Hi Yolanda, if you want to simulate a study where one obtains 2 time points (6 months apart) or 2 time points (12 months apart) and then compare the results, you would need to run these two independently through free surfers longitudinal stream (one time including 0 and 6 month, the other time including 0 and 12 months). If you are interested in measuring if for whatever reason there is a change in the slope at 6 months (e.g. a nonlinear trajectory) you can do the piecewise linear model that I described below on the full dataset.
Best, Martin On Mar 19, 2013, at 4:26 AM, Yolanda Vives <yvi...@pic.es> wrote: > Hi Martin, > > Thank you for your answer, but my idea was to compare the atrophy > rates when using scans at 6 months (0 - 6 months) and scans at 12 > months (0 - 12 months). If my templates are built with more time > points, the results will be biased, right? > > Regards, > Yolanda > > 2013/3/18 Martin Reuter <mreu...@nmr.mgh.harvard.edu>: >> Hi Yolanda, >> >> actually it is advantageous if the template is build from more time points. >> You don't need to process the images again, just do your statistical >> analysis with the time points you are interested in. >> >> But: why restrict the statistics to a subset of the data? If you are >> interested in yearly change, you can compute that from all time points. If >> you are interested in seeing if the atrophy rate increases over time, use a >> piecewise linear model (node at 6 months) and see if the slopes increase, >> etc. Removing time points from the statistics is usually a bad idea as you >> loose power. >> >> Best, Martin >> >> >> On 03/18/2013 01:15 PM, Yolanda Vives wrote: >>> >>> Dear FreeSurfer experts, >>> >>> I have run a longitudinal study considering multiple time points (7 >>> scans/subject on average) with FS 5.1. However, I am interested in the >>> percent changes between two time points only (between 0 and 6 months >>> and between 0 and 1 year). Should I run the longitudinal stream from >>> step 2 (template) two more times (1.- considering only 0 and 6 months >>> scans and 2.- considering only 0 and 12 months scans) or is it correct >>> even if my templates were built with all the time scans together? >>> >>> Thank you for your help, >>> Yolanda >>> _______________________________________________ >>> Freesurfer mailing list >>> Freesurfer@nmr.mgh.harvard.edu >>> https://mail.nmr.mgh.harvard.edu/mailman/listinfo/freesurfer >>> >>> >> >> -- >> Dr. Martin Reuter >> Assistant in Neuroscience - Massachusetts General Hospital >> Instructor in Neurology - Harvard Medical School >> MGH / HMS / MIT >> >> A.A.Martinos Center for Biomedical Imaging >> 149 Thirteenth Street, Suite 2301 >> Charlestown, MA 02129 >> >> Phone: +1-617-724-5652 >> Email: >> mreu...@nmr.mgh.harvard.edu >> reu...@mit.edu >> Web : http://reuter.mit.edu >> >> >> >> The information in this e-mail is intended only for the person to whom it is >> addressed. If you believe this e-mail was sent to you in error and the >> e-mail >> contains patient information, please contact the Partners Compliance >> HelpLine at >> http://www.partners.org/complianceline . If the e-mail was sent to you in >> error >> but does not contain patient information, please contact the sender and >> properly >> dispose of the e-mail. >> > > --------------------------------- Dr. Martin Reuter Assistant in Neuroscience - Massachusetts General Hospital Instructor in Neurology - Harvard Medical School MGH / HMS / MIT A.A.Martinos Center for Biomedical Imaging 149 Thirteenth Street, Suite 2301 Charlestown, MA 02129 Phone: +1-617-724-5652 Email: mreu...@nmr.mgh.harvard.edu reu...@mit.edu Web : http://reuter.mit.edu
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