Absorption and Scattering Properties in a Highly Absorbing Turbid Medium

Abstract

The use of NIR spectroscopy to determine, within the diffusion approximation, the absorption and scattering coefficients (μa and μs') of a highly-absorbing turbid medium is very difficult. In this paper, we investigate whether this difficulty results theoretically from the invalidation of the diffusion approximation or technically from the weak light intensity and small signal to noise ratio caused by the high absorption. First, we employ the transport theory in a simplified form to predict the ratio μa/μs' when the diffusion approximation becomes invalid. We show that the diffusion theory breaks down when the ratio of μa/μs' is equal to ~1/10. This implies that the diffusion approximation is still valid theoretically for a very high absorbing organ, such as liver. Then, as an example of NIR spectroscopy with highly absorbing tissue, we present in vivo hemoglobin saturation studies on cuffed human forearms using continuous, amplitude modulated, and pulsed light.