Optical property measurement from layered biological media.

摘要

Near infrared (NIR) photon reflectance spectroscopy is applied to measurement of blood concentration and its oxygen saturation within biological tissue. The measurement relies upon the changes in photon absorption of hemoglobin in the tissue as changes occur in the hemoglobin concentration and oxygen content. In the present study, NIR light is introduced at the skin surface and the optical properties (absorption and scattering) within the underlying tissue are determined from the resulting surface reflectance. Typically the tissue is modeled as a homogeneous mixture of bloodless tissue and blood, and the model incorporates the physical relationship between the surface reflectance and the optical properties of the tissue. The skin and underlying tissue, although heterogeneous, have a characteristic layered structure. These layers can be differentiated optically. The modeling and the inverse problem of measuring the optical properties in each of the tissue layers from the surface reflectance have been the subject of much attention by a number of investigators. Nonetheless, quantification of the relationship between surface reflectance and the optical properties of layered tissue has not been well understood nor well described.; In the forward problem, tissue optical properties yield surface reflectance profiles (SRPs). Surface reflectance profiles, or SRPs, from diffusive media consisting of two layers are calculated using numerical solutions to the Boltzmann equation. Experimental SRPs are also measured in vitro from a test medium and in vivo from the calf of human subjects. This study provides a new approach to solving the inverse problem of determining optical properties from SRPs. To solve the inverse problem, an effective diffusion constant (K_e) is determined for the layered media. The K_e is the diffusion constant of an equivalent homogeneous medium which best fits the SRP of the layered medium. The departure from K_e of the SRP for a layered media is captured concisely, and K_e becomes a tool in describing the layered optical properties. This approach is applied clinically to measure changes in the blood concentration and oxygenation measured in vivo from normals and patients with peripheral vascular disease.; A significant finding from the modeling was to identify the functional relationship of K_e to the top and lower layer diffusion constants, and the top layer thickness. When applied to in vitro measurements from media containing homogeneous layers with known optical properties, this functional relationship predicted K_e within the 95% confidence interval of the measured K_e. For the in vivo measurements, changes in K _e with exercise are consistent with expected exercise physiology. With the incorporation of the known optical absorbance of hemoglobin in the presence of oxygen, the SRPs provide a means to measure the oxygen saturation of a deep tissue layer from the surface light reflectance.