Computational modelling of light propagation in a turbid medium illuminated by a LED array–diaphragm system

Abstract

Light sources generated by LASER and LED are progressively used in the medical field. For many applications there is the need to use light sources with a well-defined size. This can be achieved when a large LED array is associated to a thin plane mask pierced with a circular hole. In this paper three tissue illumination modes were studied. The one (reference mode) allows to illuminate the tissue with a collimated light beam of finite radius, while the two others generate an illumination that depends on the relative position of the diaphragm and LED array with respect to the tissue boundary. Finite Element Method (FEM) and Monte Carlo (MC) were used to compute the fluence rate (axial and radial profiles) inside the tissue as function of source radius, realistic source shapes, tissue optical properties, and boundary conditions. Different types of boundary conditions (i.e. Robin boundary conditions (RBC), and altered with the consideration of the optical features of the interface tissue-diaphragm: -reflecting,-absorbing,-diffusing,-and partially absorbing-diffusing) were evaluated in order to compare the three above mentioned tissue illumination modes. FEM provided accurate results in cases of flat top source profiles and for simple tissue-diaphragm interfaces (absorbing and reflecting) as well as for tissue-air boundary conditions (RBC). MC simulations showed good agreements with experiments when a LED array–diaphragm system (partially absorbing-diffusing) is positioned near the tissue (2.5 mm), while FEM underestimated the fluence rate by a large error within tissue-depths of 5–10 mean-free-paths. These investigations emphasize the need to accurately model all optical events occurring in complex tissue-illumination modes. Monte Carlo simulations further confirmed that a critical beam radius Rc (Rc≥4δ) should be used to retrieve the light penetration depth δ within the tissue, in the same conditions as those of the 1-D geometry.