dwifslpreproc - Reversed phase encode b=0 pair(s)

Dear experts,

I have two questions related to the -se_epi option to add additional spin-echo b=0 volume pairs to estimate the inhomogeneity field. In my specific case, I have a DWI image and 2 EPI images with the same phase encoding but different readout time between them. In the documentation it is specified that this must be the same, is there any problem with this not being fulfilled? How does it affect that the readout time is different?

On the other hand, I also have a DWI image in the PA direction with 8 volumes b=0, and the inverse (rDWI) in the AP direction with only 1 volume b=0. How is it better to use the b0s, calculating the average of those in the PA direction, replicating the AP volumes until obtaining the same amount of volumes in each direction (16 volumes), or only using the first volume in the PA direction and the only one in the AP direction.

Thanks in advance for your help!

Ana Harris.