Hi, fmriprep experts
We are using fmriprep 21.0.1 to process fMRI data using EPI sequence. We got two scans with different PhaseEncodingDirection: AP（j-） and PA(j), the direction of data is LAS.
We ran fmriprep on the data successfully, however, the results of SDC are weird.
Here are some results of sub-test001_task-rest_dir-AP_bold.nii.gz after processed through fmriprep 21.0.1.
Preprocessed estimation with varying Phase-Endocing (PE) blips
Hover on the panels with the mouse pointer to also visualize the intensity of the inhomogeneity of the field in Hertz.
Inhomogeneities of the B0 field introduce (oftentimes severe) spatial distortions along the phase-encoding direction of the image. Utilizing two or more images with different phase-encoding polarities (PEPolar) or directions, it is possible to estimate the inhomogeneity of the field. The plot below shows a reference EPI (echo-planar imaging) volume generated using two or more EPI images with varying phase-encoding blips.
Susceptibility distortion correction
Results of performing susceptibility distortion correction (SDC) on the EPI
It seems the SDC procedure made the image more distorted. After that we try to run the same data in fmriprep 20.2.1. The results seems better, but I am not sure if it is just less worse.
Here are some results of sub-test001_task-rest_dir-AP_bold.nii.gz after processed through fmriprep 20.2.1.
Susceptibility distortion correction
Results of performing susceptibility distortion correction (SDC) on the EPI
Also, this post suggest that maybe the PhaseEncodingDirection of feildmap were wrong. https://neurostars.org/t/bold-susceptibility-distortion-correction-sdc-using-spin-echo-fieldmaps-have-to-switch-pe-direction/22527. So I changed AP to j and PA to j- just for feildmap and run fmriprep21.0.1 again. but fmriprep failed during topup this time. I am not sure my try is useful, just for more information 
So, do you have any suggestion of this situation? If the result of 20.2.1 is accepetable? Also what could made the artifacts in fmriprep 21.0.1 and what should I do to get a more satisfatory results?