Hi, FSL experts,
We recently compared two groups (patients vs. normal controls) T1 structure difference following FSL-VBM user guide. Our patients' head sizes measured by TICV (total intracranial volume) are significantly larger than our controls'. And we do find papers report their FSL-VBM results with and without controlling for TICV.
We found following statement about TIV in this paper (Good et.al. 2002, "Automatic differentiation of anatomical patterns in the human brain: validation with studies of degenerative dementias."):
"Since we have incorporated this correction for volume change induced by spatial normalization, it is appropriate to include TIV as a confounding covariate to remove any variance due to differences in head size. Without the modulation step, spatial normalization removes differences in head size and thus TIV does not need to be included as a confounding covariate."
Our question are:
1. In current FSL-VBM analysis, is head size removed? If yes, at which step and how? Normalization or modulation?
2. Even if head size is controlled by FSL-VBM, is it still necessary to list head size as covariate, since the two groups' head sizes are significantly different?
Thank you very much. Any thoughts on this will be very helpful to us.
Haiqing Huang
Biomedical Engineering Department
University of Florida
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