Near-infrared Absorption Property of Biological Soft Tissue Constituents

Near infrared (NIR) can penetrate relatively deep into biological soft tissues. The NIR absorption property of tissue varies with tissue constituents especially water, fat, collagen, and their combination ratio. Therefore, combination ratio of tissue constituents can be evaluated by decomposing the absorption spectrum to determine the light path length in each constituent. Standardized absorption spectra of tissue constituents are required in order to carry out decomposition. Since water, fat, and protein are the major contributors at NIR spectral region. This study is to measure their absorption spectra from standardized samples as reference for quantifying tissue constituents. Five kinds of major fatty acid found in human fat were mixed in proper ratio as a standard reference. Absorption spectrum of bovine skin gelatin and elastin in hog eye lens were used as references of protein. NIR absorption spectra were measured using a Shimadzu 3101-PC spectrophotometer. The results show that temperature has a strong effect on the absorption property of water but not on fatty acid mixture. Absorption spectrum of elastin is similar to that of dry bovine gelatin. NIR spectroscopy also can be used to characterize or identify different types of soft tissue based on their major chemical composition, such as detecting a fat plaque in a muscular tissue or a tumor in a high fat content tissue.