Hi Koushik,
no, I cc the list, maybe someone has done tests or seen papers. I read somewhere on atlas construction, that 10 images are fine, but that is cross subject. Here we only have small changes within subject, so from my experience less are fine , BUT - 2 of course are not enough: in 2 the method is actually different (as the median in 2 is simply the average) . Starting with 3 a median intensity even exists for each voxel. - adding 1 new to 5 existing may work without sacrificing too much bias, but adding 10 to 5 is not a good idea. Also if the one that is added is very different from the first 5 (e.g. first 5 within a week, next 3 years later), then that will be problematic also.
Best, Martin
On 10/20/2014 12:07 PM, Govindarajan, Koushik Athreya wrote:
Hi Martin,
How are you? We had this below email conversation in March aboutthe longitudinal template and it's stability. You had mentioned that with 5 time points, my template should be stable enough to patch another timepoint. I have another study that has only 2 time points and I was reading through your 2012 Longitudinal analysis paper that had used 2 time points to compare with cross-sectional data. I was wondering if you had any suggested literature for the number of time points needed for stability of a template.
Thanks
Koushik Govindarajan
*From:*Martin Reuter [mailto:mreuter@nmr.mgh.harvard.edu] *Sent:* Friday, March 14, 2014 9:31 AM *To:* Govindarajan, Koushik Athreya; freesurfer@nmr.mgh.harvard.edu *Subject:* Re: [Freesurfer] adding time points to longitudinal base template
Hi Koushik,
there is a way to add time points without recreating the base: recon-all -long <tpNsubjid> <longbasesubjid> -addtp -all
This is what the recon-all help says about that: "If a new timepoint needs to be added to a longitudinal run where a base subject has already been created (from prior timepoints), then the -addtp command can be added to the -long command in order to 'fix-up' the longitudinal stream to accept the new timepoint. Note that the base subject is *not* recomputed using the new timepoint. This potentially introduces a bias, and it is recommended to NOT add a time point this way! Instead recreate the base from all time points and run all longitudinals again."
So it's up to you. But given you already have 5 time points in your base and add only 1, you may be fine. The base should be pretty stable with 5 and if the 6th is not too different, it should not matter much. I don't think anyone has ever done a thorough analysis about adding time points.
Best, Martin
On 03/11/2014 10:08 AM, Govindarajan, Koushik Athreya wrote:
Dear FS experts, I have a serial study in which I have run cross-sectional and longitudinal analysis on 5 time points. Now, I want to add a 6^th time point to my dataset. Do I have to rerun my base template creation step from scratch or is there a way to just add further time points to an existing template? Thanks Koushik _______________________________________________ Freesurfer mailing list Freesurfer@nmr.mgh.harvard.edu <mailto:Freesurfer@nmr.mgh.harvard.edu> https://mail.nmr.mgh.harvard.edu/mailman/listinfo/freesurfer <https://urldefense.proofpoint.com/v1/url?u=https://mail.nmr.mgh.harvard.edu/mailman/listinfo/freesurfer&k=yYSsEqip9%2FcIjLHUhVwIqA%3D%3D%0A&r=4ebx%2BU4XzNnu4XA8t53Lt0n%2FTCuxbfN4Z1deUmAFsLg%3D%0A&m=DVyhlZm48BQ7pEr20iqqosHWjgQ31wzMoC9FDk9YpEo%3D%0A&s=48648d97d14c3c96469c77ba99084506ab3a9eb1d6046d5937b5a35a037bce5d>-- Martin Reuter, Ph.D. Assistant in Neuroscience - Massachusetts General Hospital Instructor in Neurology - Harvard Medical School MGH / HMS / MIT
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