An accurate homogenized tissue phantom for broad spectrum autofluorescence studies: a tool for optimizing quantum dot-based contrast agents

We are investigating the use of ZnS-capped CdSe quantum dot (QD) bioconjugates combined with fluorescence endoscopy for improved early cancer detection in the esophagus, colon and lung. A major challenge in using fluorescent contrast agents in vivo is to extract the relevant signal from the tissue autofluorescence (AF). The present studies are aimed at maximizing the QD signal to AF background ratio (SBR) to facilitate detection. These contrast optimization studies require optical phantoms that simulate tissue autofluorescence, absorption and scattering over the entire visible spectrum, while allowing us to control the optical thickness. We present an optical phantom made of fresh homogenized tissue diluted in water. The homogenized tissue is poured into a clear polymer tank designed to hold a QD-loaded silica capillary in its center. Because of the non-linear effects of absorption and scattering on measured autofluorescence, direct comparison between results obtained using tissue phantoms of different concentration is not possible. We introduce mathematical models that make it possible to perform measurements on diluted tissue homogenates and subsequently extrapolate the results to intact (non-diluted) tissue. Finally, we present preliminary QD contrast data showing that the 380-420 nm spectral window is optimal for surface QD imaging.