hi everyone, we are using fmriprep for the first time and have encountered some difficulties regarding the susceptibility correction by applying our field map. The unwarped images seem to be more distorted after performing the susceptibility distortion correction on the EPI. We are using a phase-difference map (+2 magnitude images).
Our first guess was that fmriPrep uses the wrong phase encoding direction. However, in our _phasediff.json and _bold.json, the parameter PhaseEncodingDirection is set to ‘j-‘, which matches the anterior-posterior phase encoding direction of our sequences.
Maybe some of you had encountered a similar problem at some point and could help with some advice.
Dear Steven, thank you very much for your reply. The images were obtained from a Siemens scanner. Nevertheless, I have tried to change the PhaseEncodingDirection to ‘j’ in order to see whether this might result in a better correction. The correction is different but not better. This seems not to be the source of the problem. Btw, we have the same issue with all of our subjects. I have checked consistency in units (although I suppose, it would have been a much worse output if time units were inconsistent).
I don’t know whether it helps, but I have attached the images of the applied fmap corrections for both direction settings.
Please excuse the late reply. Yes we also tried to run it without the field maps to check whether the problem indeed can be attributed to the field maps. And the fixed vs moving images look better without the fmap correction.
Sure! Do you mean using --use-syn-sdc? And I suppose I should remove the IntendedFor from my fmaps (or the whole folder) or will anything related to fmaps be ignored anyway?
I am having this same problem (similarly using a phase-diff map + 2 magnitude images) where fmap correction makes images look worse. Also using a Siemens scanner and using Anterior to Posterior direction (PhaseEncodingDirection is ‘j-’ in the .jsons for both fmaps and bold). Was a solution for this problem found?
Before fmap correction (top) and after fmap correction (bottom)
Given that fMRIprep recently updated (v22.1.0) with a new SCDFlows version, please try this fMRIprep version. If the error persists, please open a new issue under the “Software Support” category, making sure to fill out information in the pre-populated template.