Effect of optical tissue properties during real-time intraoperative optical imaging of liver metastases in humans. Effect of optical tissue properties during real-time intraoperative optical imaging of liver metastases in humans. The consequences of absorption and scattering on the image are essential for understanding the limitations of optical image-guided surgery: At the surface, fluorescent agents will appear as a bright and sharply delineated spot. However, the target will always be surrounded by a halo of fluorescent light that was directed into the tissue after emission, scattered around locally, and emitted from the surface at some distance from the target location. Moreover, due to absorption and scattering, an identical fluorescent agent that is located deeper within the tissue will have lower signal intensity and will be imaged as an indistinct blob. An example is shown of a liver metastasis that is delineated by fluorescence signal (A–C). When a thick layer of greater omentum covers the area of interest, fluorescence signal is not detected (D–F). A second liver metastasis is indicated in a different patient (G–I). However, when the liver is flipped around and the opposite side is imaged, high absorption of the liver that is saturated with blood results in an indistinct fluorescent blob (J–L). Stijn Keereweer et al. Clin Cancer Res 2013;19:3745-3754 ©2013 by American Association for Cancer Research