Review of shortwave infrared imaging and spectroscopy in tissue [Invited].

The shortwave infrared (SWIR) wavelength band (1,000-2,000 nm) has recently garnered increased interest due to its beneficial attributes for tissue imaging and spectroscopy. Lower scattering, increased sensitivity to endogenous water and lipid content, and the reduced influence of melanin offer new measurement opportunities and application areas. SWIR sensor technology is advancing at a rapid pace, fueling growth in academic and commercial instrument development. Here, we review recent progress in SWIR tissue imaging and spectroscopy, provide resources on tissue optical properties at SWIR wavelengths, and offer guidance on the utilization of models, methods, and instruments for SWIR measurements in tissue. One key finding is that while some efforts have exploited the lower scattering in the SWIR to achieve deep tissue imaging, this advantage is highly dependent on the specific choice of wavelength and the measurement geometry. Photon attenuation limits these advantages at wavelengths where water absorption is strong, especially in the context of higher-noise SWIR detectors. However, a growing number of clinical applications are emerging, especially those requiring in vivo water and lipid measurements, such as monitoring of tissue hydration, edema, lipid content, and others. There are further opportunities to expand this work toward more disease states while leveraging the low absorption of melanin in the SWIR. Since the field is currently limited by relatively few sources of tissue optical property data at SWIR wavelengths, we have compiled tabulated extinction values of SWIR chromophores from the literature. There is also a need for continued development of techniques for modeling photon propagation in tissue at SWIR wavelengths due to more moderate levels of tissue scattering compared to absorption, and methods to establish stable tissue-mimicking phantoms. The SWIR wavelength region is coming of age in the biophotonics community, with components and systems creating new opportunities in basic research and clinical applications once current challenges can be overcome.