For the first question is this something like what you are building @RafaelPalomar ?
Regarding your second question, that’s a mode I also want to implement for a PET project I’m involved with. Probably it can just be a simple segment editor effect that would be used as follows: use the Paint (or any tool) to draw anything that is possibly lesion and then the effect would calculate the max value of the background in that segment and then only keep the places where the background are 41% of that max. I’m not sure when I get around to it, but I will probably add it to the PETIndiC extension. As part of that I’ll also try to fix up some of the PET-related extensions that aren’t building with the current preview version of Slicer.