[Gate-users] Reducing computing time for realistic (industrial) CT simulations

[MELISSAS Maximilien (SAFRAN)]

Dear Gate-users
I am using Gate to simulate a CT acquisition chain (GE tomex L 450) used for CT analysis of aeronautical parts.
The main issue I am facing is computing time, especially if I want to simulate a full scan (2400 projections at I= 2ma, V = 350 kV. (One of) my “test” setup is an epoxy cylinder located 50 cm from the tube, with the detector at 1m from the tube, if I propagate 500 000 (as a benchmark) photons the “user”computing times are the following:
31.15 seconds without optimization (Only setting the simulation as /fast/ to avoid losing one hour computing a 2048x2048 pixels grid)
30.8 seconds  if I play with MaxStepSize (Actually, if I do /gate/physics/SetMaxStepSizeInRegion world  15.0 mm, is the step size reset to a normal value when entering another material ? )
31.71 seconds if I set VRT to 10 (as in the benchmarkCT/vrt example)
This important CPU time makes me fear that I cannot compute nor a realistic amount of photon, neither a realistic number of projections, so I need to find some way to reduce computing time while keeping useful informations)
Is-there any option similar to thinning/weighting (but for primaries since I am not interested in secondary electrons) to increase luminosity without increasing computing-time?  Is there any other way to reduce the simulation time?  Does the vrt increase the weight of a photon hitting the detector and thus increase virtually the number of photons? (I haven't tried yet to generate images with the vrt enabled)
Another question, even though the default source is conical, I'd like to take into account the Gaussian-shape of the X-ray spot, is there advanced setting of the source in the macros? If not-is there any Cpp example?
A last question, how do I find the history of a hit, when parsing the ntuple, can I find information about the hit's history, whether the photon has been scattered, and through which material has it been propagated ?
i.e. how do I make the difference between
0   0.0116    0.013     -150     0.156        0        0         0  world_phys initStep
 1    0.433    -4.18      -12     0.156        0      138       138 epoxyCylinder_phys Transportation
 2    0.458    -4.42    -3.97     0.156        0     8.02       146 CCylinder_phys Transportation
 3    0.482    -4.66     3.97     0.156        0     7.95       154 epoxyCylinder_phys Transportation
 4    0.506     -4.9     11.8     0.156 0.000291     7.81       162 epoxyCylinder_phys Compton
 5    0.539    -4.87       12     0.156        0    0.216       162 world_phys Transportation
 6       22     12.5      150     0.156        0      141       303  pixel_phys Transportation
And
0  -0.0249   -0.017     -150     0.114        0        0         0  world_phys initStep
 1    -4.21    0.707    -11.2     0.114        0      139       139 epoxyCylinder_phys Transportation
 2    -4.88    0.823       11     0.114        0     22.2       161  world_phys Transportation
 3    -9.06     1.55      150     0.114        0      139       300  pixel_phys Transportation
I guess I should set some actors, but I do not find any clear examples in the documentation.

Thanks for your answers/help,

 Cheers,
Maximilien

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