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Dear Annick, I acquired multiband BOLD data and I want to use topup to correct for distortions. I have plenty of options. I acquired: 1- 1 measurement of BOLD with phase encoding P to A 2- 1 measurement BOLD A to P the first thing I should say is that I think it would be better for you to have 1 and 2 to be SE-EPI P->A and A->P, instead of GE-EPI. The reason for that is that GE-EPI has dropout in addition to distortions, and that will make it harder to estimate the field. Having said that, we haven’t yet investigated just how big a difference this makes, and you might have seen a recent thread where on person thinks that his GE-EPI maps are better than his SE-EPI maps. 3- 530 measurements of BOLD P to A (run 1) 4- 530 measurements of BOLD A to P (run 2) 5- 530 measurements of BOLD P to A (run 3) 6- 530 measurements of BOLD A to P (run 4) 7- 530 measurements of BOLD P to A (run 5) 8- 530 measurements of BOLD A to P (run 6) 9- 530 measurements of BOLD P to A (run 7) The session lasted about 90 minutes. Although people didn't move too much during the runs, it feels likely that head position changed somewhat from beginning to end. People tended to move lower body parts during breaks. A few took a break outside the scanner between run 4 and run 5. My question: Instead of creating a single topup image from acquisition 1 & 2, I was wondering if there could be benefits to picking 2 images that are close in time to the to-be corrected data. To do this, I could pick the last measurement of the previous run (including the 1 measurement of BOLD P to A, #2 above), and create an image with the first measurement of the run (e.g. run 1). The topup image would be applied to that run (e.g. run 1). So each run would get its own 2 images (last measurement of the previous run + first measurement of the run). I think that you are right that picking the pair closest in time to the data you want to correct is in general a good idea. In the slightly longer run we hope to have a way to estimate how the field changes over time, but for now we don’t have that option. You need to be careful with how you select those pairs though. You said that people tend to move during breaks, and even leave the scanner. Moving of “lower body parts” tend to result in a rotation around the x-axis/translation along the z-axis of the brain. It may not be a good idea to combine for example the last volume if scan 4 with the first of scan 5 if the subject has left the scanner in between. Jesper