Hi Mike,
you can't change this retrospectively. It means that different protocols were used when scanning (or different processing after scanning, before passing it to freesurfer-in that case it could be fixed). It would be good to try to understand what exactly changed between acquisitions (is this on the same scanner/headcoil)? If it is the same scanner and no parameters that affect the image intensity changed, I would not worry too much about this small difference in voxel size. You may also want to check if this happens in all subjects, or only some. And if in some, are they all in one of your groups or distributed across groups, to avoid potential bias.
Best, Martin
On 04/04/2016 12:07 PM, Angstadt, Mike wrote:
We have a large number of scans that have been processed through the standard cross-sectional stream. Subjects have a number of repeated scans, so we now want to take advantage of the longitudinal stream.
When first beginning to run subjects through longitudinal template creation, we noticed a warning message in some cases in the log:
WARNING: Image geometries differ across time, maybe due to aquisition changes?
This can potentially bias a longitudinal study! Will continuein 10s.
Looking into it, there does indeed appear to have been a change in the reconstruction stream that led to images at different times during the study to have slightly different voxel dimensions, changing from 1x1x1.2 to 1.01x1.01x1.2mm voxels.
Our question is if there is some way to fix this prior to starting the longitudinal stream but without having to redo the original cross-sectional processing and editing that has been completed?
--
Mike Angstadt
Research Computer Specialist / PANLab Lab Manager
Department of Psychiatry / University of Michigan
(734) 936-8229
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