Hi @roeysc - I can’t speak to fmriprep particulars, but can hopefully give some overall insight.
The total readout time for the synthesized image should indeed be 0 (infinite bandwidth).
This next statement has some caveats: The total readout time for the acquired (distorted) image doesn’t matter, and we typically default to something like 0.05. If you were to use topup to estimate the field, then applytopup to apply this field to the unwrapping process, as long as you give applytopup the acquisition parameters (i.e., the total readout time you gave topup) it will correctly scale the field.
However, if the units/magnitude of the field map matters for subsequent processing, then the total readout time matters. I’m not sure how fmriprep chooses to estimate and apply the field, so this value possibly does matter.
We also also recommend using reverse PE or field maps when those are available, and only using the synthesis method when these are unavailable. Hope that helps answer some of the questions.