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

1. The temporal average is simply the average thickness: avg = 0.5 * (thick1 + thick2)
2. The rate of change is the difference per time unit, so rate = ( thick2 - thick1 ) / (time2 - time1). 
3. The percent change (pc1) is the rate with respect to the thickness at the first time point: pc1 = rate / thick1. 
4. 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 

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.