In order to correct for distortions you either need images with reversed phase-encode directions (for use with topup) or a fieldmap (magnitude/phase pair, for use with BBR). We cannot calculate the distortions directly from a T2 image or b=0 images (unless you have a set of b=0 images where some have different phase-encode directions).

On 23 Sep 2014, at 08:58, Gabriel Gonzalez Escamilla <[log in to unmask]> wrote:

Dear all,

I have images with different acquisition parameters (different b-values and number of directions). I want toperform the preprocessing using FSL, I've seen that the standard pre-process has changed and that now is possible to correct for the EPI distortions, using TOPUP and EDDY.

My problem is that I have not a second DWI acquisition with inversed PA, nor magnitud and phase images. Is it possible to correct the distortions by using the b0?, or when possible by using a high-resolution T2?

Many thanks in advanced for your time and answer,
Gabriel