Hi Sima, since in a longitudinal study the observed information in the time points are correlated, care needs to be taken in the statistical analysis. People usually use Linear Mixed Models for that purpose. I am not a statistician and therefore the wrong person to ask. You should contact your local bio-statistician as there are several design choices that can be made depending on your case.
For a simple analysis we usually recommend to compute a single number statistic for each subject (e.g. thickness difference or percent change) and then use that across groups. Best, Martin On Wed, 2011-01-19 at 12:55 +0000, sima chalavi wrote: > Dear Freesurfer experts > > I have some longitudinal data from tp1 and tp2 for three different > groups (A,B,C) and 2 different genders (G1,G2). > > I followed the longitudinal stream and processed all the images in FS > 4.5. > > For further analysis, I saw the post from Martin and Doug at > http://www.mail-archive.com/freesurfer@nmr.mgh.harvard.edu/msg14266.html in > which they suggested to first do the mris_preproc for each time point and > then calculate the differences using mris_calc. > > What I did was instead of doing mris_preproc for each tp separately I > made a FSGD file and put all the information about our different > classes including baseline (BL) and followup (FU), see below, and then > run mris_preproc which gives me ?h_thickness_both_tp_fwhm10.mgh files. > > GroupDescriptorFile 1 > > Title diagnosis_gender_glmdir > > MeasurementName thickness > > Class A_G1_BL > > Class A_G1_FU > > Class A_G2_BL > > Class A_G2_FU > > Class B_G1_BL > > Class B_G1_FU > > Class B_G2_BL > > Class B_G2_FU > > Class C_G1_BL > > Class C_G1_FU > > Class C_G2_BL > > Class C_G2_FU > > > After mris_preproc for example I made the following contrast to > compare the changes from BL to FU and then I ran mri_glmfit: > > contrast to compare (A_BL vs C_BL) vs (A_FU vs C_FU): > > contarst_AvCBLvAvCFU=[1 -1 1 -1 0 0 0 0 -1 1 -1 1] > > > >> mri_glmfit - - y ?h_thickness_both_tp_fwhm10.mgh - - fsgd > diagnosis_gender_BL_FU.fsgd - - C contarst_AvCBLvAvCFU.mtx - - surf > fsaverage ?h - - cortex - - glmdir glmdir > > > So using this approach I wanted to compare the difference between the > 2 tps using the contrasts and not calculating first the differences > using mris_calc and then runnig glm. However, I am not sure if I am > doing the right thing. Could you please send me your feedbacks. > > Thanks in advance, > > Sima Chalavi. > > > -- > Sima Chalavi, Ph.D. student > > King's College London > Institute of Psychiatry (IoP) > Box P040, > De Crespigny Park > London SE5 8AF. > Tel:+44-2078485702 > > BCN Neuroimaging Center (NIC) > University Medical Center Groningen > Antonius Deusinglaan 2 > P.O. Box 196 > 9713 AW Groningen > www.neuroimaging-DID.com > _______________________________________________ > Freesurfer mailing list > Freesurfer@nmr.mgh.harvard.edu > https://mail.nmr.mgh.harvard.edu/mailman/listinfo/freesurfer _______________________________________________ Freesurfer mailing list Freesurfer@nmr.mgh.harvard.edu https://mail.nmr.mgh.harvard.edu/mailman/listinfo/freesurfer 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.
