[Gate-users] Replacing gps with phase-space file of gps emissions not working...

[Steven Marsh]

Hi All

A quick query about the use of phase-space files (PSF).  A summary of the following is that I can collect projection data for my SPECT simulation if I use, for example a spherical source of Tc-99m, but as soon as I collect a PSF from the spherical gps source and then use that PSF as the source, I no longer get counts on my SPECT camera. This is not what I was expecting and assume I have made an error so I have included what I believe to be the essential pieces of code below...

So, I can use the following source definition and successfully collect photon counts (an image of a circle)  in my simulated SPECT system

/gate/source/addSource source gps
/gate/source/source/gps/particle gamma
/gate/source/source/gps/type Volume
/gate/source/source/gps/shape Sphere
/gate/source/source/gps/radius 25 mm
/gate/source/source/gps/angtype iso
/gate/source/source/gps/energytype UserSpectrum
/gate/source/source/gps/setSpectrumFile data/energy_spectrum_Tc99m.txt
/gate/source/source/setForcedHalfLife 242343.36 s
/gate/source/source/setActivity {ACTIVITY} MBq
/gate/source/source/gps/centre 0 0 0 mm


I can also encapsulate this source within a sphere and produce a PSF of photons exiting the sphere

/gate/actor/addActor                         PhaseSpaceActor PhS_g
/gate/actor/PhS_g/attachTo                   phsSphere
/gate/actor/PhS_g/save                                     output/PhS-Gamma-{ACTIVITY}MBq.root
/gate/actor/PhS_g/useVolumeFrame             true  #Not using world coordinates
/gate/actor/PhS_g/storeOutgoingParticles     true
/gate/actor/PhS_g/enableParticleName         false
/gate/actor/PhS_g/enableProductionVolume     false
/gate/actor/PhS_g/enableProductionProcess    false
/gate/actor/PhS_g/addFilter                  particleFilter
/gate/actor/PhS_g/particleFilter/addParticle gamma
/gate/actor/PhS_g/enableZPosition            true
#/gate/actor/PhS_g/saveEveryNSeconds          20


Inspection of the PSF produced with root suggests that this works correctly (many 0.14 MV photons exit the sphere and these are visualisable when used as a source in the next bit of code) - but the problem arises when I place the PHS for the sphere into the identical SPECT simulation and use it to replace the original point source.  The issue is I get essentially no counts on the SPECT camera (a few random dots appear only).  My code for including the PSF is the following (note, when I include this I remove the gps definition for the source above)

/gate/source/addSource                            src_Heart phaseSpace
/gate/source/src_Heart/addPhaseSpaceFile          data/PhSData/PhS-Gamma-{ACTIVITY}MBq.root
/gate/source/src_Heart/setParticleType            gamma
/gate/source/src_Heart/attachTo                   boundingVolume
/gate/source/src_Heart/useRandomSymmetry          true

/gate/source/src_Heart/setForcedHalfLife  21624.12 s  #6.0 hrs
/gate/source/src_Heart/setForcedUnstableFlag true
/gate/source/src_Heart/setForcedHalfLife 21624.12 s
/gate/source/src_Heart/setActivity {ACTIVITY} MBq


Am I doing things correctly?  I have tried to disentangle it all and it appears to me as though the SPECT detector (as defined in the gate-examples and modified and tested successfully for tc-99m) recognizes and counts photons from the gps as per above but not for the PSF source as I have included it.

As always any assistance to show me the error of my ways is greatly appreciated :)

Best wishes
Steve









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