[Gate-users] GATE coincidences detected with source outside the FOV of scanner

[Peter Neilson]

Hello All, 

  I’m working with GATE to benchmark some sensitivity calculations of a new type of PET scanner against some previous designs. However I’ve run into a bit of an odd phenomenon. When my source is located outside the FOV of the scanner, I am still detecting some coincidences, leading me to have a sensitivity around 1.5% and higher when it should be zero. I’m attempting to implement the NEMA NU4 procedure with a 18F positron source, as shown below. 

/gate/source/addSource F18SphereSource
# Have been changing activity levels to look at convergence in sensitivity calculation
/gate/source/F18SphereSource/setActivity 100000. becquerel
/gate/source/F18SphereSource/gps/particle e+
/gate/source/F18SphereSource/setForcedUnstableFlag true
/gate/source/F18SphereSource/setForcedHalfLife 6586 s
/gate/source/F18SphereSource/setType backtoback
# Fluorine specific energy type
/gate/source/F18SphereSource/gps/energytype Fluor18

/gate/source/F18SphereSource/gps/type Surface
/gate/source/F18SphereSource/gps/shape Sphere
/gate/source/F18SphereSource/gps/radius 0.15 mm
/gate/source/F18SphereSource/gps/centre 0.0 0.0 {axialOffset} cm
/gate/source/F18SphereSource/gps/angtype iso

Where axialOffset would be outside the PET scanner.

I’ve found altering things like my minSectorDifference and multiple coincidence policy to alter the number of coincidences, but nothing can remove it completely, My digitizer shown below 

#       A D D E R
# Add compton scattering
/gate/digitizer/Singles/insert adderCompton
/gate/digitizer/Singles/insert adder
#       R E A D O U T
/gate/digitizer/Singles/insert readout
# Energy winner is necssary for when using depth 2
/gate/digitizer/Singles/readout/setPolicy TakeEnergyWinner
# This will readout at the block level
/gate/digitizer/Singles/readout/setDepth 2

#      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
# Crystals are characterized by manufacturer and organized
#       E N E R G Y   C U T 
/gate/digitizer/Singles/insert thresholder
/gate/digitizer/Singles/thresholder/setThreshold 450. keV
/gate/digitizer/Singles/insert upholder
/gate/digitizer/Singles/upholder/setUphold 650. keV
#       D E A D    T I M E
/gate/digitizer/Singles/insert deadtime
/gate/digitizer/Singles/deadtime/setDeadTime 320 ns
/gate/digitizer/Singles/deadtime/setMode paralysable
/gate/digitizer/Singles/deadtime/chooseDTVolume crystal

#       C O I N C I    S O R T E R
/gate/digitizer/Coincidences/setInputName Singles
/gate/digitizer/Coincidences/minSectorDifference 3
/gate/digitizer/Coincidences/setWindow 4.73 ns
/gate/digitizer/Coincidences/setOffset 0. ns
/gate/digitizer/Coincidences/MultiplesPolicy takeAllGoods


For these calculations in GATE, is there some type of correction factor or other specification that I am not thinking of?

Thanks,
-Peter Neilson
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