Fluorescence-mediated detection of a heterogeneity in a highly scattering media

Tumors usually become localized absorbers at near infrared (NIR) wavelengths due to the increase in hemoglobin amount around the tumor, which is caused by angiogenesis. When a tumor is small and/or deeply seated, the contrast by the hemoglobin only, however, may not be strong. For such situation, contrast agents may be helpful, because they are preferentially accumulated in the tumor due to the unorganized tumor vasculature. In this study, indocyanine green (ICG) was used as a contrast enhancer. ICG is safe, absorbs NIR, and also generates fluorescence. A breast tissue-like model, embedded with a tumor model (1.2 x 0.7 x 0.5 cm) with/without ICG at a 1 cm depth, was constructed and the surface was scanned by a NIR time-resolved spectroscopy instrument. Enhanced contrast by ICG was confirmed in both absorption and fluorescence. For absorption, transmittance contrast was approximately two times higher than reflectance. In reflectance, the contrast by fluorescence was approximately four times higher than absorption. This study result shows that the information on both the absorption and fluorescence by ICG can be effectively used in detecting a tumor. A study of the ICG effect on deeper absorber detection is in progress.