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Keywords: Geometry
Forum: Geometry
Re: Question Is it possible to change the geometry during the tracking of a particle? (Wang Yue)
Re: Warning Re: Is it possible to change the geometry during the tracking of a particle? (John Apostolakis)
Date: 20 Feb, 2007
From: Wang Yue <Wang Yue>

Context of the question: GATE for system design in SPECT

Problem: tracking in parametrised volumes

In SPECT a detector is typically made of a scintillator capturing gammas emitted from a patient tracer. In front of such a detector is a lead block with circa 150.000 air holes. In parallel beam applications all these holes have an identical form and are aligned along a rectangular grid. We can model such a parallel beam collimator using arrays (repeaters) or using G4 replicas. Both options are available in GATE but for distributed computing only replicas are used since building the geometry goes fast (almost no overhead on a cluster).

However, next to parallel beam applications, we are also applying fan and cone beam collimators; respectively oriented towards a focal line and a focal spot. Hence, every individual hole has another form/orientation which can be precalculated and described in a closed analytical expression. To model such collimators we use G4 parametrised volumes. Building the geometry goes fast and flawless. The tracking however is multiple orders slower, probably because every time a particle hits the collimator every form and distance is recalculated.

Temporary solution:

We are designing a "flying hole array" which only takes the 20 neighbouring holes around the interaction site into account. This is however a limitation if the energy of the gammas increases which allows them to travel through many of the lead lamella, crossing over multiple holes (not known before how many holes).

So before to implement the flying hole array we want to make sure the property of the geometry in Geant4. Now we know the geometry can't be changed within a run. If you have any advise or suggestion, we really appreciate it.

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1 Feedback: Re: Is it possible to change the geometry during the tracking of a particle?   (John Apostolakis - 20 Feb, 2007)
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