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

[Zhengzhi Liu]

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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