[Gate-users] Energy spectra of coincidences is inconsistent

[Paola Solevi]

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> 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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