Hi Ela - Do you know what phase-encode direction your DWIs were collected with (e.g. AP, PA, LR, etc.)? Then add it to the config file like this: "set pedir = (AP)"

Otherwise, if your data are stored in BIDS format, there's a new script that will figure all this out and generate the configuration file automatically:
https://github.com/freesurfer/freesurfer/blob/dev/scripts/dmri_bids_config

Anastasia.
From: freesurfer-bounces@nmr.mgh.harvard.edu <freesurfer-bounces@nmr.mgh.harvard.edu> on behalf of Salar Dini, Elaheh <elaheh.salardini@yale.edu>
Sent: Thursday, December 9, 2021 10:31 AM
To: freesurfer@nmr.mgh.harvard.edu <freesurfer@nmr.mgh.harvard.edu>
Subject: [Freesurfer] trac-all -prep error
 

        External Email - Use Caution        

Dear freesurfer developers and Dr. Yendiki,

I am trying to run trac-all -prep -c dmriEla, dmriEla being my config file. I am attaching the config file here. If I perform registration-based correction with eddy_correct, doeddy=1, things start to run. However, if I use doeddy=2, model-based correction with eddy, I get the following error:
ERROR: for B0 distortion correction with reverse-polarity DWIs   
ERROR: or for model-based eddy-current distortion correction,  
ERROR: must specify PE direction
No Trac-all.log file is created.  If needed, I can also send you a few anonymized slices from the acquired DTI series, so you can check the header. Moreover, I can send you the acquisition sequence properties as a PDF file if needed. 

Is there a way to fix this problem? In other words, can we do something to be able to use model-based correction instead of registration-based questions since it seems to be a more sophisticated method? 

I'm running freesurfer on Linux Centos7, the version of freesurfer is 7.2.0 and the installed package is freesurfer-linux-centos7_x86_64-7.2.0.tar.gz.

I really do appreciate your time and help.
Ela