[Gate-users] Energy spectra of coincidences is inconsistent

[Patrick Hallen]

Hello,

I've discovered a weird inconsistency in the energy spectra of the two
photons of the coincidences. Attached you can find the histograms of
Coincidences/energy1 and Coincidences/energy2, i.e. the energy spectra
of the two 511 keV photons of a coincidence. I would expect, that the
two spectra would look the same (except for statistic fluctuations), but
you can clearly see, that the spectrum of the second photon has distinct
statistically significant peaks in the compton region, which are lacking
in the spectrum of the first photon.

I've observed this feature first with the simulation of our cylindrical
PET scanner, which lead me to create a minimal example to reproduce this
(see attached minimal.mac). The minimal example consists of just two
opposing blocks of LYSO scintillator and a mono-energetic 511 keV
back-to-back photon source. The attached spectrum is the result of this
simulation, but the spectra for our more complex cylindrical PET
simulation looks similar.

I've tried changing different things in the simulation, like the
radiation source (F18, C11) or the detector material (Water), with the
same result: The spectra of the two photons are significantly different
from each other. I could also reproduce this with the provided example
PET_CylindricalPET_System.mac, after I reduced the energy threshold to
include the compton region.

I am observing this with GATE 7.0 and GEANT4 9.6.3, both compiled from
source and using vGATE 3.0.

There seems to be some weird bias in the selection which of the two
photons is the first and which the second photon. By looking at the GATE
code, I couldn't find any obvious source of this. Usually one would
expect that the energy deposition of the two photons are totally
uncorrelated, which would result in the same energy spectra (except for
statistical fluctuations).

I suspect a bug, which might influence important PET performance
parameters such as the sensitivity, which depends on the energy
threshold. For a preclinical PET scanner one usually choses a small
energy threshold to increase the sensitivity, since the compton scatter
most often takes place in the detector and not in the object. When the
energy in the compton region would be wrong, this could potentially
result in a wrong simulated sensitivity.

Kind regards,
Patrick Hallen
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/vis/disable
#/control/execute visu.mac

/gate/geometry/setMaterialDatabase ./GateMaterials.db

/gate/world/geometry/setXLength 10. cm
/gate/world/geometry/setYLength 10. cm
/gate/world/geometry/setZLength 10. cm
/gate/world/setMaterial Air
/gate/world/vis/forceWireframe

/gate/world/daughters/name cylindricalPET
/gate/world/daughters/insert cylinder
/gate/cylindricalPET/placement/setTranslation 0.0 0.0 0.0 cm
/gate/cylindricalPET/setMaterial Air
/gate/cylindricalPET/geometry/setRmax 3 cm
/gate/cylindricalPET/geometry/setRmin 1 cm 
/gate/cylindricalPET/geometry/setHeight 4 cm
/gate/cylindricalPET/vis/forceWireframe
/gate/cylindricalPET/vis/setColor yellow

/gate/cylindricalPET/daughters/name scintillator
/gate/cylindricalPET/daughters/insert box
/gate/scintillator/placement/setTranslation 20 0 0 mm
/gate/scintillator/geometry/setXLength 10 mm
/gate/scintillator/geometry/setYLength 30 mm
/gate/scintillator/geometry/setZLength 30 mm
/gate/scintillator/setMaterial LYSO
/gate/scintillator/vis/setColor blue
/gate/scintillator/vis/forceWireframe

/gate/scintillator/repeaters/insert ring
/gate/scintillator/ring/setRepeatNumber 2

/gate/geometry/rebuild

/gate/systems/cylindricalPET/rsector/attach scintillator
/gate/scintillator/attachCrystalSD

# check for partially overlapping volumes
/geometry/test/run

#	D I G I T I Z E R 
/gate/digitizer/Singles/insert adder
/gate/digitizer/Singles/insert readout

#/gate/digitizer/Singles/insert blurring
#/gate/digitizer/Singles/blurring/setResolution 0.02
#/gate/digitizer/Singles/blurring/setEnergyOfReference 511. keV 

#	COINCIDENCES SORTER 
/gate/digitizer/Coincidences/setWindow 2. ns
/gate/digitizer/Coincidences/minSectorDifference 1

# PHYSICS
# manual physics lists are deprecated now, but is this pre-defined list a good
# choice for PET?
/gate/physics/addPhysicsList emstandard_opt3

/gate/run/initialize

/gate/source/addSource twogamma
/gate/source/twogamma/setActivity 10 kBq
/gate/source/twogamma/setType backtoback
/gate/source/twogamma/gps/particle gamma
/gate/source/twogamma/gps/energytype Mono
/gate/source/twogamma/gps/monoenergy 0.511 MeV
/gate/source/twogamma/gps/centre 0 0 0 mm
/gate/source/twogamma/gps/angtype iso 

/gate/random/setEngineSeed auto

#	O U T P U T
/gate/output/root/enable
/gate/output/root/setFileName minimal
/gate/output/root/setRootNtupleFlag 0
/gate/output/root/setRootHitFlag 0
/gate/output/root/setRootSinglesFlag 1
/gate/output/root/setRootCoincidencesFlag 1
/gate/output/root/setSaveRndmFlag 0

#	S T A R T 
/gate/application/setTimeSlice     1000 s
/gate/application/setTimeStart     0. s
/gate/application/setTimeStop      1000 s
/gate/application/startDAQ
exit