[Gate-users] Weird Blips in F18 data

Thu Dec 2 16:52:40 CET 2021

Hi everyone

I'm running a relatively simple simulation of a NEMA IQ phantom, looking at the dose deposited within it by F18. My sim has a CT scan loaded as the geometry (with HU determined from the StoichHU example) and the F18 scan as the source. Physics are loaded as GQSP_BERT_EMV.  The macro is split into 4 to run on 4 separate cores, then merged together afterwards using hadd for the root files. My source definition is Fluor18 with e+ particle, 6585s setforcedhalflife and true for setforcedunstableflag (unfortunately my code is on a different machine to my emails so I can't cope and paste directly).

I'm getting some interesting results in my tracklength and edepspectrum root files (see attached). The tracklength seems to get a little bit of a blip at pretty much 1mm, which I'm finding hard to explain. Additionally, on the Edep there appears to be an event (or possibly several) which deposits 20GeV from an energy of around 0.5MeV. The reason that I think it might be several events is that the same point is visible on the individual root files as well as the merged file. My actual edep and dose images look pretty good.

These examples are from merged root files from 4x25000 events so probably not amazing statistically but are quick enough for me to run and trouble shoot. I've ran slightly more events and still see the same effects, but I will run a longer simulation (4x200M or something) overnight to see if I get the same effects (and rule out a statistics related issue).

Does anyone have any clues or suggests are to where to look to troubleshoot this?

Thank you

Mark Baker
Principal Clinical Scientist (Interim)
Imaging Physics (Ionising)
The Clatterbridge Cancer Centre NHS Foundation Trust
(he, him)

CCCW Tel:  0151 556 5030
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Honorary Lecturer, Dept. of Physics
University of Liverpool

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