Estimation of the optical path length factor for functional imaging of an exposed cortex by principal component analysis

Activation of the cerebral cortex induces a localized change in the volume and oxygenation of the blood. Because the change in spectral reflectance of the cortex depends upon the concentration changes in oxy- and deoxy haemoglobin, multi-spectral imaging has been applied to investigate the functional activity of the exposed cortex related to oxy- and deoxy haemoglobin. However, brain tissue is a highly scattering medium, and the reflectance of cortical tissue depends on the mean optical path length of the detected light. The linear spectrographic analysis method without wavelength-dependent path length scaling may produce unreliable results in multi-spectral image analysis. In this study, we propose a method of estimating the wavelength-dependent path length factor from the principal component analysis of the multi-spectral images of the exposed cortex. The optical path-length factor estimated from the first principal component of the multi-spectral image of the cortical model and the absorption spectrum of haemoglobin agrees with that predicted by Monte Carlo simulation. The tendency of the optical path-length factor of the pig brain estimated from the first principal component of the multi-spectral images is almost the same as that of the cortical model.