New advances in biomedical applications of multiphoton imaging technology

In contrast to single photon excitation fluorescence imaging, laser scanning confocal imaging, and wide-field imaging, the multi-photon imaging has advantages of minimal invasion and deeper penetration by using near-infrared (NIR) laser source. Moreover, it can carry out three-dimensional high-spatial-resolution imaging of biological tissues due to its natural optical tomography capability. Since its advent, multi-photon imaging has become a powerful tool in biomedicine and achieved a series of significant discoveries in cancer pathology, neurological diseases and brain functional imaging. In the past decade, as a major form of multi-photon imaging techonoogy, two-photon excited fluorescence microscopy imaging has a great potential in biomedical applications. In order to satisfy the practical biomedical applications, multi-photon imaging technologies have made significant breakthroughs in improving the deficiencies of traditional 2PEF in multi-color imaging, functional imaging, live imaging and imaging depth, such as multicolor two-photon excitation fluorescence microscopy, two-photon fluorescence lifetime imaging microscopy, two-photon fiber endoscopic imaging, and three-photon microscopy imaging technology. For example, multicolor two-photon excitation fluorescence microscopy is demonstrated to achieve simultaneous imaging of multiple fluorophores with multiple wavelenth excitation lasers or continuous spectrum. In addition, the two-photon fluorescence lifetime microscopic imaging provides a method to achieve high-resolution three-dimensional imaging of biological tissue with multi-dimensional information including fluorescence intensity and lifetime. In addition, two-photon optical fiber endoscopic imaging with small system size and mimal invasion is developed and used to image the tissue inside the deep organ. Finally, two-photon excitation fluorescence microscopy technique still has relatively strong scattering for brain functional imaging in vivo. Therefore, the imaging depth is limited by the signal-to-background ratio. Three-photon microscopic imaging technique can achieve higher imaging depth and a desired signal-to-noise ratio by extending the wavelength from 1600 nm to 1820 nm because the attenuation of the excitation light in this wavelenth range is much smaller. In this article, we briefly introduce the principles and applications of these multi-photon imaging technologies, and finally provide our view for their future development.