Scattering dominates light propagation in biological tissue, and therefore restricts both resolution and penetration depth in optical imaging within thick tissue. As photons travel into the diffusive regime-typically 1 mm beneath human skin, their trajectories transition from ballistic to diffusive due to increased number of scattering events, which makes it impossible to focus, much less track, photon paths. Consequently, imaging methods that rely on controlled light illumination are ineffective in deep tissue. This problem has recently been addressed by a novel method capable of dynamically focusing light in thick scattering media via time reversal of ultrasonically encoded (TRUE) diffused light. Here, using photorefractive materials as phase conjugate mirrors, we show a direct visualization and dynamic control of optical focusing with this light delivery method, and demonstrate its application for focused fluorescence excitation and imaging in thick turbid media. These abilities are increasingly critical to understanding the dynamic interactions of light with biological matter and processes at different system levels, as well as their applications for biomedical diagnosis and therapy.

中文翻译：

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用时间反转超声编码光聚焦荧光激发并在厚散射介质中成像

散射支配着生物组织中的光传播，因此限制了厚组织内光学成像的分辨率和穿透深度。当光子进入扩散区（通常在人体皮肤下方 1 毫米处）时，由于散射事件数量的增加，它们的轨迹从弹道过渡到扩散，这使得无法聚焦，更不用说跟踪光子路径了。因此，依赖受控光照明的成像方法在深层组织中无效。这个问题最近已通过一种新方法得到解决，该方法能够通过超声波编码 (TRUE) 漫射光的时间反转在厚散射介质中动态聚焦光。在这里，使用光折变材料作为相位共轭镜，我们展示了这种光传输方法对光学聚焦的直接可视化和动态控制，并展示了其在厚混浊介质中聚焦荧光激发和成像的应用。这些能力对于理解光与不同系统级别的生物物质和过程的动态相互作用以及它们在生物医学诊断和治疗中的应用越来越重要。