Purpose: To investigate indocyanine green (ICG) as a molecular beacon for malignant lesions within the liver and evaluate the ability of a developed handheld imaging system to allow measurement of ICG fluorescence within focal hepatic lesions with high target-to-background ratios in a mouse model.
Materials and methods: All animal experiments were approved by the institutional animal care committee. A handheld optical molecular imaging device was constructed to pass through the introducer needle of a standard percutaneous biopsy kit. An ex vivo phantom system was constructed to quantify tissue attenuation properties of ICG in liver parenchyma. Subsequently, intrahepatic colorectal cancer metastases were generated in nude mice, and epifluorescence imaging of ICG, as well as histologic analysis of the explanted livers, was performed at 3 weeks after implantation (n = 6). Epifluorescence imaging with the handheld imaging device was then performed on intrahepatic colorectal metastases after the administration of ICG (n = 15) at 3, 6, and 24 hours after injection. Target-to-background ratios were calculated for each time point. Subsequently, a core biopsy of intrahepatic colorectal metastases was performed by using a standard clinical 18-gauge biopsy needle.
Results: There was avid localization of ICG to the focal lesions at all time points. Similarly, fluorescence within the tumors was greater than that within normal liver, as detected with the handheld imaging system (mean target-to-background ratio ± standard deviation, 3.9 ± 0.2 at 24 hours). A core biopsy of tumor and normal adjacent liver by using a standard biopsy needle demonstrated a sharp margin of fluorescence intensity at the tumor-liver interface.
Conclusion: The custom-designed molecular imaging device, in combination with ICG, readily allowed differentiation between normal and malignant tissue in a murine model of intrahepatic colorectal metastasis.