In vivo optical metabolic imaging of human skin exposed to high altitude using a femtosecond fiber-laser multiphoton FLIM tomograph

High-resolution optical metabolic imaging (OMI) of in vivo human skin with a lateral resolution of 300 nm was performed using a multimodal multiphoton tomograph based on an ultracompact 80 MHz air-cooled Er:doped fiber laser operating at 780 nm, integrated into the imaging head, in combination with time-correlated single photon counting (TCSPC) with picosecond temporal resolution. In particular, the mean autofluorescence lifetime of the two-photon excited reduced nicotinamide adenine dinucleotide (NADH), mainly located in the mitochondria of the living epidermis, was depicted as a pseudo-colored fluorescence lifetime image (FLIM). Free (unbound) NADH exhibits a short lifetime, whereas a long nanosecond lifetime corresponds to protein-bound NADH. The ratio free to bound NADH, derived from a biexponential fit of the fluorescence decay curves on a pixel-by-pixel basis, can be used as an indicator of cellular metabolism. The oxidized coenzyme NAD+ is non-fluorescent [1, 2]. Here we report the first two-photon OMI-FLIM measurements performed in human subjects inside a hypobaric chamber simulating high-altitude conditions [3]. Furthermore, the cycling effect at an altitude of 3000 m was investigated in two volunteers who climbed altitudes exceeding 6000 meters (Aconcagua, 6961m), in comparison with sea-level athletes. Interestingly, the 780 nm-excited NADH autofluorescence intensity increased during the reduction of ambient air pressure, accompanied by measurable changes in the mean autofluorescence lifetime. These preliminary low-air-pressure OMI results are promising with respect to future studies aimed at developing an OMI-based sensor for evaluating the treatment success in patients with mitochondrial dysfunction, as well as providing a novel optical tool for sports medicine.