Hi FSL community,
We’re looking to use spin-echo HCP style fieldmaps, and are comparing the application of a fieldmap calculated with topup in two methods:
1. `applytopup` - Using the fieldmap and the single phase correction as shown in the help text
2. FEAT - Using FEAT’s stream and inputting the fieldmap directly.
When comparing the outputs, it seems that FEAT produces a “gentler” correction than applytopup, including taking signal out of OFC less aggressively. Can you tell us the exact steps FEAT is performing in its correction (or give suggestions on how to dig into it), and if there are any adjustments that can be made to applytopup to make it resemble FEAT more closely?
For example, we’ve seen in various documents that FEAT expects a fieldmap in radians / second, but applytopup doesn’t specify. Is there some extra unit checking that adjusts the fieldmap to within sane / expected values that might explain this?
Thanks in advance for any detail you could give us on how to unify these and apply correction accurately. Happy to share snapshot examples of it would help,
Erik
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