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Question Re: Is it possible to change the geometry during the tracking of a particle? 

Keywords: run geometry
Forum: Run Management
Re: None Is it possible to change the geometry during the tracking of a particle? (Wang Yue)
Re: None Re: Is it possible to change the geometry during the tracking of a particle? (Makoto Asai)
Date: 14 Mar, 2007
From: Wang Yue <Wang Yue>

In our case, change position or ratation is not that important, we would like to implement new geometry(a collimator hole array) for each incoming particle(Context below). It seems not possible?

Context of the question: GATE for system design in SPECT

problem: enormous slowdown in the simulation of geometry of fanbeam collimator

> 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 (not implemented yet):
>
> 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).

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