Abstract

In most biological tissues the absorption does not result from a homogenous distribution of absorbing materials. Absorbers are often localized e.g., in blood vessels, which are randomly distributed. An understanding how this localization of absorbers influences the near infrared reflectance signal is a prerequisite for the correct interpretation of this measurements. For example time resolved reflectance measurements from the head have been used to determine the brain blood oxygenation.¹ However, little attention has been paid to the fact that blood is a highly absorbing fluid (µ_a ~ 4-cm^–1), which is localized in blood vessels, which are embedded in a very low absorbing background medium (brain µ_a ~ 0.1 cm^–1). Furthermore, blood vessels (capillaries, veins, and arteries) have different diameters and the blood oxygenation differs from vessel type to vessel type. The question arises, how strongly do these different vessels influence the result of the reflectance measurements and thus the determination of the blood oxygenation?

© 1994 IEEE