Thank you Martin. 

Just to clarify, then it is not valid to compare values of single point analyses (e.g. hippocampal volume from freesurfer analysis of time point 1 subtracted from hippocampal volume of from freesurfer analysis of time point 2).

Best wishes,

-Salil


On Mon, Nov 11, 2013 at 7:58 AM, Martin Reuter <mreuter@nmr.mgh.harvard.edu> wrote:
Hi Salil,

please send questions to the free surfer mailing list.

If you used the longitudinal stream for processing the etiv should be the same in all time points. It is computed in the 'base' = within subject template once for the entire series.
It seems that you did not run your images through the longitudinal stream if your etiv is different across time points.

For longitudinal processing see here:

Bes,t Martin

On Nov 10, 2013, at 3:37 PM, Salil Soman <salsoman@stanford.edu> wrote:

Dear Dr.  Reuter,

I have performed freesurfer 5.3 with hippo subfields Analysis on patients that have 2 consecutive scans  (pre and post surgery).  I have generated Segstats statistics for each individual time point.  I have noticed the etiv is different on all of the cases for each time point (scan 1 is different from 2).  This has led me to worry that it may not be legitimate to perform analyses of changes in volumes of structures by computing differences in values between time points).  I am writing to inquire if you were familiar with /had an opinion regarding this matter.

Best wishes,

Salil Soman, MD, MS
Postdoctoral Research Fellow - Stanford Radiological Sciences Laboratory
Fellow - Palo Alto War Related Illness and Injury Study Center
WOC Neuroradiology Attending - Veterans Affairs Palo Alto Health Care System


---------------------------------
Dr. Martin Reuter
Assistant in Neuroscience - Massachusetts General Hospital
Instructor in Neurology   - Harvard Medical School
MGH / HMS / MIT

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--
Salil Soman, MD, MS
Postdoctoral Research Fellow - Stanford Radiological Sciences Laboratory
Fellow - Palo Alto War Related Illness and Injury Study Center
WOC Neuroradiology Attending - Veterans Affairs Palo Alto Health Care System