In vivo optical reflectance measurement of human brain tissue with calculation of absorption and scattering coefficients

Parkinson's disease is a neuro-degenerative disease affecting the globus pallidus (GP), a deep brain gray matter structure surrounded by white matter. During a pallidotomy a thin radio frequency probe is inserted into the GP to generate a small lesion. A fiber optic reflectance probe was developed and used during surgery. This instrument provides real-time display of the optical reflectance spectra as well as assisted lesion localization. Our 1.5-mm probe contains seven 100-μm fibers, one delivers light and six return the reflected light to a spectrometer. During clinical studies, the probe was placed against the surface of the brain and the spectrum between 350-850 nm was recorded. Measurements were repeated at 1-mm increments from the surface of the brain to 60-mm deep (GP level). This provided optical reflectance signals from both gray and white matter. Clinical results show that gray matter reflectance is approximately 50% of white matter between 650-800 nm. By calculating the slope between 700-850 nm, the signals can be differentiated between gray and white matter. We can quantify the absorption and scattering coefficients of the locally measured brain tissue by fitting the two-flux theory of Kubelka and Munk with our measurements.