Hello
Le 26/09/2012 23:22, Jesper Andersson a écrit :
> Hi again,
>
>>> There is also the issue of susceptibility dephasing in the gradient echo
>>> that causes unrecoverable signal loss, which is much less present in spin
>>> echo data.
>> Well, isn't that a problem for all gradient echo sequences independent of topup (in other words prelude and fugue will also not recover that signal)?
> Yes and no, and this is a crucial point. When you use a "traditional" fieldmap to correct a GE-EPI it is true that it cannot help you to correct the lost signal, but at least it tells you how to "shift" the existing signal (because the fieldmap is defined everywhere).
>
> If you were to use PE-flipped GE-EPI images to calculate the fieldmap (a la topup) it won't even be defined in the areas with no/little signal.
>
> One could always turn that argument around and say that in areas with little no signal there is anyway nothing to shift, but I would always prefer to use the SE-EPI topup derived fieldmaps (as Matt described us doing in the HCP project).
I understand that you advice a SE-EPI topup, But if one acquire fmri
time series with Gradient Echo :
can we correct it with a SE-EPI topup derived fieldmaps ? the distortion
would not be the same ?
thanks for those precisions
Romain
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