Long-wavelength oblique back-illumination microscopy for deep <i>in vivo</i> imaging

Oblique back-illumination microscopy (OBM) is a label-free imaging technique that captures differential forward scattering in reflection mode to generate high-contrast pseudo-transmission images of cells and microvessels. While OBM benefits from multiple light scattering to detect forward-scattered signals, its imaging depth is constrained by tissue scattering between the objective lens and the imaging plane. In this study, we introduce a long-wavelength OBM system operating at 1650 nm-significantly longer than previous implementations-to mitigate scattering effects and extend imaging depth. Compared to a similar system using an 800 nm light source, our 1650 nm OBM achieves markedly deeper <i>in vivo</i> imaging of the mouse brain. This advancement in high-contrast, deep-tissue imaging holds promise for more detailed investigations into the pathophysiology of living biological systems.