It may or may not be something that needs to be fixed. The PVC is just a linear 
model where the regression coefficients are the uptake in each ROI. As with any 
model, it adjusts the regression coefs to minimize the error in the fit. If 
some end up being less than zero when you really expect all to be > 0, then 
this probably indicates an error in the (linear) model. This error could come 
from several sources. For example, the registration could be off. The PSF might 
not be right. Or the uptake might not be constant across the ROIs. You should 
definitely start by checking the registration. If the uptake in Nuc Acc is very 
small relative to other ROIs, then I would just leave it the way it is, ie, 
DON'T just set it to 0 when you go to a group analysis.

On 12/2/2019 4:48 PM, miracle ozzoude wrote:

        External Email - Use Caution

Hello Experts,

I ran the PETsurfer pipeline using AV45 pet and performed pvc using pons as 
reference point. When i looked at the gtm.stats.dat file for one of my 
subjects, the SUVr for right and left nucleus-accumbens areas were in the 
negative (left = -0.361; right = -0.046).

This is strange because SUVR shouldn't be negative. How do i go about fixing 
this error? Below, is my pvc command.
mri_gtmpvc --i pet.nii --reg register.lta --psf 6 --seg gtmseg.mgz 
--default--seg-merge --auto-mask PSF .01 --mgx .01 --o subject_gtmpvc --rescale 



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