[Gate-users] Scatter dependent on range cut – bug?

Tue May 17 23:34:06 CEST 2022

Dear Gaters,

I am simulating a cone beam CT setup where I output the Hits / Singles registered in the detector, which I then split into primary and scatter contribution based on the nPhantomCompton, nPhantomRayleigh and parentID parameters. These indicate the number of Compton or Rayleigh scatter events in the phantom and whether a particle was created by the source or via creation of secondary particles.
We noticed that the scatter-to-primary ratio in our simulation was far below the values reported in literature (more than a factor 10). I was able to show that for certain range cuts not every scattered photon exhibited a nPhantomCompton or nPhantomRayleigh value that would have indicated that it was indeed scattered. In the following I will try to explain what I mean by that and which behaviour of the simulation I can’t explain.

In the following figure you will see on the left the simulation setup:

  *   A monoenergetic source emitting a pencil beam of 120 keV photons,
  *   A water cube serving as a phantom,
  *   A phase space actor in front of a realistic (two-dimensional) detector,
  *   And a CTdetector of a realistic material (CsI), to which a crystalSD detector is attached.

With the output of the crystalSD detector (Hits/Singles) and the phase space actor, one can simply plot the coordinates of the intersection point of the incident photons with the detector plane. Any photon that does not go through the centre axis must then either have been scattered in the water phantom or be a secondary. (*)
Additionally, the crystalSD detector outputs the nPhantomCompton, nPhantomRayleigh and parentID parameters, which I used initially to separate primaries from scatter.

[cid:9a7f76a7-3d0a-424c-8aa1-7949eb736772 at tudelft.nl]
(*) Note: In the Hits dataset, I only evaluate the first entry for a given eventID, so scatter inside the detector does not influence this evaluation (I guess?). In the data of the phase space actor, only photons flying in the direction of the detector are evaluated.

In the following the range cut inside the detector is now varied. I then evaluated:

  *   How many photons end up in the detector, which is evaluated by counting the number of unique eventIDs in the Hits dataset (label “incident photons”),
  *   How many photons are flagged by Gate as scatter or as secondary particle, based on the nPhantomCompton, nPhantomRayleigh and parentID parameters (label “Gate: scatter&secondaries”),
  *   How many photons are outside the centre axis, based on the their intersection with the phase space actor (label “off-centre in phase-space”),
  *   and how many photons are outside the centre axis, based on their intersection with the realistic detector (i.e., using their coordinates as registered in the Hits output, label “hits off-centre").

The following plot shows the results:
[cid:14b54afc-b31f-48fb-9659-a17b10f63102 at tudelft.nl]

As one can see, below a certain range cut the number of events flagged by Gate as scatter or secondaries (via the  nPhantomCompton, nPhantomRayleigh and parentID parameters) drops considerably and deviates from the evaluation based on the number of photons that are off-centre. I have no explanation for this phenomenon.

Now let’s zoom into the the number of incident photons:

[cid:5501ee9b-b86a-4228-ab4e-09738f7a0c5c at tudelft.nl]
Interestingly, the number of photons incident on the detector varies with the range cut, which to me is unexpected. Sure, the number of interactions inside the detector increases with decreasing range cut, but this should have no effect on the number of incident photons.

Here is a zoom into the number of off-axis photons:
[cid:23ee200c-e97d-46b5-9154-f3de77f74974 at tudelft.nl]
I don’t have an explanation why the phase space actor and the crystalSD have a slightly different number of off-axis photons, but this is at the moment not my main concern. Again, one can see that their number changes with decreasing range cut.

Last but not least, I kept the range cut in the detector at the default value and instead changed the range cut inside the phantom. Here is the result:
[cid:76e39550-b51b-4970-9e54-4ff33f64ac4e at tudelft.nl]
In this case, the three different ways to extract the number of scatter and primaries agree more or less. Again, one can clearly see a change in the number of total incident photons with smaller range cuts.

So far, I am unable to explain the behaviour shown above, and I am not sure whether I did something wrong in my Gate simulation or whether this is a bug. Please find attached the Gate macros that I used to create this minimum example, and also the Python script I used to evaluate the resulting data. FYI, I use Gate version 9.1, and I also observe this behaviour with Gate version 9.0.
Any help or suggestions would be greatly appreciated. If you think that this is a bug, then I am happy to submit a bug report on GitHub.

Thanks a lot for your time in advance!

Kind regards,
David Leibold

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