Fluorescence imaging has been widely used in biomedical imaging and clinical intraoperative navigation. When fluorescence propagates in biological media, absorption attenuation and scattering disturbance will cause fluorescence energy loss and signal-to-noise ratio decrease, respectively. Generally speaking, the degree of absorption loss determines whether we can "see", and the number of scattered photons determines whether we can "see clearly". In addition, the autofluorescence of some biomolecules and signal light are collected by the imaging system and eventually become the background of the image. Therefore, for biofluorescence imaging, scientists are trying to find a perfect imaging window with low photon absorption and sufficient light scattering.
Since 2009, academician Hongjie Dai of Stanford University in the United States has discovered that the optical biological tissue window of 1000-1700 nm (NIR-II, NIR-II) is compared with the traditional 700-900 nm (NIR-I). Window, the light scattering of biological tissue is lower, and the imaging effect of living body is better.
Theoretically, because the optical path of scattered photons in biological media is longer than ballistic photons, tissue light absorption will preferentially consume multiple scattered photons, thereby suppressing the scattered background.
Recently, Professor Qian Jun’s research group of Zhejiang University and his collaborators discovered that compared with the near-infrared zone 1, the absorption of biological tissue in the near-infrared zone window is significantly increased, and the bioimaging effect is closely related to the light absorption of water. On the basis of reducing the scattering effect, the research group believes that the increase in water absorption is also the key to improving the effect of near-infrared in vivo fluorescence imaging.
Based on the absorption characteristics of near-infrared photons by water, the research group further refined the definition of the second region of near-infrared to 900-1880 nm. Among them, the research group found that the high water absorption of 1400-1500 nm, when the fluorescent probe is bright enough, the imaging effect is the best, and even exceeds the recognized near-infrared second-b imaging (1500-1700 nm, NIR- IIb). Therefore, the 1400-1500 nm band that has been neglected is defined as the near-infrared two x (NIR-IIx) window. Focusing on the near-infrared two-x window, the research team has achieved deep-depth mouse cerebral vascular imaging and multi-functional deep organ imaging. In addition, through simulation calculations, the research group defined 2080-2340 nm as another imaging window in the near-infrared band—NIR-III (NIR-III).
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