It's no coincidence. The smearing effect corresponds to parts of the
FOV of the spatially normalised data where there is no data available
in the original images. The algorithm effectively fills in these
parts of the image using values from the closest available voxels.
Usually, the data in these parts of the image would be set to zero -
which would probably be more wrong.
It should be easy to fix it in the stats by supplying an explicit mask.
Best regards,
-John
On 22 December 2010 03:27, Thackery Brown <[log in to unmask]> wrote:
> I have collected a series of fMRI data for analysis in SPM8. BOLD scans were acquired 3x3x4mm with a slice orientation aligned with the anterior-posterior commissure. All subjects had a little clipping (3 or 4mm) off of the very top of the parietal cortex due the the bounding box size/positioning during acquisition.
>
> Data were preprocessed up to normalization in SPM8 using a standard preprocessing stream. When I look at the raw BOLD images up to the motion corrected images, the brains look perfectly fine - save for signal dropout there are no strange distortions in the images. When I run MNI normalization and smoothing (6mm FWHM) using SPM8's built in functionality, nothing bad comes of it and the swua images look fine as well.
>
> However, when I run normalization/smoothing using DARTEL instead of SPM's built in functionality, I get a strange vertical expansion artifact coming out of the top of the functional brain - seemingly around the point of the scanner bounding box cut-off, although perhaps this is just a coincidence.
>
> I've attempted to attach images of 1) a "healthy" raw image, 2) a "healthy" normalized and smoothed image using SPM8's functionality, and 3) a distorted normalized and smoothed image generated by DARTEL for one subject
>
> If anyone has any experience with this I'd really appreciate the help!
>
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