Ups, sorry, missed your mail.
you can get the difference by two ways:
- you simply subtract the thickness maps in each subject by yourself and then use that for the analysis
- you put 0 and 1 into the longitudinal qdec table for the two time points. When the time distance is 1, the rate will be the difference.
Best, Martin
On Oct 24, 2013, at 4:30 PM, Qi Wu qwu1975@gmail.com wrote:
Hi Freesurfer users,
For the longitudinal stream in FS5.1, there are only 4 options of measures (do-avg, do-rate, do-pc1 and do-spc). However, if I need other measures, such as the difference between two time points (thick2-thick1), how should I do to get the values and still be able to do the voxel-wise analysis for regional difference in cortical thickness (with glmfit)?
Many thanks
Keith Postdoc research fellow Toronto Western Hospital
The temporal average is simply the average thickness: avg = 0.5 * (thick1 + thick2) The rate of change is the difference per time unit, so rate = ( thick2 - thick1 ) / (time2 - time1). The percent change (pc1) is the rate with respect to the thickness at the first time point: pc1 = rate / thick1. The symmetrized percent change (spc) is the rate with respect to the average thickness: spc = rate / avg. _______________________________________________ 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: mreuter@nmr.mgh.harvard.edu reuter@mit.edu Web : http://reuter.mit.edu