have less bias (at least for brain asymmetry) if we collect resting-state fMRI data with both phase encoding directions, so that all the data will not have the identical signal loss and distortion? Thanks Longchuan On Friday, March 7, 2014 5:25 PM, Matt Glasser <[log in to unmask]> wrote: You need to collect some spin echo volumes with phase reversed averages, ideally with the same geometry and echo spacing as the gradient echo volumes. You cannot combine timeseries from multiple averages in the same way you do with diffusion data because timeseries data are not simple averages. Peace, Matt. From: Longchuan Li <[log in to unmask]> Reply-To: FSL - FMRIB's Software Library <[log in to unmask]> Date: Friday, March 7, 2014 4:17 PM To: <[log in to unmask]> Subject: [FSL] Correcting resting-state fMRI using topup Hi, FSL expert I have a question regarding correcting resting-state fMRI data using "topup" and would appreciate your comments on it. Currently, I have two options for collecting rsfMRI (300 volumes) and want to have the spatial distortion in the data removed: (1) collecting two sessions with 150 volumes in each session, and the two sessions with opposite phase encoding directions; (2) collecting one session with 300 volumes, and acquiring only a few volumes with the opposite phase encoding direction for distortion correction. Which protocols are more sound? and what are the major pros and cons for them (especially in the data processing steps)? Your comments will be greatly appreciated| Longchuan