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

[Simon Stute]

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
> _______________________________________________
>
>
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