The use of spatially resolved fluorescence and reflectance to determine interface depth in layered fluorophore distributions

The possibility of using spatially resolved fluorescence and reflectance measurements to recover tissue optical properties, fluorophore concentration and the thickness of a superficial layer in a two-layer geometry was investigated. A diffusion theory model was used to fit reflectance and fluorescence data generated using Monte Carlo simulations or experimentally obtained using tissue-simulating phantoms. Initial analysis fitting diffusion theory generated data suggested that it should be possible to recover all parameters from a single set of spatially resolved fluorescence and reflectance measurements. However, when Monte Carlo or experimental data were fitted the results were less impressive. Overall, it was shown that there is a strong coupling between interface depth, fluorophore concentration and tissue absorption, especially at larger depths. The recovery of all input parameters from a single set of spatially resolved measurements was limited to interface depths less than 3 mm, which is a reasonable range for measuring fluorophore in skin. When the tissue optical properties and fluorophore concentrations were known, then the interface depth could be monitored with good accuracy in simulated serial measurements. These results may also point to deficiencies in the diffusion theory model that introduce significant errors in the fitted results.