<div dir="ltr">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).<div>
<br></div><div style>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.....</div><div style>this did the trick:</div><div style><div>/gate/physics/SetMaxTrackLengthInRegion crystal 0.001 mm</div>
<div>/gate/physics/ActivateSpecialCuts e-</div><div><br></div><div style>i vaguely remember this being discussed a few years back......</div><div style><br></div><div style>mlewis</div><div style><br></div></div></div><div class="gmail_extra">
<br><br><div class="gmail_quote">On Tue, Dec 10, 2013 at 6:44 PM, Marc Chamberland <span dir="ltr"><<a href="mailto:MarcChamberland@cmail.carleton.ca" target="_blank">MarcChamberland@cmail.carleton.ca</a>></span> wrote:<br>
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Hi Matthew,
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<div>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.</div>
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<div>Marc</div>
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<div>__________________________</div>
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<div>Marc Chamberland, MSc</div>
PhD candidate<br>
Department of Physics<br>
Carleton University<br>
Ottawa (ON)</div>
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<div>On Dec 10, 2013, at 17:55, Matthew Lewis <<a href="mailto:matthew.lewis@gmail.com" target="_blank">matthew.lewis@gmail.com</a>> wrote:</div>
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<div>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</div>
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<div>i tried to do this with the following:</div>
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<div># 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</div>
<div>/gate/physics/addProcess PhotoElectric</div>
<div>/gate/physics/processes/PhotoElectric/setModel StandardModel</div>
<div>#/gate/physics/addProcess Compton<br>
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<div>#/gate/physics/processes/Compton/setModel StandardModel</div>
<div>#/gate/physics/addProcess RayleighScattering<br>
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<div>#/gate/physics/processes/RayleighScattering/setModel PenelopeModel</div>
<div>/gate/physics/addProcess ElectronIonisation<br>
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<div>/gate/physics/processes/ElectronIonisation/setModel StandardModel e-</div>
<div>/gate/physics/processes/ElectronIonisation/setModel StandardModel e+</div>
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<div>/gate/physics/processList Enabled</div>
<div>/gate/physics/processList Initialized</div>
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<div>#disable adder and blurring</div>
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<div>#/gate/digitizer/Singles/insert adder</div>
<div>/gate/digitizer/Singles/insert readout</div>
<div>/gate/digitizer/Singles/readout/setDepth 1</div>
<div>#/gate/digitizer/Singles/insert blurring<br>
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<div>#/gate/digitizer/Singles/blurring/setResolution 0.01</div>
<div>#/gate/digitizer/Singles/blurring/setEnergyOfReference 511. keV</div>
<div>/gate/digitizer/Singles/insert thresholder<br>
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<div>/gate/digitizer/Singles/thresholder/setThreshold 350. keV</div>
<div>/gate/digitizer/Singles/insert upholder</div>
<div>/gate/digitizer/Singles/upholder/setUphold 650. keV</div>
<div>/gate/digitizer/Coincidences/setWindow 10. ns<br>
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<div>/gate/digitizer/Coincidences/minSectorDifference 1</div>
<div>/gate/digitizer/name delay<br>
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<div>/gate/digitizer/insert coincidenceSorter</div>
<div>/gate/digitizer/delay/setWindow 10. ns</div>
<div>/gate/digitizer/delay/setOffset 500. ns</div>
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<div>source is backtoback gamma 0.511 MeV at 0 0 0 </div>
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<div>If I look at the coincidence events (globalPosX1, etc.), I was hoping the LORs would all go through the origin.</div>
<div>the mean of all the intercepts is close to 0, but there is a residual 0.2 mm PSF.</div>
<div>Can anyone tell me where this is coming from? something in the digitizer?</div>
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<div>mlewis</div>
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