Dear Experts,
My problem is a systematic jump of the distortion corrected (unwarped) EPI scans, using FieldMap in SPM8. The unwarped EPI scan seems to jump about 1 to 3 voxel (voxel spacing: 3.5 x 3.5 x 3.5) in anterior direction, with respect to the warped EPI scan. In addition, the unwarped scan appears slightly stretched, so that the shift at the frontal pole of the brain appears to be about 1 voxel bigger, compared to the occipital pole. This happens with all our subjects.
Our field map has different voxel size (spacing: 2.6 x 2.6 x 2.5) and slice number (64 slices instead of 47), but same orientation (i.e. ACPC aligned) and shimming parameters (i.e. adjust volume was copied from EPI scan to field map, on a Siemens Verio 3T).
I tried different parameter sets, but this shift is always present, while there is very few or none "real" distortion correction visible in the unwarped EPI scan.
Did anybody experience similar problems, or do you have any suggestion what I could have done wrong?
Thanks a lot,
Benno
|