Development of 200-channel mapping system for tissue oxygenation measured by near-infrared spectroscopy

Near-infrared spectroscopy (NIRS) is a very useful technique for noninvasive measurement of tissue oxygenation. Among various methods of NIRS, continuous wave near-infrared spectroscopy (CW-NIRS) is especially suitable for real-time measurement and for practical use. CW-NIRS has recently been applied to in vivo reflectance imaging of muscle oxygenation and brain activity. However, conventional mapping systems do not have a sufficient mapping area at present. Moreover, they do not enable quantitative measurement of tissue oxygenation because conventional MRS is based on the inappropriate assumption that tissue is homogeneous. In this study we developed a 200-channel mapping system that enables measurement of changes in oxygenation and blood volume and that covers a wider area (30 cm x 20 cm) than do conventional systems. The spatial resolution (source-detector separation) of this system is 15 mm. As for the effects of tissue inhomogeneity on muscle oxygenation measurement, subcutaneous adipose tissue greatly reduces measurement sensitivity. Therefore, we also used a correction method for the influence of the subcutaneous fat layer so that we could obtain quantitative changes in concentrations of oxy- and deoxy-hemoglobin. We conducted exercise tests and measured the changes in hemoglobin concentration in the thigh using the new system. The working muscles in the exercises could be imaged, and the heterogeneity of the muscles was shown. These results demonstrated the new 200-channel mapping system enables observation of the distribution of muscle metabolism and localization of muscle function.