[gate-users] Time Resolution and More

[David Guez]

Mikhail Shilov wrote:

>Greetings All,
>
>I'm trying to understand some confusing results I get from running Gate
>simulation for ecat PET scanner.
>First of all I've noticed that when I do:
>
>/gate/digitizer/Singles/deadtime/setDeadTime  # ns
>
>and then do:
>
>/gate/digitizer/Singles/deadtime/describe
>
>It prints out time which is 1000. times the time I specified in the
>first command. Has anyone noticed that before?
>Though, it looks like it's using the specified value in the simulation
>but I'm not sure.
>  
>
Actually, the dead time module transforms the specified dead time value
into a integer number of picoseconds (may be to avoid floating point
comparisons problems, but I'm not sure...). Anyway, the describe
method should make the inverse translation, and this feature
should be easily implemented, by replacing the method

void GateDeadTime::DescribeMyself(size_t indent)
{
  G4cout << GateTools::Indent(indent) << "DeadTime: " << 
G4BestUnit(m_deadTime,"T
ime") << G4endl;
}


by the following
void GateDeadTime::DescribeMyself(size_t indent)
{
  G4cout << GateTools::Indent(indent) << "DeadTime: " << 
G4BestUnit(m_deadTime*picosecond,"T
ime") << G4endl;
}


in the src/GateDeadTime.cc file.

>Also, when I look at the energy spectrum of the recorded singles it
>extends well beyond 511 keV to up to 2 keV.
>The only explanation I find is that the digitizer module integrates the
>hits in the detector over some time windows and adds up their energy. I
>couldn't find any information on the time resolution or simulation of
>the 'pile up' in the crystals in the manual. 
>Is 'timeResolution' module responsible for this ? 
>Why does it add up the hits if I didn't insert this module in the
>digitized chain and what is the default time resolution?
>Thanks
>  
>

Do you mean that you're not using any energy resolution ?
The depth of your readout module (is it exists), which fix the
hierachical level into which the hits INSIDE a given event are summed up
should not be too large. In the extreme case, a readout level set to
the base depth wil lead to adding hits of the whole detector,
and thus, to a mean event energy equal to 2*511 keV corresponding to
the two 511 keV photons of the e+ desintegration.
Oterwise I don't really have a response to you r problem,
but your point about the pile up feature motivate my response... are you
looking for such a module ? It may be not so hard to implement and I
may do it if demands exists and tests are possible.

    By

>Mikhail A. Shilov
>The Johns Hopkins University
>Department of Radiology
>Division of Medical Imaging Physics
>Tel:  443-287-7315
>_______________________________________________
>gate-users mailing list
>gate-users at lphe1pet1.epfl.ch
>http://lphe1pet1.epfl.ch/mailman/listinfo/gate-users
>  
>


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**  David GUEZ                                                 **
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