[Gate-users] Difference between simulated and experimental LOR in inveon

[Santos, Roque]

Hello all.

I am validating a GATE model of Siemens Inveon. In order to do this, I am placing a 12.7 mm cube source of Na22 (7uCi at September 2016) at different position along the z axis (depth of the Bore) of the real machine. At these positions I'm taking a 10 second scan and I am using the IAW (inveon software) to create the histogram and the image of the scan. This software creates log files that have the number of singles and the number of LOR (under the name of events). I am comparing this number of events with the number of LOR generated in GATE.

For the simulation I'm using Jen Gregor's Inveon model. I changed the coincidence policy to killAll and the energy widow to 425-625 keV. The last change was done because this is the energy window that we use in our group work. The rest of the model is the same as it is distributed by Dr. Gregor. A brief description of the digitizer features: dead time: depth of response=1, crystal resolution min=0.15 crystal resolution max=0.35, Quantum Efficiency=0.85, Energy reference= 511 keV, dead time= 7000000. ps, coincidence window=3.438 ns.

I am using the standard model for the Photoelectric, Compton and Rayleigh scattering. The standard model is also used for the electron ionization and Bremsstrahlung processes. For the positron annihilation I'm using the eMultipleScattering model and the Radioactive decay is on.

I modeled the source as a point source centered in the cube, and I'm using my own positron distribution (however the results do not change too much when I used the default one).

The results that I got showed that the simulation produces more LOR than the experimental. I'm attaching pictures of this situation

[cid:2f2de02f-72db-46ae-8002-aae003bb2e17][cid:b909bae3-f62d-41fb-adbe-b395655a0dc7]
The first one is the LOR and the second one is the efficiency. Both shows a difference of almost twice. The model that I'm using is a validated model. I looked for other references with the same problem, but so far I just could find one.

Does anybody have an idea of how can I get the same LOR experimental and simulated? or Why is this happening? Any idea will be explored.

Thank you very much for your help.

Greetings,

Roque Santos
University of Tennessee, Knoxville
Departement of Nuclear Engineering


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