3-hour sitting Medicine NIRS Neurosciences TRS VSRAD absorption aging biological tissue brain brain atrophy brain oxygenation breast cancer cerebral blood volume cerebral hemoglobin oxygen saturation characteristic length and time scales of photon transport chemotherapy circumference cognitive function compression stocking datatypes diffuse light diffuse optical spectroscopy diffuse optical tomography diffuse optics diffusion and delta-Eddington approximations diffusion approximation diffusion equation extracellular water fluorescence diffuse optical tomography