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1. I need to be efficient, spatial discretization
For spatial discretization, Acuros partitions the computational volume (the CT image series)
into variable sized Cartesian elements.
Computational element size varies based on material properties and the
gradient of the scatter photon fluence
.
The spatial derivatives in the LBTEs are replaced by finite differences at multiple points
within each computational element
Supplementary equations are used to describe fluence variation between these points and
preserve particle balance (DFEM - linear discontinuous Galerkin finite-element method)
Hence the angular fluence is known everywhere in each element, not only at the points
where spatial derivatives are evaluated
BUT
spatial discretization errors can ensue at points in high fluence gradients
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