A systematic study on fluorescence contrast in near infrared diffuse transmittance imaging with indocyanine green

Abstract

Near infrared fluorescence imaging is a sensitive, noninvasive technique for diagnostic applications in biomedical optics. The main purpose of this work is thus to explore how to improve the contrast of images obtained by near infrared light using a fluorescent extrinsic agent. Among different fluorophores, indocyanine green has been mostly studied because it is approved for use in humans. In this work, simulations and experimental studies on phantoms (systems that optically emulate biological tissues) are used to systematically investigate the influence of the increased intrinsic tissue absorption by adding indocyanine green. The experiments reproduce the situation of fluorescence imaging of carcinomas in the human breast, where the natural absorption due to neovascularization is increased by the injection of this fluorophore. Assuming measurements in transmission geometry, the breast is modeled by a homogeneous background medium containing a tumor-like inclusion (or lesion) with two- or threefold increased absorption. Fluorescence contrast is simulated over a broad range of dye concentrations using diffusion theory. Selected concentrations ratios are applied in experimental studies with laser excitation of indocyanine green fluorescence and with a charge-coupled device camera for fluorescence detection. Both simulations and experiments show that the intrinsic absorption of the inclusion strongly reduces the number of detected fluorescence photons and that the fluorescence contrast can be canceled or become even negative. It was found that for typical optical properties and geometrical conditions, in fluorescence imaging of breast cancer, a dye ratio of about 10:1 (lesion:background) is required to turn from negative to positive fluorescence contrast. Since such a high ratio is difficult to attain, raw fluorescence images need to be normalized by the intrinsic lesion absorption (without indocyanine green (ICG)) to enhance the presence of the dye in the lesion.