SPECT quantification issue

Hi Slicer Users,

I desperately need some help here. I would like to quantify a few SPECT images and wanted to calculate the calibration factor of a the protocol/camera by looking at a phantom scan.

When I place a few segments in the phantom (outside the hotter spheres) I get values of around 6000-6700 counts. In other programs like VivoQuant or Hermes Affinity, the results are more like 3500 counts.

Does anybody know why there is this discrepancy?

Thank you so much,
Jan