Optical Properties of Tissues Using SWIR Light

Abstract

The behavior of light passing through biological materials has been studied theoretically and experimentally to develop new noninvasive biomedical applications. Light through turbid media, such as tissue, consists of a combination of ballistic and diffusive photons. Ballistic photons propagate in the forward direction and do not scatter, preserving the characteristics of the incident beam. Diffusive photons, on the other hand, scatter multiple times and become attenuated as they propagate through turbid media. This causes a loss of information and thus a reduction in image quality. The optical properties of tissue can be described in terms of scattering and absorption events, and can be calculated using experimental data (represented by spectral profiles) and theoretical models. The attenuation of light in turbid media can vary due to tissue chemical composition, such as water content (major absorption bands at 1450 and 1930 nm) and lipids (absorption bands at approximately 1200 or 1450 nm for O–H groups), and are dependent on the density, structure, size, and shape of the sample. Light in the shortwavelength infrared (SWIR) region (from ~1000 to 2500 nm) has been utilized to penetrate tissues more deeply compared to shorter wavelengths. This comparatively deeper penetration is due to low absorption of light by molecules in select regions: wavelengths at approximately 1100–1350, 1600–1870, and 2100–2300 nm. These three SWIR spectral regions are referred to as the therapeutic near-infrared (NIR) windows II, III, and IV, respectively.