Dear FreeSurfer community,

I wanted to interpret the BrainSegVol.to.eTIV ratio that is outputted from FreeSurfer version 5.2.0. In my sample of normally developing adolescents of ages 8 to 21, this ratio is between 0.65 and 0.85 for 98% of subjects. However, the value is less than 0.65 for 2% of subjects.

Could this reflect actual feature of brain volume to eTIV?

Or is this a reflection of poor scans, poor overall FreeSurfer processing, or inaccurate eTIV estimates for these subjects? Subjects with BrainSegVol.to.eTIV ratios less than 0.65 tended to all be females. They had Brain Segmentation volumes falling in normal ranges of females in the full sample. However, their eTIV was on the upper-end among eTIVs of females in the sample. If the eTIVs for these subjects were over-inflated, then it makes sense the ratios are especially low.

Thank you.

Best,

Chintan