[Gate-users] problematic behavior of deadtime in PET detector

[Albert Grace Lieu]

Dear GATE users,

Has anyone experienced a similar issue with the deadtime module in the PET
detector? Could you please generously share your thoughts or give any help?

Greatly appreciated.

Zhengzhi

On Fri, May 15, 2020 at 9:05 AM Albert Grace Lieu <albertnew2018 at gmail.com>
wrote:

> *As my previous email account expired, please reply to this one if you
> can. Thank you very much. *
>
> Honored Gate experts and developers,
>
> I am experiencing some issues with deadtime in the PET detector when
> validating my simulated PET system in GATE against my real PET system. My
> PET system is similar to a typical standard ring PET system. I have built a
> corresponding GATE model to it as close as I can. But the count rate in the
> GATE model is twice higher than the physical system. I think it is due to
> the weird behavior of deadtime in digitizer settings. Then I did some
> simple validation simulations to prove that. In my real system, 11.3 mci
> (equivalent to 19 kBq/ml) is high enough to paralyze the system, i.e., the
> collected coincidence rate would increase then decrease as decays go by.
> However, in the GTE model, I have not observed such a trend yet. Here is a
> simple example:
>
> [image: Screenshot from 2020-05-14 13-43-13.png]
> When I doubled the initial dose from 19 kBq/ml (11.3 mci) to 38 kBq/ml
> (22.6 mci), Gate also doubled the count of singles. To me, the deadtime
> module is not working. Because 19 kBq/ml is high enough to paralyze the
> detector, when the dose is doubled, it shall output ways less than twice of
> the singles.
>
> Another simulation I did to show GATE mode reacts to deadtime is varying
> the deadtime:
>
> [image: Screenshot from 2020-05-14 14-10-41.png]
> As you can see from the results above, the counts of singles barely change
> when deadtime increases from 10 ns to 100000 ns, which is very weird
> behavior to me. Also, 19 kBq/ml (11.3 mci) shall be high enough to paralyze
> the system.
>
> Please provide any clue or any help to correct it. Thank you very much.
> Sincerely,
> Zhengzhi
>
>
>
> Following is my digitizer settings:
> #=====================================================
> #   D I G I T I Z E R: DETECTOR ELECTRONIC RESPONSE
> #=====================================================
> #      A D D E R
> /gate/digitizer/Singles/insert                        adder
>
> #       R E A D O U T
> /gate/digitizer/Singles/insert                        readout
> #/gate/digitizer/Singles/readout/setPolicy             TakeEnergyCentroid
> #TakeEnergyWinner
> /gate/digitizer/Singles/readout/setDepth              5
>
> #      E N E R G Y   B L U R R I N G
> /gate/digitizer/Singles/insert                        blurring
> /gate/digitizer/Singles/blurring/setResolution        0.15
> /gate/digitizer/Singles/blurring/setEnergyOfReference 511. keV
>
> #       E N E R G Y   C U T
> /gate/digitizer/Singles/insert                        thresholder
> /gate/digitizer/Singles/thresholder/setThreshold      395. keV
> /gate/digitizer/Singles/insert                        upholder
> /gate/digitizer/Singles/upholder/setUphold            600. keV
>
> /gate/digitizer/Singles/insert timeResolution
> /gate/digitizer/Singles/timeResolution/setTimeResolution 0.700 ns
>
> #       D E A D    T I M E
> /gate/digitizer/Singles/insert deadtime
> /gate/digitizer/Singles/deadtime/setDeadTime 1000. ns
> /gate/digitizer/Singles/deadtime/setMode paralysable
> /gate/digitizer/Singles/deadtime/chooseDTVolume Crystal
>
>
> #=====================================================
> # C O I N C I D E N C E    S O R T E R
> #=====================================================
> /gate/digitizer/Coincidences/setWindow                5. ns
> /gate/digitizer/Coincidences/minSectorDifference      1
> /gate/digitizer/Coincidences/setOffset                0. ns
> #/gate/digitizer/Coincidences/setMultiplePolicy        killAll
> /gate/digitizer/Coincidences/describe
>
> /gate/digitizer/name                                  delayedCoinc
> /gate/digitizer/insert                                coincidenceSorter
> /gate/digitizer/delayedCoinc/setOffset                90. ns
> /gate/digitizer/delayedCoinc/setWindow                5. ns
> /gate/digitizer/delayedCoinc/minSectorDifference      1
> /gate/digitizer/delayedCoinc/describe
>
> /gate/digitizer/name                                  finalCoinc
> /gate/digitizer/insert                                coincidenceChain
> /gate/digitizer/finalCoinc/addInputName               delayedCoinc
> /gate/digitizer/finalCoinc/addInputName               Coincidences
> /gate/digitizer/finalCoinc/usePriority                true
> /gate/digitizer/finalCoinc/insert                     deadtime
> /gate/digitizer/finalCoinc/deadtime/setDeadTime       10. ns
> /gate/digitizer/finalCoinc/deadtime/setMode           paralysable
> /gate/digitizer/finalCoinc/describe
>
>
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