Hybrid optical parametrically oscillating emitter-enabled photoacoustic imaging of water: enhanced contrast, dynamic range, and multifaceted applications

Abstract

Water photoacoustic microscopy (PAM) enables water absorption contrast mapping in deep biological tissue, which further allows a more detailed architecture analysis and facilitates a better understanding of metabolic and pathophysiological pathways. The strongest absorption peak of water in the near-infrared region occurs at 1930 nm, where the first overtone of the O-H bond lies. However, general light sources operating in this band hitherto still suffer from low optical signal-to-noise ratio and suboptimal pulse widths for photoacoustic signal generation. These lead to not only PAM contrast deterioration but also a high risk of sample photodamage. Consequently, we developed a hybrid optical parametrically-oscillating emitter (HOPE) source for an improved water PAM image contrast, leading to noninvasive and safer bioimaging applications. Our proposed source generates 1930 nm laser pulses with high spectral purity at a repetition rate of 187.5 kHz. The pulse width is flexibly tunable from 4 to 15 ns, and the maximum pulse energy is 700 nJ with a power stability of 1.79%. Leveraging these advancements, we also demonstrated high-contrast water PAM in multifaceted application scenarios, including tracking the dynamic of water distribution in a zebrafish embryo, visualizing the water content of a murine tumor xenograft, and mapping the fluid distribution in an edema mouse ear model. Finally, we showcased 1750-nm/1930-nm dual-color PAM for quantitative imaging of lipid and water distributions with reduced cross talk and imaging artifacts. Given all these results, we believe that our HOPE source can heighten water PAM's relevance in both biological research and clinical diagnostics. The Authors.