Local optical characterization of biological tissues in vitro and in vivo
Two methods for measuring the optical properties of tissue, i.e. reduced scattering and absorption coefficient, were developed. The first one was designed for in vitro investigations. It is based on the measurement of the spatially resolved transmittance through a tissue slab (typically 5 mm thick). The second one was designed for local in vivo investigations. It is based on the measurement of the spatially-resolved diffuse reflectance, close to the source (< 2 mm). A sterilizable probe, of diameter of 2.2 mm was realized for measurements in clinic, during surgery or endoscopy. A model of the light propagation in turbid media was developed based on Monte Carlo simulations. It allows us to establish the correspondence between the optical coefficients to the experimental data (spatially-resolved transmittance or reflectance). The influence of the scattering phase function on the reflectance and transmittance was carefully studied. In particular, the role of the first and the second moment of the phase function were shown for short distances measurements (close to one transport mean free path). Similarity relations, taking into account the first and second moment of the phase function were proposed and assessed. The model was tested extensively by measurements on tissue-like phantoms, with known optical properties. Further assessments of the model were performed by comparison with time-resolved measurements. Investigations on human brain, breast and cervix tissues were performed. Tissue structure (i.e. heterogeneities not taken into account by the model) were found to play a significant role in the light propagation. Normal and malignant brain tissue were investigated in vitro, and in vivo during surgery. Significant differences were found between different type of tissues, and were explained in terms of chromophore content (hemoglobin, water) and specific features of scattering properties.