[gate-users] Energy Spectrum Woes - Dual Head and 4 Head Designs

[Angela M K Foudray]

Recently my labmates and I have been looking into two head and 4 head 
PET system designs using LSO crystals for scintillators, F18 in water
phantoms for the source+phantom... I can give you more details if you
like... but we have been increasingly alarmed over the energy spectra
that we have been getting.  We have been looking at the energies output
in the Coincidence file of the two photons seperately.  We have been
getting very confounding results.  First off, the two spectra are not
"the same" (and of course, I don't mean small differences you're going
to get from Monte Carlo) - they're just not similar.  There is a high
energy peak near 700 keV and a low energy peak near 200 in one of them
that we can't explain, and there's a compton peak near 350 keV in only
the one that doesn't have the strange 200 keV peak.  If there was a
weird 700 keV peak and a weird 200 keV peak in *both* of them, I would
be slightly less worried - maybe - but the spectra just are completely
different.  I understand that "photon one" (the first one listed in the
coincidences file) is simply the first photon of a pair that, with a
particular time resolution, happened to be "detected" first - with a
great number of counts, the spectrum from photon one should be, to the
eye, nearly exactly the same as the energy spectrum for photon two,
statistically.  

I have attached a picture of the two energy spectra for a particular
simulation (they *always* come out the same, the spectrum for photon one
has a weird low energy peak and small compton peak, as well as a weird
high energy peak) displaying the current bane of our simulations.

What is going on?

If you have *ANY* insight... hit me with it!

Ange
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