A technique for optimised navigation in regular geometries

The simulation of a DICOM file describing the geometry of a patient through a 3-dimensional grid of several million voxels represents a big challenge in terms of time and memory consumption. We have developed a fast technique to navigate in these regular voxelised geometries in the GEANT4 framework. It takes advantage of the regular structure of the geometry to optimise the location of voxels at tracking time. An option to skip on the fly the voxel boundaries when two contiguous voxels share the same material reduces the number of steps and therefore the simulation time. When the number of materials in the phantom is of the order a few dozens, enough to reach the required precision in medical physics applications, this technique is several times faster than the optimised navigation algorithms implemented in GEANT4, while keeping optimal memory and initialisation time.