Time-resolved monte carlo simulation of optical and isotopic photons in hybrid phantoms

We have developed a novel Monte Carlo simulation code for time-resolved simultaneous migration of optical and isotopic photons through 4D hybrid phantom media. Phantoms consist of parameterized regions with spatial varying material, activity, and wavelength-dependent optical properties (scatter, attenuation, anisotropy) which can be represented by implicit and higher-order surfaces as well as voxelized tomographic data. Since interaction frequency and subsequently mean free path lengths for optical photons differ significantly from isotopic photon propagation, two alternative ray-tracing strategies are proposed. This new approach allows for direct investigation and comparison not only of nuclear (PET, SPECT) and optical (ODT) imaging modalities but also for direct comparative analysis of simulation strategies such as Monte Carlo versus finite element simulation because of identical source and absorber geometries. The proposed algorithm has been used in our group to support small animal instrumentation development and to generate flux/projection images to advance reconstruction strategies, particular for tomographic approaches in optical imaging. The framework can also be used to develop and optimize acquisition protocols due to the ability of direct investigation of tracer/marker kinetic distributions including dual-modality labeling protocols