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Monitoring Absorption Changes in a Layered Diffusive Medium by White-Light Time-Resolved Reflectance Spectroscopy

Diffuse spectroscopy of turbid media has assumed a crucial role in the characterization of biological tissues. In particular, broadband time-resolved optical spectroscopy allows the direct determination in a single measurement of both the optical parameters of the tissue and the concentration of its main constituents. Moreover, the possibility of performing parallel wavelength measurements allows the recording of data in real time, providing a system that is able to perform dynamic measurements. We used a white-light time-resolved spectroscopy system to monitor absorption changes in a layered diffusive medium. Measurements were performed in reflectance geometry, with a 2-cm source-detector distance, on a two-layer liquid phantom with optical properties similar to those of human tissues. By varying the concentrations of three inks with different spectral features, we changed the absorption coefficient of the layers to mimic functional brain activation and the systemic response in the scalp. Data were analyzed by a time-resolved spectrally constrained fitting method based on a homogeneous model of photon diffusion. Although this approach is based on a homogeneous model and employs a single source-detector distance, the technique is able to monitor changes in the lower layer, while it is scarcely affected by variations in the upper layer. These results were confirmed by numerical simulations based on a perturbation approach to diffusion theory. Preliminary in vivo measurements have been performed on healthy volunteers to monitor oxy- and deoxy-hemoglobin changes in the brain during a motor task. Although the overall sensitivity of the technique is reduced, in vivo results are in general agreement with the findings of the dedicated system for functional brain activity.