[gate-users] Re: gate-users Digest, Vol 10, Issue 26

[Long Zhang]

Dear Claude,

You are right. The difference is due to the geometrical cuts when GATE
build sinograms from coincidence events for ECAT system.

However, my suggestion is to directly remove those cuts. e.g. by using
/gate/output/sinogram/RadialBins N_crystals/ring-1
and do not use the max ring difference parameters when generating the
sinogram or set it to number of rings -1. Thus the sinogram will use
all events.

Another question is the two runs. The random numbers generation is
based on a seed file. So if this file is the same, the two runs will
be the same except one with TruesOnly flag on. So there is no
statistical problems.

Hope this helps.

Regards,

Long


> 
> I don't know if this is the explanation, but there is not only a
> geometrical cut in /gate/output/sinogram on the ring difference, but
> also on the radial coordinate of the LOR.
> 
> /gate/output/sinogram/RadialBins 256
> 
> If you use a root program and want to compare it to the sinogram output,
> you should also apply this radial cut (for more details, see user's
> guide, section data output / sinogram output).
> 
> Regards,
> 
> Claude
> 
> Qianmei Zhang wrote:
> 
> >
> > Hello Gater User,
> >
> > I met a puzzle about the random coincidence.
> >
> > In order to model the effect from random coincidnence for a Ecat 953
> > system (16 rings), i run two cases, one without and the other with the
> > flag:/gate/output/sinogram/TruesOnly , and i got two Ecat7 output files,
> > by the intergral of counts of the subtraction of the two file, the total
> > random number was obtained.  Since the maximum ring difference i set is
> > 13, the number should be a little bit of smaller than the actual random
> > coincidence number.
> >
> > I also modified the C program in the benchmark and by run it with the
> > root output file from the first case, i can obtain the number of  random
> > as well (if the two eventIDs in coincidence are different, they  belong
> > to random coincidnence )
> >
> > For  the same  decay of  the source, in a very short run, from the first
> > method, the number i got is 272, after corrected by the effect from the
> > ring difference, it will be 356.  and the number from the second method
> > is 500.  The difference is out of statistical uncertainty. I also run it
> > for a longer time, but it seems same.  By the way, for this case, the
> > deadtime is 3000ns and coincidence window is 12ns and the source
> > activity is 0.8mCi.
> >
> > Can anyone here explain it for me, thanks.
> >
> > Qianmei
> >
> >
> >
> >
> >
> >
> > _______________________________________________
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> >
> >
> 
> --
> Claude Comtat                                   comtat at ieee.org
> Image Acquisition and Processing group          tel: +33 1 69 86 78 01
> Frédéric Joliot Hospital Facility               fax: +33 1 69 86 77 49
> French Atomic Energy Commission                 www-dsv.cea.fr/shfj
>