[gate-users] small bug about coincidence sorter

[David Guez]

yuxuan.zhang at di.mdacc.tmc.edu wrote:

>
> hi,
>
> Recently I've written another coincidence sorting program from the 
> ASCII output
> of the singles.dat file because I need
> get some other information that GATE coincidence sorter can not provide.
> After I finished the program, I compared the results from my program 
> and the
> results from GATE, then I noticed thare were some differences between 
> them.
> Especially under the high activity condition (large amount of random 
> coincidence).
> For example, with one model, using GATE output of the coincidences are:
>
>  Total  True   Random   Scatter   NECR
>  21761  4014    15056      2691    740
>
> the output from my program:
>
>  Total  Ture    Random   Scatter  NECR
>  22600  4011     15833      2756   712
>
> Though the differences of the NECR is only 3%~4%, but still I wanted 
> to find out the causes.
>
> After I compared each event that was different in these two outputs 
> and checked the original
> singles data file, I found out in which condition this will happen.
>
> Usually there are two kinds of situation:
>
> example 1, there are some enevts in the single events list:
>
> ID_of_enevt        Time_Stamp        
> 8243                4.42524487908583266e-05
> 8249                4.42824325537103663e-05
> 8250                4.42823796782774862e-05
> 8257                4.43126886297442500e-05
>
> in this case, if we set the timewindow as 10 ns, event 8249 and 8250 
> should be a coincidence,
> but from the GATE output, there's no this coincidence.
>
> example 2, for these events:
>
> 10593                5.66959683503792577e-05  
> 10595                5.66958859230306874e-05  
> 10595                5.66959047255570771e-05  
> 10610                5.67960969563888841e-05  
>
> there are three events 10593, 10595 and 10595  in the timewindow, 
> according the sorting criterion,
> if there are more 2 events within the timewindow, all these events 
> should be rejected. However,
> GATE program give a coincidence for 10595 and 10595.
>
> So we can see that in both cases, the sequence of the events appears 
> in the singles data file is not
> exactly according to the time that the events was detected 
> (time_stamp). Sometimes, a single event with
> a larger time_stamp (later one) can be in front of another event with 
> a smaller time_stamp
> (early one). It seems that when this happans, the GATE sorting program 
> just ignores the first
> events.
>
> Hope this bug can be fixed in the next generation of GATE. And then 
> GATE will provide more accurate results.
>
>
>
> Yuxuan
>
>------------------------------------------------------------------------
>
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>  
>
Hello,
I think that this problem is due to the event per event treatment of 
GEANT4. For very high activities, the time of flight of the photons may 
be lower than the duration between two consecutive events. In the case 
where the photon created during the first event is more scattered, or 
was created near to the center of the FOV, while the photon of the 
second event is near from the detector, an has no scattering, the 
detection order may differ from the photon creation order. But due to 
the event per event treatment of GEANT4, the data are treated following 
the photon creation order.
For a PET diameter L, this effect should start to be significant for 
activities ~ c/L (600 MBq, for L=50 cm). The scattering effect can 
reduce this limit.
So, this bug, as far as I understand it, may be very difficult to 
completely fix. However, in the next release of GATE, some efforts were 
made to minimize this kind of effect, but in a non systematic way. It 
would be very interesting to repeat your test with this version, when it 
will be available.

Thanks for your comment,

    David

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**  David GUEZ                                                 **
**  ^^^^^^^^^^                                                 **
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