Method for measuring locally and superficially the scattering and absorption properties of turbid media

We present a method and apparatus for local and superficial measurement of the optical properties of turbid media. The depth probed is on the order of 1 transport mean free path of the photon. The absorption coefficient, reduced scattering coefficient and the phase function parameter gamma =(1-g2)/(1-g1) are optical parameters computed from a single measurement of the spatially resolved reflectance close to the source. Images of superficial structures of the medium can be obtained by performing multi-site measurements. An important application of this technique is the characterization of biological tissues, for example for medical diagnostic purposes. Measurements on biological tissues can be achieved using a probe of diameter less than 2mm, and the average volume probed is on the order of 1 mm. Separate images of absorption and tissue structure can be achieved with a resolution of approximately one transport mean free path of the considered tissue.

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.

EPFL1998

Monte Carlo study of diffuse reflectance at source-detector separations close to one transport mean free path

Christian Depeursinge

The spatially resolved reflectance of turbid media is studied at short source–detector separations (approximately one transport mean free path) with Monte Carlo simulations. For such distances we found that the first and second moments of the phase function play a significant role in the reflectance curve, whereas the effect of higher-order moments is weak. Second-order similarity relations are tested and are found efficient at reducing the number of relevant parameters necessary to predict the reflectance. Indeed, only the four following parameters are necessary: the refractive index, the absorption coefficient, the reduced scattering coefficient, and a phase function parameter γ that depends on the first and second moments of the phase function. For media of known γ, the absorption and reduced scattering coefficients can be determined from the intensity and the slope of the log of the reflectance, measured at a single distance. Other empirical properties of the reflectance are derived from the simulations, using short-distance measurements, which provide clues for determining the scattering and absorption properties. In particular, the slope of the square root of the reflectance does not depend on the absorption coefficient but depends on both the reduced scattering coefficient and the phase function parameter γ.

1999

Local optical characterization of biological tissues in vitro and in vivo

EPFL1998

Method for measuring locally and superficially the scattering and absorption properties of turbid media

In Vivo Local Determination of Tissue Optical Properties: Applications to Human Brain

Local optical characterization of biological tissues in vitro and in vivo

Monte Carlo study of diffuse reflectance at source-detector separations close to one transport mean free path

In Vivo Local Determination of Tissue Optical Properties: Applications to Human Brain

Local optical characterization of biological tissues in vitro and in vivo

Monte Carlo study of diffuse reflectance at source-detector separations close to one transport mean free path