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.

  • $\begingroup$ It might be helpful to go read the Wikipedia page and then edit your question for details not provided there. $\endgroup$ Commented Mar 7, 2018 at 16:18
  • $\begingroup$ It is positron, not photon. Actually a pretty clever trick. They put in an "activated" fluorine and it decomposes to oxygen. The half-life is only about 18 hours. Unfortunately I have had a couple PET scans. $\endgroup$ Commented Mar 8, 2018 at 16:54
  • $\begingroup$ Wiki says the half life of the Fl is 110 minutes. $\endgroup$ Commented Mar 8, 2018 at 20:06

1 Answer 1


I happen to be a cancer patient and have had the PET-CT.

The way they explained it to me is after giving you a rapidly decaying radioactive sugar with a half-life of about one hour which has one carbon switched with fluoride.

Then the hungry cancer cells break the sugar, positrons are released which in a deadly collision with electrons release two gamma rays which are colinear and in opposite direction.

The machine is counting only these colinear gamma rays and will trace them back to their origin's x,y,z. This point will be shown as a red dot superposed on a regular ct of patients body that is used as a reference to his/her anatomy.

Problem is because of ethnicity and nutrition and diet some false positive shadows may show up.

  • $\begingroup$ Thank you very much for your answer, I think it is a good summary of what is going on during the PET measurement. $\endgroup$
    – LynxLynx
    Commented Mar 8, 2018 at 15:39

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