“…Fluorescent compounds are traced easily in tissues ( 3 , 4 , 23 , 24 ) using laser scanning microscopy. Nonfluorescent substances require fluorescent labeling (altering the diffusion and partition properties), use of fluorescent phantom compounds (having similar physical and chemical properties), or a probing modality beyond fluorescence, like Raman scattering ( 25 , 26 , 27 ). Raman techniques applied to biological samples often suffer from considerable unspecific backgrounds.…”