[Freesurfer] SynthSeg hippocampal quality control bias

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Dear Freesurfers,

In using SynthSeg, I have observed that the quality control value for the hippocampus+amygdala seems to be directly correlated with the hippocampal volume. Higher QC values = higher volumes. There also seems to be an age effect on the quality control result, where QC begins to fall in the elderly, just like the volume would.

This is not as obviously the case for thalamus QC or gray matter QC with thalamus and gray matter volumes.

Is there a systematic bias with the algorithm of SynthSeg? Is the volume used as input to calculate the QC result? Is there a systematic bias or real relationship here?

Any insight would be appreciated.

Sincerely, Matt