[Gate-users] Energy spectra of coincidences is inconsistent

[Pietrzyk, Uwe]

Hi Patrick,

how did you generate the plot with the two energy histograms?
Did you use a root-macro and could you send it to me/us to check,
whether I can reproduce it. It would save some time to possibly answer
your findings.

Thanks and kind regards,

Uwe Pietrzyk
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On 04 Feb 2015, at 10:17, Patrick Hallen <patrick.hallen at rwth-aachen.de<mailto:patrick.hallen at rwth-aachen.de>> wrote:

Hi Paola,

unfortunately it isn't caused by missing time blurring. I've had removed
the time blurring from the attached example, to keep it as minimal as
possible. It has always been included in our "real" PET simulation,
which shows the same behavior.
When I add time blurring to the attached minimal example, the two energy
spectra stay the same.

Best
Patrick

On 02/03/2015 05:23 PM, Paola Solevi wrote:
Dear Patrick,
could it be due to the time stamp?
I see you don't apply any time blurring to your Singles so you preserve
in your Singles list the ordering coming from the tracking of the photons.
Try to apply the time blurring to your Singles to check if the effect
smooths out.
Hope it helps,
p.



On Tue, Feb 3, 2015 at 5:11 PM, Patrick Hallen
<patrick.hallen at rwth-aachen.de<mailto:patrick.hallen at rwth-aachen.de> <mailto:patrick.hallen at rwth-aachen.de>>
wrote:

   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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