Noninvasive Assessment of Cerebral Hemodynamics and Tissue Oxygenation during Activation of Brain Cell Function in Human Adults Using Near Infrared Spectroscopy

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Abstract
Near Infrared Spectroscopy (NIRS) is an optical technique that measures tissue absorbance of light at several wavelengths in the 700-1000 nm spectral region. From the obtained values, changes in the concentration of oxygenated hemoglobin (HbO2), deoxygenated hemoglobin (HbR),corpuscular blood volume (HbO2 + HbR) and cytochrome C-oxidase oxygenation are determined. Since near infrared light penetrates biological tissue (and even hone) well, the method can be performed noninvasively through the intact skull as first proposed by Jöbsis in 1977 (Jöbsis 1977). Since this first report, most applications have focussed on neonates, in whom transmission studies through the entire head are possible (Brazy et al. 1985, Cope and Delpy 1988, Ferrari et al. 1986, Reynolds et al. 1988). The larger diameter of the adult head prevented such studies in adults with sufficient signal to noise ratio. However, using the signal reflected from the brain (instead of transilluminated through the brain) is another way to obtain measurements of light absorbance that follow the Lambert-Beer Law (Faris et al. 1991). This approach permits the examination of regions of the adult human brain (McCormick et al.1991a,Elwell et al. 1992). Using this method, so far mainly severe global alterations in the general oxygenation state of the brain such as hypoxia have been assessed (McCormick et al. 1991b).