Imaging deep and non-invasively through biological tissue is one of the main challenges of modern microscopy. As a light beam travels through tissue, it is attenuated both by scattering and absorption events. The behavior of both processes differs spectrally: with increasing wavelength, scattering decreases while absorption increases but possesses local dips in the near-infrared spectral range. Imaging deeper in tissue thus can be achieved using a light source centered within these regions of decreased water absorption, called biological optical windows, as has been shown both in multiphoton microscopy and optical coherence tomography (OCT) at 1.3 µm and 1.7 µm. In their latest work, Zhu et al. explore the optical window centered at 2.1 µm for OCT brain imaging, and demonstrate its potential for non-invasive imaging through the skull, a dense and highly scattering tissue with moderate water concentration. The authors highlight a significant reduction in the attenuation coefficient of bone tissue at 2.1 µm compared to 1.3 µm, enabling imaging of superficial cerebral cortical vasculature through the intact skull in anesthetized rats. Lastly, owing to the high water absorption at 2.1 µm, the authors present another feature of this optical window by performing direct spectroscopic measurements of water content of bone and cortical tissue.

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