We have developed a new noninvasive method for measuring hemoglobin concentration called the “near-infrared spectroscopic imaging method.” This method is based on Lambert-Beer's law and on estimating the path length of light transmitted in a blood vessel using the analysis of near-infrared optical images that visualize peripheral blood vessels in the human finger. To investigate the validity of this method, basic studies were conducted using a prototype device, finger phantoms and a human finger. The experimental results of these basic studies were consistent with a theoretical equation derived from a simple light transmission model under the assumption that the cross-sectional shape of a blood vessel is completely circular. As the result, our noninvasive method was shown to be valid. To apply this method to human fingers, reproducibility and correlation tests were conducted on volunteers. Reproducibility (CV %) with five volunteers was less than 3.5%. Good correlation was obtained between predicted hemoglobin concentrations using this method and reference values obtained by the SLS hemoglobin method using an automated blood cell counter (r=0.86, n=54). These results on volunteers suggest that our noninvasive method can be applied to human fingers. As our method has the advantage of not requiring blood sampling, we will evaluate the usefulness of this noninvasive method for anemia screening.
