Hi, Newbie (gotta love that name!)-
First, I'm an SPM newbie, so I invite correction, instruction, and/or
validation from the list...
From my understanding, SPM contrasts are only concerned with whether,
e.g., scan A values are larger than scan B values. It doesn't care if
those values are positive or negative. It's likely that if you created
BP images with ImCalc using DVR-1 at each voxel, and re-run the same
analysis, you'd get relatively similar areas of activation, even if the
BP values are negative. The subtraction of "1" from DVR doesn't change
the relative nature of the relationship of a voxel in scan A to that
same voxel in scan B.
As to why the BP values are going negative (I'm assuming you mean the BP
values for each scan are negative, and not the difference in BP between
the scans), some additional information would be helpful. For example:
What tracer is this? Are baseline DVR values low in general? What
kinetic model and analysis method are being used to generate the
parametric DVR images? Are you using a plasma input function or a ref
region? How large are the ROIs? Are they close to areas of nonspecific
binding or ventricles? (e.g., is there any chance that a partial volume
effect is diluting your DVR values?)
Karmen Yoder, Ph.D.
Indiana University School of Medicine
P.S. The latter stuff is pretty off-topic for the list, it may be a
good idea to respond directly to me, so as not to bore the fMRI people
too much! :-) There's some really savvy PET people on the list, so
hopefully they'll provide some guidance.
>From: SPM (Statistical Parametric Mapping)
>[mailto:[log in to unmask]] On Behalf Of PET Newbie
>Sent: Thursday, September 06, 2007 8:15 PM
>To: [log in to unmask]
>Subject: [SPM] Calculating DVR's in PET
>Looking for some advice.
>I have a mood induction experiment with many subjects, 1 group, and 2
>In SPM I have created T-Contrasts to produce activation (condition A -
>Condition B), and have identified desired ROI's.
>Everything seems to have gone smoothly up until this point.
>Then, I try to extract the DVR's from the respective Subject's
>calculate BP with BP=DVR-1. This is where things look a bit
>get some individual subject negative values and if I average per
>condition, for some ROI's I get negative average BP's as well. I can't
>make any sense of this. I would expect that I should only
>BP's (at least positive averages) for regions that were found to be
>significant on the SPM analyses. Anyone have any suggestions.
>Thanks so much,