[Gate-users] ideal PET system

[Matthew Lewis]

The problem is that the digitizer ultimately looks at the photoelectron
track for the energy deposition. this will always be offset  from the true
photoelectric absorption position and is an additional source of variance
(which frankly I don't remember ever seeing mentioned in books or papers -
granted I am simulating worst case scenario which is coincidences between 2
PE events, which is pretty rare in PET system).

so, to get the LORs to go exactly through the source position, one needs to
cut the electron track so that it is essentially zero.....
this did the trick:
/gate/physics/SetMaxTrackLengthInRegion crystal 0.001 mm
/gate/physics/ActivateSpecialCuts e-

i vaguely remember this being discussed a few years back......

mlewis



On Tue, Dec 10, 2013 at 6:44 PM, Marc Chamberland <
MarcChamberland at cmail.carleton.ca> wrote:

>  Hi Matthew,
>
>  Have you tried setting the digitizer readout depth to the crystal level?
> I'm not really if it will help, but it's worth a try.
>
>   Marc
>
>
>
>  __________________________
>
>  Marc Chamberland, MSc
> PhD candidate
> Department of Physics
> Carleton University
> Ottawa (ON)
>
>  On Dec 10, 2013, at 17:55, Matthew Lewis <matthew.lewis at gmail.com> wrote:
>
>
>
>  is there anyway to model an ideal PET system where the (x,y,z)
> coordinates in the detector crystals have no variance? that is, each
> coincidence event contains two points where the connecting line passes
> through the source up to round-off error
>
>  i tried to do this with the following:
>  # setup simple physics - turn off scattering - this is horribly
> inefficient since it now requires 2 simultaneous PE at 511 keV to generate
> coincidence, but i get about 5000 in 1M primaries
> /gate/physics/addProcess PhotoElectric
> /gate/physics/processes/PhotoElectric/setModel StandardModel
> #/gate/physics/addProcess Compton
>  #/gate/physics/processes/Compton/setModel StandardModel
> #/gate/physics/addProcess RayleighScattering
>  #/gate/physics/processes/RayleighScattering/setModel PenelopeModel
> /gate/physics/addProcess ElectronIonisation
>  /gate/physics/processes/ElectronIonisation/setModel StandardModel e-
> /gate/physics/processes/ElectronIonisation/setModel StandardModel e+
>  /gate/physics/processList Enabled
> /gate/physics/processList Initialized
>
>  #disable adder and blurring
>  #/gate/digitizer/Singles/insert adder
> /gate/digitizer/Singles/insert readout
> /gate/digitizer/Singles/readout/setDepth 1
> #/gate/digitizer/Singles/insert blurring
>  #/gate/digitizer/Singles/blurring/setResolution 0.01
> #/gate/digitizer/Singles/blurring/setEnergyOfReference 511. keV
> /gate/digitizer/Singles/insert thresholder
>  /gate/digitizer/Singles/thresholder/setThreshold 350. keV
> /gate/digitizer/Singles/insert upholder
> /gate/digitizer/Singles/upholder/setUphold 650. keV
> /gate/digitizer/Coincidences/setWindow 10. ns
>  /gate/digitizer/Coincidences/minSectorDifference 1
> /gate/digitizer/name delay
>  /gate/digitizer/insert coincidenceSorter
> /gate/digitizer/delay/setWindow 10. ns
> /gate/digitizer/delay/setOffset 500. ns
>
>  source is backtoback gamma 0.511 MeV at 0 0 0
>
>  If I look at the coincidence events (globalPosX1, etc.), I was hoping
> the LORs would all go through the origin.
> the mean of all the intercepts is close to 0, but there is a residual 0.2
> mm PSF.
> Can anyone tell me where this is coming from? something in the digitizer?
>
>  mlewis
>
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