# Calculating acquisition time of PET

Hello Everyone
I am trying to calculate sensitivity of a PET scanner, which is defined as number of true coincidences per second per unit of activity. So for that I need to calculate the acquisition time for which my PET scanner has ran. I can calculate the time elapsed during the completion of simulation using clock() function. Is this elapsed time same as acquisition time of PET scanner ? I am asking this because my values for sensitivity aren’t matching with the published results. Kindly help me out ?

No. That is telling you the elapsed time on the clock in your computer, or hanging on the wall of your office, in other words, how long did you have to wait for your simulation job to finish.

Geant4 runs jobs based on number of events, not based on time. If you know the true activity of the source you’re simulating (for example, a 5 microcurie capsule of Tc-99m will generate 185,000 events in one second of real time. So if you ran your simulation with a Tc-99m source, then `/run/beamOn 185000` that would correspond to one second of a 5 uCi source, or one minute of an 83 nCi source, and so on.

TL;DR: You need to convert your number of events into acquisition time based on the source you are simulating.

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Dear @mkelsey, you are correct. I modified my code accordingly now the results are matching with the published one. Thank you so very much for your help. This was a big relief !

Dear @mkelsey, in the same context I have one more doubt. I was calculating NECR (Noise Equivalent Count Rate), in the article activity was given in kBq/mL in this case. For simulating it, I needed the total activity so I multiplied it by the volume of source. But on doing the analysis, my result is not matching.
I think this time also my acquisition time is wrong due to wrong estimation of activity. How should I estimate the total activity in the scanner ?
Kindly help

Not sure. You may need to consult the literature to be clear on how specific activity is defined or computed for real-world sources. What you did (specific activity x total volume) is what I would do.

Aren’t PET scans done by introducing a source (via injection or implantation) into a larger person/animal/phantom? In that case, the volume of interest would be “how much was in the syringe”.

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