IMG-141. Improving intraoperative tumor visibility with shortwave infrared fluorescence imaging

Fluorescence-guided surgery (FGS) uses fluorescent tumor-specific biomarkers to enhance contrast between tumor tissue and normal brain. 5-aminolevulinic acid-protoporphyrin IX (ALA-PpIX) is the standard of care fluorophore used for high grade gliomas. ALA-PpIX absorbs light in the violet-blue part (~405 nm) and emits in the red part (~635 nm) of the spectrum. However, these characteristics pose a problem for effective tumor imaging because of strong tissue absorption at excitation and strong scattering at emission, leading to low excitation penetration depths and blurred emission images. In general, both tissue absorption and scattering drop towards longer, near-infrared (NIR) wavelengths (700-1000 nm). Therefore, using biomarkers such as indocyanine green (ICG), that absorb and emit NIR light, allows imaging deeper inside tissue with sharper tumor features. Our objective was to explore if FGS in even longer wavelengths—specifically, in the shortwave infrared (SWIR) band (1000-1700 nm)—allows even sharper localization of tumor tissue. We built a fluorescence imaging system consisting of a 780 nm excitation light and a cooled SWIR camera with >85% peak quantum efficiency. For imaging, we used longpass filters with increasing cut-on wavelengths—800 nm, 1000 nm and 1300 nm—to select the desired fluorescence wavelength band. We injected an orthotopic rat glioblastoma model intravenously with ICG, extracted the brain 24 hours post-injection and imaged it with the above system. We assessed the effect of filtering at these successively longer wavelength bands on the fluorescence image. We observed a clear improvement in the quality of tumor localization towards longer wavelengths, with features such as tumor crossing over hemispheres and dense vasculature reliably visible only in the SWIR image. Imaging fluorescence in the SWIR spectrum therefore shows promise in improving tumor localization and providing higher signal-to-noise ratios (due to higher penetration depth) and signal-to-background ratios (due to less scattering).