The spatial variation in reflectance such as the blood-vessel pattern can be observed in the image of cerebral cortex. This spatial variation is mainly caused by the difference in concentrations of oxy- and deoxyhemoglobin in the tissue. We analyze the reflectance spectra obtained from multispectral images of pig cortex by principal component analysis to extract information that relates to physiological parameters such as the concentrations of oxy- and deoxyhemoglobin and physical parameters such as mean optical path length. The light propagation in a model of exposed pig cortex is predicted by Monte Carlo simulation to estimate the interpretation of physiological and physical meanings of the principal components. The spatial variance of reflectance spectra of the pig cortex can be approximately described by the first principal component. The first principal component reflects the spectrum of hemoglobin in the cortical tissue multiplied by the mean optical path length. These results imply that the wavelength dependence of mean optical path length can be experimentally estimated from the first principal component of the reflectance spectra obtained from multispectral image of cortical tissue.
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.
Optical imaging of an exposed cortex for brain function measurement is an attractive method for both clinical and physiological investigations. Multi-spectral imaging of the exposed cortical tissue enables measuring the activity-dependent changes in oxy- and deoxy haemoglobin independently. Because light propagation in the cortical
tissue strongly depends upon wavelength, the blurring by a scattering effect on multi-spectral images depends upon wavelength as well. It is important for more accurate measurement to correct this wavelength-dependent blurring in the multi-spectral images of the exposed cortex. In this study, the relative point spread functions which represent the difference in blurring by wavelength were predicted from the multi-spectral images of a blood vessel in the cortical
tissue. The multi-spectral images of the cortical model are calculated by Monte Carlo simulation and wavelength-dependent point spread functions are estimated from the cross section of the blood vessel in the images. The tendency of the wavelength-dependence of relative point spread functions is almost the same as that of the point spread functions predicted from the light propagation in the cortical model. The relative point spread functions estimated from wide blood vessels are broader than those estimated from a narrow blood vessel.