Review: Tissue Optical Clearing Window for Blood Flow Monitoring

The tissue optical clearing (TOC) technique could significantly improve the biomedical optical imaging depth, but most current investigations are limited to in vitro studies. For in vivo applications, the TOC method must provide a rapid treatment process, sufficient transparency, and safety for animals, which makes it more difficult. Recently developed innovative optical clearing methods for in vivo use show great potential for enhancing the contrast and resolution of laser speckle contrast imaging (LSCI) for blood flow monitoring. This paper gives an overview of recent progress in the use of TOC for vascular visualization with LSCI. First, the principle of TOC-induced improvement of LSCI and a quantitative analysis method for evaluating the improvement are described briefly. Second, the paper introduces transparent windows, including various skin windows and a cranial window, that permit LSCI to monitor dermal or cortical blood flow, respectively, with high resolution and contrast. Third, preliminary investigations of the safety of TOC demonstrate that the transparent skin window is switchable, which enables LSCI to repeatedly image blood flow. However, research on in vivo TOC is currently less advanced than that on in vitro TOC. Future work should focus on developing a highly effective, safe method and extending its applications.