DTI image preprocessing

Hello&
I have a DTI dataset and each subject has 64 gradients images and one b0 image.
I want to preprocess them based on an article that used FSL for that but, I don’t know how to do one line highlighted in the pic below I attached.
this line “First, we use the FMRIB Software Library (FSL) [40] to correct the b0 image distortion.”
I will appreciate anyone that can help me and explain how I should do that line in FSL.

It sounds like they are using topup, it mentions ‘distortion-corrected b0 (output of topup)’ on the FSL pages. Details of the topup tool are at the link below.

https://fsl.fmrib.ox.ac.uk/fsl/fslwiki/topup

There’s also an existing topup discussion on the forum :

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