[gate-users] WARNING message in PET simulation

[Claude Comtat]

Please see the following mail exchange.

Claude

Mar Vla wrote:

> Hi everyone,
> 
> I am running a PET simulation and during the simulation I get this 
> warning message several times....
> 
>  
> 
> [GateTrajectoryNavigator::FindAnnihilationGammasTrackID] : WARNING : 
> positron found and photonID vector size : 1
> 
>  
> 
> Nothing seems to be wrong, the simulation code ends, the output seems to 
> be OK...
> 



*****************************************************************************


> Hi Theo and Jin
> I can confirm the remarks by Jin
> 
> You get these (warning) messages when the positron has not annihilated 
> before reaching the World boundary.
> This happens often when the source is in Air, as Jin suggested.
> But having the source in Air is in most cases totally unrealistic, as it 
> is normally contained in some liquid or solid. 
> 
> I would suggest to define the source geometry (volumes and materials) 
> correctly (for both liquid and container) and to position the source 
> accurately in it.
> 
> Due to the relatively low positron energy, most of the positrons will be 
> stopped either in the liquid or in the (glass/plastic/...) container. 
> Some can escape, so a few warnings are OK. in any case, the results will 
> be realistic, as in the real experiment some positrons will escape as 
> well.
> 
> Best regards,
> -- Giovanni ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ 
>> Hi Theo:
>> 
>> Yes, I think to have electron range cuts would help but I doubt if the 
>> simulation results would be very different. As I guess the warning means 
>> that the positron is out of the camera's field of view and thus it's 
>> annihilation photons would not be counted as coincidence events.  But 
>> they do contribute to Random and Scatter counts though. Pls correct me 
>> if I am wrong.
>> My suggestion is that a few of these warnings would be OK, if you didn't 
>> get tons of it, which reduces your actual activity dose.
>> Good luck and I want to hear more about this as well.
>> 
>> Jin
>> 
>> Mutanga Theodore wrote:
>> 
> 
>> >Hi Jin,
>> >I did as you suggested and I still had some warnings, maybe I have to set
>> >some electron range cuts instead ?
>> >What do you think ?
>> >Has anyone been doing measurements in air ?
>> >Theo
>> >
>> >
>> >
>> > > >
> 
>> >>That is because of the much longer positron range in air, which is about
>> >>170 cm for F18. But the average range in water is only 0.06 cm (F18). If
>> >>you can use a small water sphere to hold your source, it may help to
>> >>stop the warning message.
>> >>
>> >>Jin
>> >>
>> >>Mutanga Theodore wrote:
>> >>
>> >> > >>
> 
>> >>>Hello Gaters,
>> >>>
>> >>>I had a setup for simulating and measuring the resolution of the
>> >>>scanner,since the measurements had to be done in Air, I defined a phantom
>> >>>which is a cylinder the same size as the inner diamter of the scanner.
>> >>>
>> >>>When I set the material as Air I have this warning messages
>> >>>
>> >>>:[GateTrajectoryNavigator::FindAnnihilationGammasTrackID]  WARNING :
>> >>>positron found and photonID vector size: 1
>> >>>
>> >>>I don't really know what this means but when I use Water as the medium in
>> >>>the phantom I don't get this error , how does it affect my simulation ?
>> >>>
>> >>>Cheers
>> >>>
>> >>>Theodore
>> >>>
>> >>>_______________________________________________