We describes a label-free optical imaging method for brain imaging through the intact skull of a living mouse with virtually no loss in spatial resolving power and its application to a two-photon fluorescence imaging.
Research trends in endoscopy have been to reduce the dimension of the system for minimally invasive diagnostics and to improve spatial resolution to the microscopic level for the detailed investigation of specimens. In developing endoscopes that meet these needs, ultrathin imaging probes such as graded index lenses and fiber bundles have been widely used. And a single imaging probe is used for both illumination and detection to maintain the small diameter of the probe unit. However, this causes a fundamental problem, that is the back-reflection noise from the surface of the imaging probes. This back-reflection noise can overwhelm signals from target objects with weak contrast, which is the case for biological tissues, and degrade image contrast to such an extent that the objects remain unresolved.
Here, we present an endomicroscope free from back-reflection noise generated at an ultrathin imaging probe and yet guaranteeing microscopic spatial resolution. In our method, we send illumination through single individual core fibers in the image fiber bundle, and detect signal light by the other core fibers. By blocking the back-reflection occurring only at the core used for the illumination, we remove the back-reflection noise before it reaches the detector sensor. The transmission matrix of the fiber bundle is measured and used to reconstruct a pixelation-free and high-resolution image from the raw images captured by the other fibers, which are blurred and pixelated. We demonstrated that the proposed imaging method improved 3.2 times on the signal to noise ratio produced by the conventional illumination-detection scheme.
To exploit photonics technologies for in vivo studies in life science and biomedicine, it is necessary to efficiently deliver light energy to the target objects embedded deep within complex biological tissues. However, light waves diffuse randomly inside complex media due to multiple scattering, and only a small fraction reaches the target object. Here we present a method to counteract the random diffusion and to focus ‘snake-like’ multiple-scattered waves to the embedded target. To realize this, we experimentally identified time-gated reflection eigenchannels that have extraordinarily large reflectance at a specific flight time where most of the multiple-scattered waves have interacted with the target object. By injecting light to these eigenchannels, we achieved more than 10-fold enhancement in light energy delivery compared to ordinary wave diffusion cases. This method works up to depths of approximately 2 times the transport mean free path at which target objects are completely invisible by ballistic optical imaging. This work will lay a foundation for enhancing the working depth of imaging, sensing, and light stimulation.
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