Chapter 5 - Camera-based blood oxygen measurement

Abstract

The major chromophores in biological tissues are oxygenated hemoglobin and deoxygenated hemoglobin, which show distinctive optical absorption properties in the visible wavelength range. The absorption spectra of oxygenated hemoglobin and deoxygenated hemoglobin differ since the binding of oxygen to hemoglobin changes the light absorption spectrum of hemoglobin. Peripheral tissues and cells demand a continuous supply of oxygen, which are delivered via blood circulation. Delivery of oxygen to peripheral tissues can be evaluated from the diffuse reflectance spectrum based on the absorption spectra of oxygenated hemoglobin and deoxygenated hemoglobin. The percentage of oxygenated hemoglobin in a volume of tissue is called the tissue oxygen saturation (StO2) or hemoglobin oxygen saturation, which is a useful indicator for monitoring peripheral tissue oxygen consumption, hypoperfusion, cyanosis, and tissue viability. Analysis of diffuse reflectance spectra provide useful information on tissue activities and functions that are related to biological chromophores. Spectral imaging with diffuse reflectance has been employed for visualizing the spatial distribution of blood oxygen saturation in living tissue using a series of discrete optical interference filters, a liquid crystal wavelength tunable filter, and a hyperspectral imaging system. For practical uses, more simple, cost-effective, and portable equipment is needed. Imaging with a digital red–green–blue (RGB) camera is one of the promising tools to satisfy those demands. Several techniques with RGB camera-based imaging have been used for the non-invasive characterization of biological tissues. This can be achieved using a general-purpose digital color camera and a white-light source such as a white-light emitting diode (LED), which enable cost-effective, easy-to-use, battery-powered, portable, remotely administrable, and/or point-of-care solutions. RGB camera-based diffuse-reflectance imaging technique meets medical and healthcare needs. In this chapter, a simple and affordable imaging technique to evaluate blood oxygen saturation and hemoglobin concentration in biological tissues by using a digital RGB camera is described. Several examples of potential applications to biomedical imaging are presented.