[Gate-users] Potential bug with short time slice / low activity

[Simon Stute]

Hi again,

Thank you Guillem for having tested further and reporting the results.
We will dig on this to find the problem and hopefully a solution.

And about your specific concern, why do you need to use so many time slices
?

Regards,
Simon


On Wed, Mar 12, 2014 at 5:36 PM, Guillem Pratx <pratx at stanford.edu> wrote:

> Hi Simon,
>
> I looked at the number of positron decays in root (total_nb_primaries) and
> again the number of observed positrons far exceeds what one would expect
> (~330 positrons observed for a 1Bq, 100s acquisition with 500 time slices).
> This result is verified for different random seeds. With 10,000 time
> slices, I even get >1200 positron decays when one would only expect only
> ~100.
>
> I do apply standard 18F decay to my e+ source (see below):
>
> /gate/source/addSource F18PointSource
> /gate/source/F18PointSource/setActivity 1 becquerel
> /gate/source/F18PointSource/gps/particle e+
> /gate/source/F18PointSource/setForcedUnstableFlag true
> /gate/source/F18PointSource/setForcedHalfLife 6586 s
> /gate/source/F18PointSource/gps/energytype Fluor18
> /gate/source/F18PointSource/gps/type Point
> /gate/source/F18PointSource/gps/angtype iso
> /gate/source/F18PointSource/gps/centre 0. 0. 0. cm
> /gate/source/F18PointSource/attachTo movpoint
> /gate/source/list
>
>
> And yes, I use a list of random seeds (simply 1, 2, 3, ... etc.).
>
> I think this behavior should be easy to reproduce with any PET simulation.
> This would at least show that it is not an isse specific to my macro.
>
> Thanks for your help,
>
> Guillem Pratx, PhD
> _______________________________________________
>
>    Assistant Professor Office A247
>    Radiation Oncology 1050 Arastradero Rd
>    Stanford University  Palo Alto, CA 94304
>    http://pratxlab.stanford.edu           (650) 724-9829
> _______________________________________________
>
> On Mar 12, 2014, at 3:13 AM, Simon Stute <gate.stute at gmail.com> wrote:
>
> Hi Guillem,
>
> I do not have the answer for that problem, but few suggestions.
>
> Try to use the command "/gate/application/verbose 2". This will print out
> the number of EMITTED particles from the source at the end of the
> acquisition. If this is not the expected one, one has to dig in this
> direction (in this case that may be a bug), otherwise the problem may be
> related to the detection process.
>
> As the problem disappears at bigger activities or fewer number of runs, I
> would suspect some kind of rounding error accumulations (if the problem
> comes from the number of emitted particles) ...
>
> Also, are you applying any decay to your source ?
> What kind of source are you using (backtoback, e+ or ion) ?
> Apologize for the silly question, but did you change the random seed from
> a replicate to another ?
>
> Regards,
> Simon
>
>
> On Tue, Mar 11, 2014 at 11:51 PM, Guillem Pratx <pratx at stanford.edu>wrote:
>
>> Dear GATE users and developers,
>>
>> I have noticed a strange behavior with PET simulations that use low
>> counts and short time slices. In my set-up, I am simulating a point source
>> inside the Inveon PET scanner. The activity of the point source is varied
>> from 1 to 1000 Bq,the total run time is 100 sec, the the time slice is 0.2
>> s (so 500 slices).
>>
>> For 1 Bq, one would expect ~ 7 counts on average but in fact I observed
>> 23 (average over 250 realizations). For higher activity, the error is
>> smaller but still statistically significant (see attached plot). The issue
>> disappears around 1000 Bq or if the number of time slices is reduced to ~
>> 10 (instead of 500). Note that if the activity of the source is 0 Bq, then
>> no counts are observed, so this is not a background issue.
>>
>> So my question is is there an explanation for this behavior, or could
>> this be a bug in how counts are generated within each time frame?
>>
>> Thank you for your help in solving this issue.
>>
>>  Guillem Pratx, PhD
>> _______________________________________________
>>
>>    Assistant Professor Office A247
>>    Radiation Oncology 1050 Arastradero Rd
>>    Stanford University  Palo Alto, CA 94304
>>    http://pratxlab.stanford.edu           (650) 724-9829
>> _______________________________________________
>>
>> <time_slice_bug.png>
>>
>> _______________________________________________
>> Gate-users mailing list
>> Gate-users at lists.opengatecollaboration.org
>> http://lists.opengatecollaboration.org/mailman/listinfo/gate-users
>>
>
>
>
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