What I have learned
The detectors of the PET actually measure the photons, which are caused by the radioactive tracer injected into the patient. Then, with the help of backprojection, we get the photon count for each voxel. I hope I understood these processes right, please correct me if this is not the case.
But as final result, we get the radioactivity concentration in kBq/mL.
How can the scanner convert the counted photons into radioactivity concentration?
My guess would be, that a phantom of known volume is used, with an inserted radiotracer of known radioactivity concentration.