If you mean between the phase up and phase down, that is possible, but one
of the benefits is that these images are acquired very quickly (faster
than a regular field map), which also would get messed up by movement. If
you mean between the field map and the EPI data you are correcting, that
is also true for both approaches, but is something that is being worked on.
Peace,
Matt.
On 12/16/16, 8:51 AM, "FSL - FMRIB's Software Library on behalf of Antonin
Skoch" <[log in to unmask] on behalf of [log in to unmask]> wrote:
>Dear Matt,
>
>I found this yours 2 years old post very interesting:
>
>"I actually prefer using spin echo field maps (phase reversed spin echo
>images matched to the gradient echo EPI acquisition) and topup, as these
>have the same distortion as the gradient echo EPIs and can be very
>precisely registered (EPI to matching spin echo image) without the
>complication of using the structural image as an intermediate. Then one
>can use BBR on the undistorted EPI image. This keeps each step separate
>for debugging and łeasy˛ for the algorithms involved to achieve robustly."
>
>This I followed with interest since I am trying to achieve precisely the
>same thing. However, one is unclear to me. You write that the gradient
>echo EPI and phase reversed matched spin echo images have the same
>distortion. But in case of movement between the series, the distortion
>would not be the same and precise match in registration could not be
>achieved. Or do you think this effect is negligible? Could you please
>comment on?
>
>Regards,
>
>Antonin Skoch
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