Absorption by water increases fluorescence image contrast of biological tissue in the shortwave infrared

• Published in: Proceedings of the National Academy of Sciences - PNAS Vol. 115; no. 37; pp. 9080 - 9085
• Main Authors: Carr, Jessica A., Aellen, Marianne, Franke, Daniel, So, Peter T. C., Bruns, Oliver T., Bawendi, Moungi G.
• Format: Journal Article
• Language: English
• Published: United States National Academy of Sciences 11.09.2018
• Subjects: Absorption; Absorptivity; Animals; Electromagnetic absorption; Fluorescence; Image contrast; Infrared imaging; Infrared imaging systems; Infrared Rays; Infrared windows; Mice; Microscopy; Models, Theoretical; Optical Imaging - instrumentation; Optical Imaging - methods; Optical properties; Penetration depth; Physical Sciences; Short wave radiation; Tissues; Water - chemistry; Water absorption; Wave attenuation; Wavelength; Wavelengths; microscopy; fluorescence; imaging; contrast; shortwave infrared
• ISSN: 0027-8424; 1091-6490
• DOI: 10.1073/pnas.1803210115

Recent technology developments have expanded the wavelength window for biological fluorescence imaging into the shortwave infrared. We show here a mechanistic understanding of how drastic changes in fluorescence imaging contrast can arise from slight changes of imaging wavelength in the shortwave infrared. We demonstrate, in 3D tissue phantoms and in vivo in mice, that light absorption by water within biological tissue increases image contrast due to attenuation of background and highly scattered light. Wavelengths of strong tissue absorption have conventionally been avoided in fluorescence imaging to maximize photon penetration depth and photon collection, yet we demonstrate that imaging at the peak absorbance of water (near 1,450 nm) results in the highest image contrast in the shortwave infrared. Furthermore, we show, through microscopy of highly labeled ex vivo biological tissue, that the contrast improvement from water absorption enables resolution of deeper structures, resulting in a higher imaging penetration depth. We then illustrate these findings in a theoretical model. Our results suggest that the wavelength-dependent absorptivity of water is the dominant optical property contributing to image contrast, and is therefore crucial for determining the optimal imaging window in the infrared.