In the aseg, 24=CSF is comprised of some indeterminant ventricle space in the middle of the brain. It is small and not well defined. In synthseg, it represents that region but is mostly comprised of extracerebral CSF as you have noted.

On 5/29/2025 5:24 PM, Emma Rodrigues wrote:

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Dear FreeSurfer Experts,

I hope this email finds you well.

I'm reaching out to request clarification regarding label 24 – CSF in the FreeSurfer software. From my observations, this label appears to correspond to a region near the third ventricle. However, it does not seem to be included in any of the standard ventricular volume measurements.

Additionally, I’ve noticed that in segmentations produced with SynthSeg, label 24 – CSF appears to correspond to the subarachnoid space, which adds some confusion. I’ve looked for documentation or forum discussions on this topic but have encountered mixed information.

Could you please clarify what anatomical region label 24 – CSF represents in the original (pre-SynthSeg) version of FreeSurfer?

Thank you in advance for your time and help.

Best regards,

Emma Rodrigues

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