[Gate-users] Energy spectra of coincidences is inconsistent
Patrick Hallen
patrick.hallen at rwth-aachen.de
Thu Feb 5 10:36:25 CET 2015
Hi,
I've attached the used plotting script. It's really a simple histogram
of the energy1 and energy2 branches in the coincidences tree. You get
the same histogram by opening the produced root file in TBrowser by
double-clicking on the branches.
So reproducing the plots is just running my supplied Gate macro and then
drawing the energy1 and energy2 branches of the resulting root file.
Kind regards,
Patrick Hallen
On 02/05/2015 09:58 AM, Pietrzyk, Uwe wrote:
>
> 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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