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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.


Question

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.

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  • $\begingroup$ It might be helpful to go read the Wikipedia page and then edit your question for details not provided there. $\endgroup$ – Carl Witthoft Mar 7 '18 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. $\endgroup$ – blacksmith37 Mar 8 '18 at 16:54
  • $\begingroup$ Wiki says the half life of the Fl is 110 minutes. $\endgroup$ – blacksmith37 Mar 8 '18 at 20:06
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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.

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  • $\begingroup$ 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 Mar 8 '18 at 15:39
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From the Becquerel Wiki page, Becquerel is defined as the "activity" per second of a radioactive material.

A photon is an elementary particle of light and light visible to humans is a form of the electromagnetic radiation, as is infrared radiation, ultraviolet radiation and x-rays etc.

So when Becquerel is in conjunction with light (photons) it is giving a photon count - how many photons were detected. Likewise, a reading of kBq/mL by a PET scanner, gives many thousands of photos are emitted per mL by the item being scanned.

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  • $\begingroup$ Thank you for showing me the Wikipedia link, obviously I totally forgot to look the Becquerel definition myself up - thank you very much, this is helping me! $\endgroup$ – LynxLynx Mar 8 '18 at 15:40

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