Measurement of the Optical Properties of the Adult Human Head with Spatially Resolved Spectroscopy and Changes of Posture

Abstract

Absolute optical properties (i.e., absorption and reduced scattering coefficients, µ _a and µ _s ’) of human tissues such as the head, calf and arm have been measured using near-infrared (NIR) phase¹, time² or spatially³ resolved spectroscopy (SRS) systems. While a simple continuous-wave (CW) system can measure Δµ _a , absolute µ _a cannot be easily measured because of the complex geometry in which measurements are made in tissues. It has been shown that the SRS technique can be used to calculate a scaled absolute µ _a , i.e. µ _s ’ µ _a where µ _s ’ is considered as a time-invariant scaling factor⁴. This paper suggests a way to use a commercially available spectrometer, namely the NIRO-300 (Hamamatsu KK.) which has both CW and SRS capabilities, to calibrate an absolute µ _a based on the changes of µ _a (i.e., Δµ _a , calculated from the CW data and a modified Beer-Lambert law) and the scaled µ _a (i.e., µ _s ’ µ _a calculated from the SRS data). Using changes of posture from the supine to the head up position, the absolute optical properties of 15 adult human heads were calculated. Issues of errors due to the inhomogeneity in real tissues and methods to minimise them are also discussed.