Hi Vincent,
yes, it is tp2-tp1 / time_delta
so thinning is negative, thickening positive. I'll correct the help
text, the tutorial is right.
It knows about the time point ordering because of the time column.
Best, Martin
On 08/02/2013 09:04 PM, vbrun...@nmr.mgh.harvard.edu wrote:
> Dear Freesurfer experts,
>
> I have followed the LongitudinalTutorial with two time points + two
> groups and now, after looking at the output of the qdec analysis, I just
> want to make sure I haven't made a sign error as for the control group
> the difference is as expected around 0 but for the experimental group I
> see negative values (= thinning?) all over the place which seems weird,
> just want to make sure I got this right:
>
> I set the time variable to 0 for the baseline scan and to 1 for the
> after-intervention-scan. Is it correct that for do-rate the calculation
> is then only - and not the other
> way around?
> I ask because in the long_mris_slopes -help it says that do-rate would
> yield the "thinning in mm/time" and if this means that a positive value
> stands for thinning then this would contradict what I understood from
> the Tutorial where it says for the rate "thick2-thick1" is calculated,
> so a positive value should mean "thickening".
>
> Also, how does long_mris_slopes know which one is tp1 and which one is
> tp2? I assume it takes the first entry in the fsid column of
> long.qdec.table.dat as tp1 and the second entry below as tp2?
>
> Hm, I see no other possibilities of making a sign error during the whole
> process, did anyone report a similar issue?
>
> Thanks for your help! Best,
> Vincent
>
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--
Martin Reuter, Ph.D.
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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