Second near-infrared (NIR-II) imaging: a novel diagnostic technique for brain diseases.

Imaging in the second near-infrared II (NIR-II) window, a kind of biomedical imaging technology with characteristics of high sensitivity, high resolution, and real-time imaging, is commonly used in the diagnosis of brain diseases. Compared with the conventional visible light (400-750 nm) and NIR-I (750-900 nm) imaging, the NIR-II has a longer wavelength of 1000-1700 nm. Notably, the superiorities of NIR-II can minimize the light scattering and autofluorescence of biological tissue with the depth of brain tissue penetration up to 7.4 mm. Herein, we summarized the main principles of NIR-II in animal models of traumatic brain injury, cerebrovascular visualization, brain tumor, inflammation, and stroke. Simultaneously, we encapsulated the in vivo process of NIR-II probes and their in vivo and in vitro toxic effects. We further dissected its limitations and following optimization measures.