The GTM is just a general linear model, so there are no constraints on sign. The GTM makes some assumptions about the nature and magnitude of the PVEs (linear, gaussian, with the FWHM that you supply). Violations of these assumptions can cause negative values. There are a lot of other things that can cause it too, such as errors in various corrections (scatter, atten, randoms, etc). Are they all showing up neg or just this one subject?

Dear Freesurfer experts,

We found that a stat file in gtmpvc output folder shows that one area has negative avg uptake value. What could explain this result?

Quoted the row from the file: 

 11   26 Left-Accumbens-area             subcort_gm         3    2.351       -0.490       2.3033

The command used is as in the log file.

Best regards,

Jak

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