Label-free microendoscopy using a micro-needle imaging probe for in vivo deep tissue imaging

We report a label-free imaging method for microendoscopy that uses a needle-type imaging probe. We inserted a thin GRIN lens that had been attached to a fiber bundle into a medical-grade needle that was used as an imaging probe. The introduction of the needle probe into biological tissue allows for direct access to deep regions that we otherwise could not achieve because of the multiple light scattering. To minimize invasiveness, we introduced the illuminating probe on the tissue surface, using an oblique back-illumination configuration. We achieved three-dimensional depth imaging by changing the depth of penetration. Since only the imaging probe goes deep into the tissue while leaving the illumination channels outside, the achievable signal depends on the location of the illumination channels. We explored this point and investigated the optimal condition for the illumination distance in a systematic way. We also applied this method to ex vivo, as well as in vivo, imaging of a mouse brain, and confirmed that we had visualized the microvasculature embedded deep within the brain.