FIX: Derive field from SyN displacements using EPI affine #421
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mgxd
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Jan 9, 2024
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👍 definitely an improvement, although the current unwarping in the images you shared looks much better than in nipreps/fmriprep#3158
Agreed. Running tests on one known to produce extreme distortions. |
oesteban
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oesteban
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Theoretically, a fieldmap in Hz is the number of voxels/second that signal is shifted from the true location. SyN-SDC calculates the shift in mm from the true location. In order to find the equivalent fieldmap, we need the size of the voxels, readout time and phase-encoding direction of the EPI image that is to be corrected.
Previously, we were deriving the fieldmap from the readout time and phase-encoding direction of the EPI image, but the voxel size of the displacement field. This causes an overestimate of the number of voxels to be shifted, and thus exaggerated correction.
Original EPI:
Current unwarp:
New unwarp: