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Hi,
I am interested in when and if reversed phase-encode directions or a fieldmap really are needed. With the small number of cases I have tried so far warping
the b0 or BOLD image directly to an undistorted T2 or even T1 weighted image works well, and has the advantage of only needing scans that we would get anyway. In some of the correction method papers I have seen the EPI distortions are so large that I can
see why they could cause problems for registration, but all of the correction methods need some kind of registration at some point.
Would anybody like to share their experience or opinions?
Rob
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Robert I. Reid, Ph.D. | Aging and Dementia Imaging Research | Opus Center for Advanced Imaging Research
Mayo Clinic | 200 First Street SW | Rochester, MN 55905 |
mayoclinic.org
From: FSL - FMRIB's Software Library [mailto:[log in to unmask]]
On Behalf Of Mark Jenkinson
Sent: Thursday, October 02, 2014 2:37 PM
To: [log in to unmask]
Subject: Re: [FSL] Distorion corrections of DWI data
Dear Gabriel,
In order to correct for distortions you either need images with reversed phase-encode directions (for use with topup) or a fieldmap (magnitude/phase pair, for use with BBR). We cannot calculate the distortions directly from a T2 image
or b=0 images (unless you have a set of b=0 images where some have different phase-encode directions).
All the best,
Mark
On 23 Sep 2014, at 08:58, Gabriel Gonzalez Escamilla <[log in to unmask]> wrote:
Dear all,
I have images with different acquisition parameters (different b-values and number of directions). I want toperform the preprocessing using FSL, I've seen that the standard pre-process has changed and that now is possible to correct for the EPI distortions,
using TOPUP and EDDY.
My problem is that I have not a second DWI acquisition with inversed PA, nor magnitud and phase images. Is it possible to correct the distortions by using the b0?, or when possible by using a high-resolution T2?
Many thanks in advanced for your time and answer,
Gabriel