Study of Minor Chromophores in Biological Tissues by Diffuse Optical Spectroscopy (Review)

Diffuse optical spectroscopy (DOS) is a rapidly advancing non-invasive diagnostic technique to investigate biological tissue, based on probing the target object with optical radiation in the visible and/or near-infrared wavelength range and detecting the diffusely scattered light from the tissue. The signals obtained through DOS provide extensive information about the biochemical composition of tissues due to the presence of light-absorbing compounds known as chromophores. To date, DOS is widely employed to detect major chromophores such as deoxygenated (Hb) and oxygenated (HbO₂) hemoglobin, water, lipids, and melanin. The concentrations of Hb and HbO₂ in biological tissues are highly significant in clinical research, as they offer valuable insights into tissue oxygenation status and enable the detection of hypoxia. However, biological tissues also contain less-studied chromophores - minor chromophores - which also contribute to the overall absorption spectrum. These include various globins, such as methemoglobin, carboxyhemoglobin, and myoglobin, as well as cytochromes and cytochrome c oxidase. Identifying minor chromophores using DOS is challenging due to their relatively low absorption contributions compared to major chromophores, as well as the limited understanding of their specific absorption spectra. Nevertheless, the simultaneous detection of both major and minor chromophores could provide a comprehensive understanding of metabolic processes within vascular, intracellular, and mitochondrial compartments of tissues. This would substantially expand the potential applications of DOS in both research and clinical studies. In this review we examine literature sources that explore the investigation of minor chromophores in biological tissues by DOS, discuss the role of major chromophores, and evaluate the potential for simultaneous detection of both major and minor chromophores with DOS.